diff --git a/.bumpversion.cfg b/.bumpversion.cfg
deleted file mode 100644
index ac1d31324..000000000
--- a/.bumpversion.cfg
+++ /dev/null
@@ -1,29 +0,0 @@
-[bumpversion]
-current_version = 2.4.0.dev0
-parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(\.(?P<release>[a-z]+)(?P<dev>\d+))?
-serialize = 
-	{major}.{minor}.{patch}.{release}{dev}
-	{major}.{minor}.{patch}
-
-[bumpversion:part:release]
-optional_value = dummy
-values = 
-	dev
-	dummy
-
-[bumpversion:part:dev]
-
-[bumpversion:file:setup.py]
-
-[bumpversion:file:conda.recipe/meta.yaml]
-
-[bumpversion:file:src/runtime/resources/clr.py]
-
-[bumpversion:file:src/SharedAssemblyInfo.cs]
-serialize = 
-	{major}.{minor}.{patch}
-
-[bumpversion:file:src/clrmodule/ClrModule.cs]
-serialize = 
-	{major}.{minor}.{patch}
-
diff --git a/.editorconfig b/.editorconfig
index 2e7c58ffe..d64f74bc1 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -19,6 +19,17 @@ indent_size = 2
 [*.{csproj,pyproj,config}]
 indent_size = 2
 
+# .NET formatting settings
+[*.{cs,vb}]
+dotnet_sort_system_directives_first = true
+dotnet_separate_import_directive_groups = true
+
+[*.cs]
+csharp_new_line_before_open_brace = all
+csharp_new_line_before_else = true
+csharp_new_line_before_catch = true
+csharp_new_line_before_finally = true
+
 # Solution
 [*.sln]
 indent_style = tab
diff --git a/.travis.yml b/.travis.yml
index 1dadbad1d..2062a35da 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,80 +1,18 @@
-dist: trusty
+dist: xenial
 sudo: false
 language: python
-  
-matrix:
-  include:
-# --------------------- XPLAT builds ------------------------
-    - python: 2.7
-      env: &xplat-env
-        - BUILD_OPTS=--xplat
-        - NUNIT_PATH=~/.nuget/packages/nunit.consolerunner/3.*/tools/nunit3-console.exe
-      addons: &xplat-addons
-        apt:
-          sources:
-          - sourceline: deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-ubuntu-trusty-prod trusty main
-            key_url: https://packages.microsoft.com/keys/microsoft.asc
-          - sourceline: deb http://download.mono-project.com/repo/ubuntu trusty main
-            key_url: http://keyserver.ubuntu.com/pks/lookup?op=get&search=0xA6A19B38D3D831EF
-          packages:
-            - mono-devel
-            - ca-certificates-mono
-            - dotnet-hostfxr-2.0.0
-            - dotnet-runtime-2.0.0
-            - dotnet-sdk-2.0.0
-            
-    - python: 3.5
-      env: *xplat-env
-      addons: *xplat-addons
-
-    - python: 3.6
-      env: *xplat-env
-      addons: *xplat-addons
-      
-    - python: 3.7
-      env: *xplat-env
-      dist: xenial
-      sudo: true
-      addons: &xplat-addons-xenial
-        apt:
-          sources:
-          - sourceline: deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-ubuntu-xenial-prod xenial main
-            key_url: https://packages.microsoft.com/keys/microsoft.asc
-          - sourceline: deb https://download.mono-project.com/repo/ubuntu stable-xenial main
-            key_url: http://keyserver.ubuntu.com/pks/lookup?op=get&search=0xA6A19B38D3D831EF
-          packages:
-            - mono-devel
-            - ca-certificates-mono
-            - dotnet-hostfxr-2.0.0
-            - dotnet-runtime-2.0.0
-            - dotnet-sdk-2.0.0
-
-# --------------------- Classic builds ------------------------
-    - python: 2.7
-      env: &classic-env
-        - BUILD_OPTS=
-        - NUNIT_PATH=./packages/NUnit.*/tools/nunit3-console.exe
-
-    - python: 3.5
-      env: *classic-env
-
-    - python: 3.6
-      env: *classic-env
-
-    - python: 3.7
-      env: *classic-env
-      dist: xenial
-      sudo: true
-      addons:
-        apt:
-          sources:
-            - sourceline: deb http://download.mono-project.com/repo/ubuntu xenial main
-              key_url: http://keyserver.ubuntu.com/pks/lookup?op=get&search=0xA6A19B38D3D831EF
-          packages:
-            - mono-devel
-            - ca-certificates-mono
+python:
+    - 3.8
+    - 3.7
+    - 3.6
+    - 3.5
+    - 2.7
 
 env:
+  matrix:
+    - BUILD_OPTS=--xplat NUNIT_PATH="~/.nuget/packages/nunit.consolerunner/3.*/tools/nunit3-console.exe" RUN_TESTS=dotnet EMBED_TESTS_PATH=netcoreapp2.0_publish/ PERF_TESTS_PATH=net461/
+    - BUILD_OPTS="" NUNIT_PATH="./packages/NUnit.*/tools/nunit3-console.exe" RUN_TESTS="mono $NUNIT_PATH" EMBED_TESTS_PATH="" PERF_TESTS_PATH=""
+
   global:
     - LD_PRELOAD=/lib/x86_64-linux-gnu/libSegFault.so
     - SEGFAULT_SIGNALS=all
@@ -84,11 +22,16 @@ env:
 addons:
   apt:
     sources:
-      - sourceline: deb http://download.mono-project.com/repo/ubuntu trusty main
+      - sourceline: deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-ubuntu-xenial-prod xenial main
+        key_url: https://packages.microsoft.com/keys/microsoft.asc
+      - sourceline: deb http://download.mono-project.com/repo/ubuntu stable-xenial/snapshots/5.20 main
         key_url: http://keyserver.ubuntu.com/pks/lookup?op=get&search=0xA6A19B38D3D831EF
     packages:
       - mono-devel
       - ca-certificates-mono
+      - dotnet-hostfxr-2.2
+      - dotnet-runtime-2.2
+      - dotnet-sdk-2.2
 
 before_install:
   # Set-up dll path for embedded tests
@@ -102,13 +45,11 @@ install:
 
 script:
   - python -m pytest
-  - mono $NUNIT_PATH src/embed_tests/bin/Python.EmbeddingTest.dll
-  - if [[ $BUILD_OPTS == --xplat ]]; then dotnet src/embed_tests/bin/netcoreapp2.0_publish/Python.EmbeddingTest.dll; fi
+  - $RUN_TESTS src/embed_tests/bin/$EMBED_TESTS_PATH/Python.EmbeddingTest.dll --labels=All
+  # does not work on Linux, because NuGet package for 2.3 is Windows only
+  # - "if [[ $TRAVIS_PYTHON_VERSION == '3.5' && $PERF_TESTS_PATH != '' ]]; then mono $NUNIT_PATH src/perf_tests/bin/$PERF_TESTS_PATH/Python.PerformanceTests.dll; fi"
 
 after_script:
-  # Uncomment if need to geninterop, ie. py37 final
-  # - python tools/geninterop/geninterop.py
-
   # Waiting on mono-coverage, SharpCover or xr.Baboon
   - coverage xml -i
   - codecov --file coverage.xml --flags setup_linux
diff --git a/AUTHORS.md b/AUTHORS.md
index 27aae63f4..5dbf1e1ce 100644
--- a/AUTHORS.md
+++ b/AUTHORS.md
@@ -2,8 +2,11 @@
 
 ## Development Lead
 
--   Barton Cline ([@BartonCline](https://github.com/BartonCline))
 -   Benedikt Reinartz ([@filmor](https://github.com/filmor))
+-   Victor Milovanov ([@lostmsu](https://github.com/lostmsu))
+
+## Former Development Leads
+-   Barton Cline ([@BartonCline](https://github.com/BartonCline))
 -   Brian Lloyd ([@brianlloyd](https://github.com/brianlloyd))
 -   David Anthoff ([@davidanthoff](https://github.com/davidanthoff))
 -   Denis Akhiyarov ([@denfromufa](https://github.com/denfromufa))
@@ -12,13 +15,17 @@
 
 ## Contributors
 
+-   Alex Earl ([@slide](https://github.com/slide))
+-   Alex Helms ([@alexhelms](https://github.com/alexhelms))
 -   Alexandre Catarino([@AlexCatarino](https://github.com/AlexCatarino))
+-   Andrey Sant'Anna ([@andreydani](https://github.com/andreydani))
 -   Arvid JB ([@ArvidJB](https://github.com/ArvidJB))
 -   Benoît Hudson ([@benoithudson](https://github.com/benoithudson))
 -   Bradley Friedman ([@leith-bartrich](https://github.com/leith-bartrich))
 -   Callum Noble ([@callumnoble](https://github.com/callumnoble))
 -   Christian Heimes ([@tiran](https://github.com/tiran))
 -   Christoph Gohlke ([@cgohlke](https://github.com/cgohlke))
+-   Christopher Bremner ([@chrisjbremner](https://github.com/chrisjbremner))
 -   Christopher Pow ([@christopherpow](https://github.com/christopherpow))
 -   Daniel Fernandez ([@fdanny](https://github.com/fdanny))
 -   Daniel Santana ([@dgsantana](https://github.com/dgsantana))
@@ -26,16 +33,23 @@
 -   David Lassonde ([@lassond](https://github.com/lassond))
 -   David Lechner ([@dlech](https://github.com/dlech))
 -   Dmitriy Se ([@dmitriyse](https://github.com/dmitriyse))
+-   Florian Treurniet ([@ftreurni](https://github.com/ftreurni))
 -   He-chien Tsai ([@t3476](https://github.com/t3476))
--   Ivan Cronyn ([@cronan](https://github.com/cronan))
+-   Inna Wiesel ([@inna-w](https://github.com/inna-w))
+-   Ivan Cronyn ([@cronan](https://github.com/cronan))
 -   Jan Krivanek ([@jakrivan](https://github.com/jakrivan))
--   Jeff Reback ([@jreback](https://github.com/jreback))
+-   Jeff Reback ([@jreback](https://github.com/jreback))
+-   Jeff Robbins ([@jeff17robbins](https://github.com/jeff17robbins))
 -   Joe Frayne ([@jfrayne](https://github.com/jfrayne))
+-   Joe Lidbetter ([@jmlidbetter](https://github.com/jmlidbetter))
+-   Joe Savage ([@s4v4g3](https://github.com/s4v4g3))
 -   John Burnett ([@johnburnett](https://github.com/johnburnett))
 -   John Wilkes ([@jbw3](https://github.com/jbw3))
 -   Luke Stratman ([@lstratman](https://github.com/lstratman))
 -   Konstantin Posudevskiy ([@konstantin-posudevskiy](https://github.com/konstantin-posudevskiy))
 -   Matthias Dittrich ([@matthid](https://github.com/matthid))
+-   Meinrad Recheis ([@henon](https://github.com/henon))
+-   Mohamed Koubaa ([@koubaa](https://github.com/koubaa))
 -   Patrick Stewart ([@patstew](https://github.com/patstew))
 -   Raphael Nestler ([@rnestler](https://github.com/rnestler))
 -   Rickard Holmberg ([@rickardraysearch](https://github.com/rickardraysearch))
@@ -43,7 +57,6 @@
 -   Sean Freitag ([@cowboygneox](https://github.com/cowboygneox))
 -   Serge Weinstock ([@sweinst](https://github.com/sweinst))
 -   Simon Mourier ([@smourier](https://github.com/smourier))
--   Victor Milovanov ([@lostmsu](https://github.com/lostmsu))
 -   Viktoria Kovescses ([@vkovec](https://github.com/vkovec))
 -   Ville M. Vainio ([@vivainio](https://github.com/vivainio))
 -   Virgil Dupras ([@hsoft](https://github.com/hsoft))
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 289b864cb..ad4016450 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,11 +1,71 @@
 # Changelog
 
-All notable changes to Python for .NET will be documented in this file.
-This project adheres to [Semantic Versioning][].
+All notable changes to Python.NET will be documented in this file. This
+project adheres to [Semantic Versioning][].
 
 This document follows the conventions laid out in [Keep a CHANGELOG][].
 
-## [2.4.0][]
+## [Unreleased][]
+
+### Added
+
+### Changed
+
+### Fixed
+
+## [2.5.0][] - 2020-06-14
+
+This version improves performance on benchmarks significantly compared to 2.3.
+
+### Added
+
+-   Automatic NuGet package generation in appveyor and local builds
+-   Function that sets `Py_NoSiteFlag` to 1.
+-   Support for Jetson Nano.
+-   Support for `__len__` for .NET classes that implement ICollection
+-   `PyExport` attribute to hide .NET types from Python
+-   `PythonException.Format` method to format exceptions the same as
+    `traceback.format_exception`
+-   `Runtime.None` to be able to pass `None` as parameter into Python from .NET
+-   `PyObject.IsNone()` to check if a Python object is None in .NET.
+-   Support for Python 3.8
+-   Codecs as the designated way to handle automatic conversions between
+    .NET and Python types
+
+### Changed
+
+-   Added argument types information to "No method matches given arguments" message
+-   Moved wheel import in setup.py inside of a try/except to prevent pip collection failures
+-   Removes `PyLong_GetMax` and `PyClass_New` when targetting Python3
+-   Improved performance of calls from Python to C#
+-   Added support for converting python iterators to C# arrays
+-   Changed usage of the obsolete function
+    `GetDelegateForFunctionPointer(IntPtr, Type)` to
+    `GetDelegateForFunctionPointer<TDelegate>(IntPtr)`
+-   When calling C# from Python, enable passing argument of any type to a
+    parameter of C# type `object` by wrapping it into `PyObject` instance.
+    ([#881][i881])
+-   Added support for kwarg parameters when calling .NET methods from Python
+-   Changed method for finding MSBuild using vswhere
+-   Reworked `Finalizer`. Now objects drop into its queue upon finalization,
+    which is periodically drained when new objects are created.
+-   Marked `Runtime.OperatingSystemName` and `Runtime.MachineName` as
+    `Obsolete`, should never have been `public` in the first place. They also
+    don't necessarily return a result that matches the `platform` module's.
+-   Unconditionally depend on `pycparser` for the interop module generation
+
+### Fixed
+
+-   Fixed runtime that fails loading when using pythonnet in an environment
+    together with Nuitka
+-   Fixes bug where delegates get casts (dotnetcore)
+-   Determine size of interpreter longs at runtime
+-   Handling exceptions ocurred in ModuleObject's getattribute
+-   Fill `__classcell__` correctly for Python subclasses of .NET types
+-   Fixed issue with params methods that are not passed an array.
+-   Use UTF8 to encode strings passed to `PyRun_String` on Python 3
+
+## [2.4.0][] - 2019-05-15
 
 ### Added
 
@@ -24,6 +84,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
 -   Added PyObject finalizer support, Python objects referred by C# can be auto collect now ([#692][p692]).
 -   Added detailed comments about aproaches and dangers to handle multi-app-domains ([#625][p625])
 -   Python 3.7 support, builds and testing added. Defaults changed from Python 3.6 to 3.7 ([#698][p698])
+-   Added support for C# types to provide `__repr__` ([#680][p680])
 
 ### Changed
 
@@ -47,7 +108,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
 -   Fixed conversion of 'float' and 'double' values ([#486][i486])
 -   Fixed 'clrmethod' for python 2 ([#492][i492])
 -   Fixed double calling of constructor when deriving from .NET class ([#495][i495])
--   Fixed `clr.GetClrType` when iterating over `System` members ([#607][p607]) 
+-   Fixed `clr.GetClrType` when iterating over `System` members ([#607][p607])
 -   Fixed `LockRecursionException` when loading assemblies ([#627][i627])
 -   Fixed errors breaking .NET Remoting on method invoke ([#276][i276])
 -   Fixed PyObject.GetHashCode ([#676][i676])
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ea0f3408e..ffeb792f4 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,11 +1,15 @@
 # How to contribute
 
-PythonNet is developed and maintained by unpaid community members so well
+Python.NET is developed and maintained by unpaid community members so well
 written, documented and tested pull requests are encouraged.
 
 By submitting a pull request for this project, you agree to license your
 contribution under the MIT license to this project.
 
+This project has adopted the code of conduct defined by the Contributor
+Covenant to clarify expected behavior in our community. For more information
+see the [.NET Foundation Code of Conduct](https://dotnetfoundation.org/code-of-conduct).
+
 ## Getting Started
 
 -   Make sure you have a [GitHub account](https://github.com/signup/free)
@@ -41,3 +45,4 @@ contribution under the MIT license to this project.
 
 -   [General GitHub documentation](https://help.github.com/)
 -   [GitHub pull request documentation](https://help.github.com/send-pull-requests/)
+-   [.NET Foundation Code of Conduct](https://dotnetfoundation.org/about/code-of-conduct)
diff --git a/LICENSE b/LICENSE
index 59abd9c81..19c31a12f 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2006-2019 the contributors of the "Python for .NET" project
+Copyright (c) 2006-2020 the contributors of the Python.NET project
 
 Permission is hereby granted, free of charge, to any person obtaining a
 copy of this software and associated documentation files (the "Software"),
diff --git a/README.rst b/README.rst
index e59ad94f6..55f0e50a1 100644
--- a/README.rst
+++ b/README.rst
@@ -1,14 +1,14 @@
-pythonnet - Python for .NET
+pythonnet - Python.NET
 ===========================
 
 |Join the chat at https://gitter.im/pythonnet/pythonnet|
 
 |appveyor shield| |travis shield| |codecov shield|
 
-|license shield| |pypi package version| |python supported shield|
+|license shield| |pypi package version| |conda-forge version| |python supported shield|
 |stackexchange shield|
 
-Python for .NET is a package that gives Python programmers nearly
+Python.NET is a package that gives Python programmers nearly
 seamless integration with the .NET Common Language Runtime (CLR) and
 provides a powerful application scripting tool for .NET developers. It
 allows Python code to interact with the CLR, and may also be used to
@@ -17,8 +17,7 @@ embed Python into a .NET application.
 Calling .NET code from Python
 -----------------------------
 
-Python for .NET allows CLR namespaces to be treated essentially as
-Python packages.
+Python.NET allows CLR namespaces to be treated essentially as Python packages.
 
 .. code-block::
 
@@ -90,11 +89,25 @@ Output:
    int32
    [  6.  10.  12.]
 
+
+
+Resources
+---------
+
 Information on installation, FAQ, troubleshooting, debugging, and
 projects using pythonnet can be found in the Wiki:
 
 https://github.com/pythonnet/pythonnet/wiki
 
+Mailing list
+    https://mail.python.org/mailman/listinfo/pythondotnet
+Chat
+    https://gitter.im/pythonnet/pythonnet
+
+.NET Foundation
+---------------
+This project is supported by the `.NET Foundation <https://dotnetfoundation.org>`_.
+
 .. |Join the chat at https://gitter.im/pythonnet/pythonnet| image:: https://badges.gitter.im/pythonnet/pythonnet.svg
    :target: https://gitter.im/pythonnet/pythonnet?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
 .. |appveyor shield| image:: https://img.shields.io/appveyor/ci/pythonnet/pythonnet/master.svg?label=AppVeyor
@@ -111,3 +124,5 @@ https://github.com/pythonnet/pythonnet/wiki
    :target: https://pypi.python.org/pypi/pythonnet
 .. |stackexchange shield| image:: https://img.shields.io/badge/StackOverflow-python.net-blue.svg
    :target: http://stackoverflow.com/questions/tagged/python.net
+.. |conda-forge version| image:: https://img.shields.io/conda/vn/conda-forge/pythonnet.svg
+   :target: https://anaconda.org/conda-forge/pythonnet
diff --git a/appveyor.yml b/appveyor.yml
index 74b9a9c8e..b58b72372 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -15,24 +15,21 @@ environment:
     CODECOV_ENV: PYTHON_VERSION, PLATFORM
 
   matrix:
-    - PYTHON_VERSION: 2.7 
+    - PYTHON_VERSION: 3.8
       BUILD_OPTS: --xplat
-    - PYTHON_VERSION: 3.5
+    - PYTHON_VERSION: 3.7
       BUILD_OPTS: --xplat
     - PYTHON_VERSION: 3.6
       BUILD_OPTS: --xplat
-    - PYTHON_VERSION: 3.7
-      BUILD_OPTS: --xplat
-    - PYTHON_VERSION: 2.7
     - PYTHON_VERSION: 3.5
-    - PYTHON_VERSION: 3.6
+      BUILD_OPTS: --xplat
+    - PYTHON_VERSION: 2.7 
+      BUILD_OPTS: --xplat
+    - PYTHON_VERSION: 3.8
     - PYTHON_VERSION: 3.7
-
-matrix:
-    allow_failures:
-      - PYTHON_VERSION: 3.4
-        BUILD_OPTS: --xplat
-      - PYTHON_VERSION: 3.4
+    - PYTHON_VERSION: 3.6
+    - PYTHON_VERSION: 3.5
+    - PYTHON_VERSION: 2.7
 
 init:
   # Update Environment Variables based on matrix/platform
@@ -57,9 +54,8 @@ build_script:
   - coverage run setup.py bdist_wheel %BUILD_OPTS%
 
 test_script:
-  - pip install --no-index --find-links=.\dist\ pythonnet
+  - pip install --find-links=.\dist\ pythonnet
   - ps: .\ci\appveyor_run_tests.ps1
-  - ps: .\ci\appveyor_build_recipe.ps1
 
 on_finish:
   # Temporary disable multiple upload due to codecov limit of 20 per commit.
@@ -72,6 +68,7 @@ on_finish:
 
 artifacts:
   - path: dist\*
+  - path: '.\src\runtime\bin\*.nupkg'
 
 notifications:
   - provider: Slack
diff --git a/ci/appveyor_build_recipe.ps1 b/ci/appveyor_build_recipe.ps1
deleted file mode 100644
index 84e0bc7c6..000000000
--- a/ci/appveyor_build_recipe.ps1
+++ /dev/null
@@ -1,39 +0,0 @@
-# Build `conda.recipe` only if this is a Pull_Request. Saves time for CI.
-
-$env:CONDA_PY = "$env:PY_VER"
-# Use pre-installed miniconda. Note that location differs if 64bit
-$env:CONDA_BLD = "C:\miniconda36"
-
-if ($env:PLATFORM -eq "x86"){
-    $env:CONDA_BLD_ARCH=32
-} else {
-    $env:CONDA_BLD_ARCH=64
-    $env:CONDA_BLD = "$env:CONDA_BLD" + "-x64"
-}
-
-if ($env:APPVEYOR_PULL_REQUEST_NUMBER -or $env:APPVEYOR_REPO_TAG_NAME -or $env:FORCE_CONDA_BUILD -eq "True") {
-    # Update PATH, and keep a copy to restore at end of this PowerShell script
-    $old_path = $env:path
-    $env:path = "$env:CONDA_BLD;$env:CONDA_BLD\Scripts;" + $env:path
-
-    Write-Host "Starting conda install" -ForegroundColor "Green"
-    conda config --set always_yes True
-    conda config --set changeps1 False
-    conda config --set auto_update_conda False
-    conda install conda-build jinja2 anaconda-client --quiet
-    conda info
-
-    # why `2>&1 | %{ "$_" }`? Redirect STDERR to STDOUT
-    # see: http://stackoverflow.com/a/20950421/5208670
-    Write-Host "Starting conda build recipe" -ForegroundColor "Green"
-    conda build conda.recipe --quiet 2>&1 | %{ "$_" }
-
-    $CONDA_PKG=(conda build conda.recipe --output)
-    Copy-Item $CONDA_PKG .\dist\
-    Write-Host "Completed conda build recipe" -ForegroundColor "Green"
-
-    # Restore PATH back to original
-    $env:path = $old_path
-} else {
-    Write-Host "Skipping conda build recipe" -ForegroundColor "Green"
-}
diff --git a/ci/appveyor_run_tests.ps1 b/ci/appveyor_run_tests.ps1
index b45440fbe..bd90943d5 100644
--- a/ci/appveyor_run_tests.ps1
+++ b/ci/appveyor_run_tests.ps1
@@ -1,8 +1,13 @@
 # Script to simplify AppVeyor configuration and resolve path to tools
 
+$stopwatch = [Diagnostics.Stopwatch]::StartNew()
+[array]$timings = @()
+
 # Test Runner framework being used for embedded tests
 $CS_RUNNER = "nunit3-console"
 
+$XPLAT = $env:BUILD_OPTS -eq "--xplat"
+
 # Needed for ARCH specific runners(NUnit2/XUnit3). Skip for NUnit3
 if ($FALSE -and $env:PLATFORM -eq "x86"){
     $CS_RUNNER = $CS_RUNNER + "-x86"
@@ -11,7 +16,7 @@ if ($FALSE -and $env:PLATFORM -eq "x86"){
 # Executable paths for OpenCover
 # Note if OpenCover fails, it won't affect the exit codes.
 $OPENCOVER = Resolve-Path .\packages\OpenCover.*\tools\OpenCover.Console.exe
-if ($env:BUILD_OPTS -eq "--xplat"){
+if ($XPLAT){
     $CS_RUNNER = Resolve-Path $env:USERPROFILE\.nuget\packages\nunit.consolerunner\*\tools\"$CS_RUNNER".exe
 }
 else{
@@ -23,6 +28,17 @@ $PY = Get-Command python
 $CS_TESTS = ".\src\embed_tests\bin\Python.EmbeddingTest.dll"
 $RUNTIME_DIR = ".\src\runtime\bin\"
 
+function ReportTime {
+    param([string] $action)
+
+    $timeSpent = $stopwatch.Elapsed
+    $timings += [pscustomobject]@{action=$action; timeSpent=$timeSpent}
+    Write-Host $action " in " $timeSpent -ForegroundColor "Green"
+    $stopwatch.Restart()
+}
+
+ReportTime "Preparation done"
+
 # Run python tests with C# coverage
 Write-Host ("Starting Python tests") -ForegroundColor "Green"
 .$OPENCOVER -register:user -searchdirs:"$RUNTIME_DIR" -output:py.coverage `
@@ -31,20 +47,51 @@ Write-Host ("Starting Python tests") -ForegroundColor "Green"
 $PYTHON_STATUS = $LastExitCode
 if ($PYTHON_STATUS -ne 0) {
     Write-Host "Python tests failed, continuing to embedded tests" -ForegroundColor "Red"
+    ReportTime ""
+} else {
+    ReportTime "Python tests completed"
 }
 
 # Run Embedded tests with C# coverage
 Write-Host ("Starting embedded tests") -ForegroundColor "Green"
 .$OPENCOVER -register:user -searchdirs:"$RUNTIME_DIR" -output:cs.coverage `
-            -target:"$CS_RUNNER" -targetargs:"$CS_TESTS" `
+            -target:"$CS_RUNNER" -targetargs:"$CS_TESTS --labels=All" `
             -filter:"+[*]Python.Runtime*" `
             -returntargetcode
 $CS_STATUS = $LastExitCode
 if ($CS_STATUS -ne 0) {
     Write-Host "Embedded tests failed" -ForegroundColor "Red"
+    ReportTime ""
+} else {
+    ReportTime "Embedded tests completed"
+
+    # NuGet for pythonnet-2.3 only has 64-bit binary for Python 3.5
+    # the test is only built using modern stack
+    if (($env:PLATFORM -eq "x64") -and ($XPLAT) -and ($env:PYTHON_VERSION -eq "3.5")) {
+        # Run C# Performance tests
+        Write-Host ("Starting performance tests") -ForegroundColor "Green"
+        if ($XPLAT) {
+            $CS_PERF_TESTS = ".\src\perf_tests\bin\net461\Python.PerformanceTests.dll"
+        }
+        else {
+            $CS_PERF_TESTS = ".\src\perf_tests\bin\Python.PerformanceTests.dll"
+        }
+        &"$CS_RUNNER" "$CS_PERF_TESTS"
+        $CS_PERF_STATUS = $LastExitCode
+        if ($CS_PERF_STATUS -ne 0) {
+            Write-Host "Performance tests (C#) failed" -ForegroundColor "Red"
+            ReportTime ""
+        } else {
+            ReportTime "Performance tests (C#) completed"
+        }
+    } else {
+        Write-Host ("Skipping performance tests for ", $env:PYTHON_VERSION) -ForegroundColor "Yellow"
+        Write-Host ("on platform ", $env:PLATFORM, " xplat: ", $XPLAT) -ForegroundColor "Yellow"
+        $CS_PERF_STATUS = 0
+    }
 }
 
-if ($env:BUILD_OPTS -eq "--xplat"){
+if ($XPLAT){
     if ($env:PLATFORM -eq "x64") {
          $DOTNET_CMD = "dotnet"
     }
@@ -54,15 +101,20 @@ if ($env:BUILD_OPTS -eq "--xplat"){
 
     # Run Embedded tests for netcoreapp2.0 (OpenCover currently does not supports dotnet core)
     Write-Host ("Starting embedded tests for netcoreapp2.0") -ForegroundColor "Green"
-    &$DOTNET_CMD .\src\embed_tests\bin\netcoreapp2.0_publish\Python.EmbeddingTest.dll
+    &$DOTNET_CMD ".\src\embed_tests\bin\netcoreapp2.0_publish\Python.EmbeddingTest.dll"
     $CS_STATUS = $LastExitCode
     if ($CS_STATUS -ne 0) {
         Write-Host "Embedded tests for netcoreapp2.0 failed" -ForegroundColor "Red"
+        ReportTime ""
+    } else {
+        ReportTime ".NET Core 2.0 tests completed"
     }
 }
 
+Write-Host "Timings:" ($timings | Format-Table | Out-String) -ForegroundColor "Green"
+
 # Set exit code to fail if either Python or Embedded tests failed
-if ($PYTHON_STATUS -ne 0 -or $CS_STATUS -ne 0) {
+if ($PYTHON_STATUS -ne 0 -or $CS_STATUS -ne 0 -or $CS_PERF_STATUS -ne 0) {
     Write-Host "Tests failed" -ForegroundColor "Red"
     $host.SetShouldExit(1)
 }
diff --git a/conda.recipe/README.md b/conda.recipe/README.md
deleted file mode 100644
index 42241999f..000000000
--- a/conda.recipe/README.md
+++ /dev/null
@@ -1,5 +0,0 @@
-# Conda Recipe
-
-The files here are needed to build Python.Net with conda
-
-http://conda.pydata.org/docs/building/recipe.html
diff --git a/conda.recipe/bld.bat b/conda.recipe/bld.bat
deleted file mode 100644
index 27b55f2de..000000000
--- a/conda.recipe/bld.bat
+++ /dev/null
@@ -1,6 +0,0 @@
-:: build it
-
-:: set path to modern MSBuild
-set PATH=C:\Program Files (x86)\MSBuild\14.0\Bin;C:\Program Files (x86)\MSBuild\15.0\Bin;%PATH%
-
-%PYTHON% setup.py install
diff --git a/conda.recipe/meta.yaml b/conda.recipe/meta.yaml
deleted file mode 100644
index 545a01268..000000000
--- a/conda.recipe/meta.yaml
+++ /dev/null
@@ -1,29 +0,0 @@
-package:
-  name: pythonnet
-  version: {{ GIT_DESCRIBE_VERSION }}
-
-build:
-  skip: True  # [not win]
-  number: {{ GIT_DESCRIBE_NUMBER }}
-  {% if environ.get('GIT_DESCRIBE_NUMBER', '0') == '0' %}string: py{{ environ.get('PY_VER').replace('.', '') }}_0
-  {% else %}string: py{{ environ.get('PY_VER').replace('.', '') }}_{{ environ.get('GIT_BUILD_STR', 'GIT_STUB') }}{% endif %}
-
-source:
-  git_url: ../
-
-requirements:
-  build:
-    - python
-    - setuptools
-
-  run:
-    - python
-
-test:
-  imports:
-    - clr
-
-about:
-  home: https://github.com/pythonnet/pythonnet
-  license: MIT
-  license_file: LICENSE
diff --git a/demo/wordpad.py b/demo/wordpad.py
index 876372100..409c8ad4d 100644
--- a/demo/wordpad.py
+++ b/demo/wordpad.py
@@ -382,7 +382,7 @@ def InitializeComponent(self):
         self.label1.Size = System.Drawing.Size(296, 140)
         self.label1.TabIndex = 2
         self.label1.Text = "Python Wordpad - an example winforms " \
-                           "application using Python for .NET"
+                           "application using Python.NET"
 
         self.AutoScaleBaseSize = System.Drawing.Size(5, 13)
         self.ClientSize = System.Drawing.Size(300, 150)
diff --git a/pythonnet.15.sln b/pythonnet.15.sln
index f2015e480..ce863817f 100644
--- a/pythonnet.15.sln
+++ b/pythonnet.15.sln
@@ -1,189 +1,393 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 15
-VisualStudioVersion = 15.0.26730.3
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.29102.190
 MinimumVisualStudioVersion = 10.0.40219.1
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.Runtime.15", "src/runtime/Python.Runtime.15.csproj", "{2759F4FF-716B-4828-916F-50FA86613DFC}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.Runtime.15", "src\runtime\Python.Runtime.15.csproj", "{2759F4FF-716B-4828-916F-50FA86613DFC}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.EmbeddingTest.15", "src/embed_tests/Python.EmbeddingTest.15.csproj", "{66B8D01A-9906-452A-B09E-BF75EA76468F}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.EmbeddingTest.15", "src\embed_tests\Python.EmbeddingTest.15.csproj", "{66B8D01A-9906-452A-B09E-BF75EA76468F}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "clrmodule.15", "src/clrmodule/clrmodule.15.csproj", "{E08678D4-9A52-4AD5-B63D-8EBC7399981B}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "clrmodule.15", "src\clrmodule\clrmodule.15.csproj", "{E08678D4-9A52-4AD5-B63D-8EBC7399981B}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Console.15", "src/console/Console.15.csproj", "{CDAD305F-8E72-492C-A314-64CF58D472A0}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Console.15", "src\console\Console.15.csproj", "{CDAD305F-8E72-492C-A314-64CF58D472A0}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.Test.15", "src/testing/Python.Test.15.csproj", "{F94B547A-E97E-4500-8D53-B4D64D076E5F}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.Test.15", "src\testing\Python.Test.15.csproj", "{F94B547A-E97E-4500-8D53-B4D64D076E5F}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Python.PerformanceTests", "src\perf_tests\Python.PerformanceTests.csproj", "{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Repo", "Repo", "{441A0123-F4C6-4EE4-9AEE-315FD79BE2D5}"
+	ProjectSection(SolutionItems) = preProject
+		.editorconfig = .editorconfig
+		.gitignore = .gitignore
+		CHANGELOG.md = CHANGELOG.md
+		README.rst = README.rst
+	EndProjectSection
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "CI", "CI", "{D301657F-5EAF-4534-B280-B858D651B2E5}"
+	ProjectSection(SolutionItems) = preProject
+		.travis.yml = .travis.yml
+		appveyor.yml = appveyor.yml
+		ci\appveyor_build_recipe.ps1 = ci\appveyor_build_recipe.ps1
+		ci\appveyor_run_tests.ps1 = ci\appveyor_run_tests.ps1
+	EndProjectSection
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{57F5D701-F265-4736-A5A2-07249E7A4DA3}"
+	ProjectSection(SolutionItems) = preProject
+		setup.py = setup.py
+	EndProjectSection
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "conda.recipe", "conda.recipe", "{7FD2404D-0CE8-4645-8DFB-766470E2150E}"
+	ProjectSection(SolutionItems) = preProject
+		conda.recipe\bld.bat = conda.recipe\bld.bat
+		conda.recipe\meta.yaml = conda.recipe\meta.yaml
+		conda.recipe\README.md = conda.recipe\README.md
+	EndProjectSection
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
+		DebugMono|Any CPU = DebugMono|Any CPU
 		DebugMono|x64 = DebugMono|x64
 		DebugMono|x86 = DebugMono|x86
+		DebugMonoPY3|Any CPU = DebugMonoPY3|Any CPU
 		DebugMonoPY3|x64 = DebugMonoPY3|x64
 		DebugMonoPY3|x86 = DebugMonoPY3|x86
+		DebugWin|Any CPU = DebugWin|Any CPU
 		DebugWin|x64 = DebugWin|x64
 		DebugWin|x86 = DebugWin|x86
+		DebugWinPY3|Any CPU = DebugWinPY3|Any CPU
 		DebugWinPY3|x64 = DebugWinPY3|x64
 		DebugWinPY3|x86 = DebugWinPY3|x86
+		Release|Any CPU = Release|Any CPU
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
+		ReleaseMono|Any CPU = ReleaseMono|Any CPU
 		ReleaseMono|x64 = ReleaseMono|x64
 		ReleaseMono|x86 = ReleaseMono|x86
+		ReleaseMonoPY3|Any CPU = ReleaseMonoPY3|Any CPU
 		ReleaseMonoPY3|x64 = ReleaseMonoPY3|x64
 		ReleaseMonoPY3|x86 = ReleaseMonoPY3|x86
+		ReleaseWin|Any CPU = ReleaseWin|Any CPU
 		ReleaseWin|x64 = ReleaseWin|x64
 		ReleaseWin|x86 = ReleaseWin|x86
+		ReleaseWinPY3|Any CPU = ReleaseWinPY3|Any CPU
 		ReleaseWinPY3|x64 = ReleaseWinPY3|x64
 		ReleaseWinPY3|x86 = ReleaseWinPY3|x86
 	EndGlobalSection
 	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Debug|Any CPU.ActiveCfg = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Debug|Any CPU.Build.0 = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Debug|x64.ActiveCfg = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Debug|x64.Build.0 = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Debug|x86.ActiveCfg = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Debug|x86.Build.0 = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMono|Any CPU.ActiveCfg = DebugMono|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMono|Any CPU.Build.0 = DebugMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMono|x64.ActiveCfg = DebugMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMono|x64.Build.0 = DebugMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMono|x86.ActiveCfg = DebugMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMono|x86.Build.0 = DebugMono|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMonoPY3|Any CPU.ActiveCfg = DebugMonoPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMonoPY3|Any CPU.Build.0 = DebugMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMonoPY3|x64.ActiveCfg = DebugMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMonoPY3|x64.Build.0 = DebugMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMonoPY3|x86.ActiveCfg = DebugMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugMonoPY3|x86.Build.0 = DebugMonoPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWin|Any CPU.ActiveCfg = DebugWin|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWin|Any CPU.Build.0 = DebugWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWin|x64.ActiveCfg = DebugWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWin|x64.Build.0 = DebugWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWin|x86.ActiveCfg = DebugWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWin|x86.Build.0 = DebugWin|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWinPY3|Any CPU.ActiveCfg = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWinPY3|Any CPU.Build.0 = DebugWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWinPY3|x64.ActiveCfg = DebugWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWinPY3|x64.Build.0 = DebugWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWinPY3|x86.ActiveCfg = DebugWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.DebugWinPY3|x86.Build.0 = DebugWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Release|Any CPU.ActiveCfg = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Release|Any CPU.Build.0 = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Release|x64.ActiveCfg = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Release|x64.Build.0 = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Release|x86.ActiveCfg = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.Release|x86.Build.0 = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMono|Any CPU.ActiveCfg = ReleaseMono|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMono|Any CPU.Build.0 = ReleaseMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMono|x64.ActiveCfg = ReleaseMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMono|x64.Build.0 = ReleaseMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMono|x86.ActiveCfg = ReleaseMono|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMono|x86.Build.0 = ReleaseMono|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMonoPY3|Any CPU.ActiveCfg = ReleaseMonoPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMonoPY3|Any CPU.Build.0 = ReleaseMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMonoPY3|x64.ActiveCfg = ReleaseMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMonoPY3|x64.Build.0 = ReleaseMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMonoPY3|x86.ActiveCfg = ReleaseMonoPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseMonoPY3|x86.Build.0 = ReleaseMonoPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWin|Any CPU.ActiveCfg = ReleaseWin|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWin|Any CPU.Build.0 = ReleaseWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWin|x64.ActiveCfg = ReleaseWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWin|x64.Build.0 = ReleaseWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWin|x86.ActiveCfg = ReleaseWin|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWin|x86.Build.0 = ReleaseWin|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWinPY3|Any CPU.ActiveCfg = ReleaseWinPY3|Any CPU
+		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWinPY3|Any CPU.Build.0 = ReleaseWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWinPY3|x64.ActiveCfg = ReleaseWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWinPY3|x64.Build.0 = ReleaseWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWinPY3|x86.ActiveCfg = ReleaseWinPY3|Any CPU
 		{2759F4FF-716B-4828-916F-50FA86613DFC}.ReleaseWinPY3|x86.Build.0 = ReleaseWinPY3|Any CPU
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.Debug|Any CPU.ActiveCfg = ReleaseWinPY3|x86
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.Debug|Any CPU.Build.0 = ReleaseWinPY3|x86
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.Debug|x64.ActiveCfg = DebugWinPY3|x64
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.Debug|x64.Build.0 = DebugWinPY3|x64
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.Debug|x86.ActiveCfg = DebugWinPY3|x86
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.Debug|x86.Build.0 = DebugWinPY3|x86
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMono|Any CPU.ActiveCfg = DebugMono|x86
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMono|x64.ActiveCfg = DebugMono|x64
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMono|x64.Build.0 = DebugMono|x64
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMono|x86.ActiveCfg = DebugMono|x86
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMono|x86.Build.0 = DebugMono|x86
+		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMonoPY3|Any CPU.ActiveCfg = DebugMonoPY3|x86
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMonoPY3|x64.ActiveCfg = DebugMonoPY3|x64
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMonoPY3|x64.Build.0 = DebugMonoPY3|x64
 		{66B8D01A-9906-452A-B09E-BF75EA76468F}.DebugMonoPY3|x86.ActiveCfg = DebugMonoPY3|x86
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 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseMonoPY3|x64.ActiveCfg = ReleaseMonoPY3|x64
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseMonoPY3|x64.Build.0 = ReleaseMonoPY3|x64
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseMonoPY3|x86.ActiveCfg = ReleaseMonoPY3|x86
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseMonoPY3|x86.Build.0 = ReleaseMonoPY3|x86
+		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWin|Any CPU.ActiveCfg = ReleaseWin|x86
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWin|x64.ActiveCfg = ReleaseWin|x64
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWin|x64.Build.0 = ReleaseWin|x64
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWin|x86.ActiveCfg = ReleaseWin|x86
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWin|x86.Build.0 = ReleaseWin|x86
+		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWinPY3|Any CPU.ActiveCfg = ReleaseWinPY3|x86
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWinPY3|x64.ActiveCfg = ReleaseWinPY3|x64
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWinPY3|x64.Build.0 = ReleaseWinPY3|x64
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWinPY3|x86.ActiveCfg = ReleaseWinPY3|x86
 		{F94B547A-E97E-4500-8D53-B4D64D076E5F}.ReleaseWinPY3|x86.Build.0 = ReleaseWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Debug|Any CPU.ActiveCfg = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Debug|Any CPU.Build.0 = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Debug|x64.ActiveCfg = DebugWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Debug|x64.Build.0 = DebugWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Debug|x86.ActiveCfg = DebugWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Debug|x86.Build.0 = DebugWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMono|Any CPU.ActiveCfg = DebugMono|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMono|x64.ActiveCfg = DebugMono|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMono|x64.Build.0 = DebugMono|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMono|x86.ActiveCfg = DebugMono|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMono|x86.Build.0 = DebugMono|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMonoPY3|Any CPU.ActiveCfg = DebugMonoPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMonoPY3|x64.ActiveCfg = DebugMonoPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMonoPY3|x64.Build.0 = DebugMonoPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMonoPY3|x86.ActiveCfg = DebugMonoPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugMonoPY3|x86.Build.0 = DebugMonoPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWin|Any CPU.ActiveCfg = DebugWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWin|x64.ActiveCfg = DebugWin|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWin|x64.Build.0 = DebugWin|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWin|x86.ActiveCfg = DebugWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWin|x86.Build.0 = DebugWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWinPY3|Any CPU.ActiveCfg = DebugWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWinPY3|x64.ActiveCfg = DebugWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWinPY3|x64.Build.0 = DebugWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWinPY3|x86.ActiveCfg = DebugWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.DebugWinPY3|x86.Build.0 = DebugWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Release|Any CPU.ActiveCfg = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Release|Any CPU.Build.0 = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Release|x64.ActiveCfg = ReleaseWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Release|x64.Build.0 = ReleaseWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Release|x86.ActiveCfg = ReleaseWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.Release|x86.Build.0 = ReleaseWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMono|Any CPU.ActiveCfg = ReleaseMono|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMono|x64.ActiveCfg = ReleaseMono|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMono|x64.Build.0 = ReleaseMono|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMono|x86.ActiveCfg = ReleaseMono|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMono|x86.Build.0 = ReleaseMono|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMonoPY3|Any CPU.ActiveCfg = ReleaseMonoPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMonoPY3|x64.ActiveCfg = ReleaseMonoPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMonoPY3|x64.Build.0 = ReleaseMonoPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMonoPY3|x86.ActiveCfg = ReleaseMonoPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseMonoPY3|x86.Build.0 = ReleaseMonoPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWin|Any CPU.ActiveCfg = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWin|x64.ActiveCfg = ReleaseWin|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWin|x64.Build.0 = ReleaseWin|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWin|x86.ActiveCfg = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWin|x86.Build.0 = ReleaseWin|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWinPY3|Any CPU.ActiveCfg = ReleaseWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWinPY3|x64.ActiveCfg = ReleaseWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWinPY3|x64.Build.0 = ReleaseWinPY3|x64
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWinPY3|x86.ActiveCfg = ReleaseWinPY3|x86
+		{6FB0D091-9CEC-4DCC-8701-C40F9BFC9EDE}.ReleaseWinPY3|x86.Build.0 = ReleaseWinPY3|x86
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
diff --git a/setup.cfg b/setup.cfg
index 38aa3eb3d..19c6f9fc9 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,9 +1,3 @@
-# [bumpversion] comments. bumpversion deleted all comments on its file.
-# Don't combine `.bumpversion.cfg` with `setup.cfg`. Messes up formatting.
-# Don't use `first_value = 1`. It will break `release` bump
-# Keep `optional = dummy` needed to bump to release.
-# See: https://github.com/peritus/bumpversion/issues/59
-
 [tool:pytest]
 xfail_strict = True
 # -r fsxX: show extra summary info for: (f)ailed, (s)kip, (x)failed, (X)passed
diff --git a/setup.py b/setup.py
index 7b12997d9..216620211 100644
--- a/setup.py
+++ b/setup.py
@@ -15,10 +15,14 @@
 import sysconfig
 from distutils import spawn
 from distutils.command import install, build, build_ext, install_data, install_lib
-from wheel import bdist_wheel
 
 from setuptools import Extension, setup
 
+try:
+    from wheel import bdist_wheel
+except ImportError:
+    bdist_wheel = None
+
 # Allow config/verbosity to be set from cli
 # http://stackoverflow.com/a/4792601/5208670
 CONFIG = "Release"  # Release or Debug
@@ -334,6 +338,7 @@ def build_extension(self, ext):
             ),
             '/p:PythonBuildDir="{}"'.format(os.path.abspath(dest_dir)),
             '/p:PythonInteropFile="{}"'.format(os.path.basename(interop_file)),
+            "/p:PackageId=pythonnet_py{0}{1}_{2}".format(PY_MAJOR, PY_MINOR, ARCH),
             "/verbosity:{}".format(VERBOSITY),
         ]
 
@@ -357,6 +362,16 @@ def build_extension(self, ext):
                 ),
                 shell=use_shell,
             )
+            subprocess.check_call(
+                " ".join(
+                    cmd
+                    + [
+                        '"/t:Python_PerformanceTests:publish"',
+                        "/p:TargetFramework=net461",
+                    ]
+                ),
+                shell=use_shell,
+            )
         if DEVTOOLS == "Mono" or DEVTOOLS == "dotnet":
             self._build_monoclr()
 
@@ -383,14 +398,13 @@ def _build_monoclr(self):
                 return
             raise
         mono_cflags = _check_output("pkg-config --cflags mono-2", shell=True)
-        glib_libs = _check_output("pkg-config --libs glib-2.0", shell=True)
-        glib_cflags = _check_output("pkg-config --cflags glib-2.0", shell=True)
-        cflags = mono_cflags.strip() + " " + glib_cflags.strip()
-        libs = mono_libs.strip() + " " + glib_libs.strip()
+        cflags = mono_cflags.strip()
+        libs = mono_libs.strip()
 
         # build the clr python module
         clr_ext = Extension(
             "clr",
+            language="c++",
             sources=["src/monoclr/pynetinit.c", "src/monoclr/clrmod.c"],
             extra_compile_args=cflags.split(" "),
             extra_link_args=libs.split(" "),
@@ -442,26 +456,20 @@ def _find_msbuild_tool(self, tool="msbuild.exe", use_windows_sdk=False):
         # trying to search path with help of vswhere when MSBuild 15.0 and higher installed.
         if tool == "msbuild.exe" and use_windows_sdk == False:
             try:
-                basePathes = subprocess.check_output(
+                basePaths = subprocess.check_output(
                     [
                         "tools\\vswhere\\vswhere.exe",
                         "-latest",
                         "-version",
-                        "[15.0, 16.0)",
+                        "[15.0,)",
                         "-requires",
                         "Microsoft.Component.MSBuild",
-                        "-property",
-                        "InstallationPath",
+                        "-find",
+                        "MSBuild\**\Bin\MSBuild.exe",
                     ]
                 ).splitlines()
-                if len(basePathes):
-                    return os.path.join(
-                        basePathes[0].decode(sys.stdout.encoding or "utf-8"),
-                        "MSBuild",
-                        "15.0",
-                        "Bin",
-                        "MSBuild.exe",
-                    )
+                if len(basePaths):
+                    return basePaths[0].decode(sys.stdout.encoding or "utf-8")
             except:
                 pass  # keep trying to search by old method.
 
@@ -513,26 +521,20 @@ def _find_msbuild_tool(self, tool="msbuild.exe", use_windows_sdk=False):
     def _find_msbuild_tool_15(self):
         """Return full path to one of the Microsoft build tools"""
         try:
-            basePathes = subprocess.check_output(
+            basePaths = subprocess.check_output(
                 [
                     "tools\\vswhere\\vswhere.exe",
                     "-latest",
                     "-version",
-                    "[15.0, 16.0)",
+                    "[15.0,)",
                     "-requires",
                     "Microsoft.Component.MSBuild",
-                    "-property",
-                    "InstallationPath",
+                    "-find",
+                    "MSBuild\**\Bin\MSBuild.exe",
                 ]
             ).splitlines()
-            if len(basePathes):
-                return os.path.join(
-                    basePathes[0].decode(sys.stdout.encoding or "utf-8"),
-                    "MSBuild",
-                    "15.0",
-                    "Bin",
-                    "MSBuild.exe",
-                )
+            if len(basePaths):
+                return basePaths[0].decode(sys.stdout.encoding or "utf-8")
             else:
                 raise RuntimeError("MSBuild >=15.0 could not be found.")
         except subprocess.CalledProcessError as e:
@@ -593,21 +595,21 @@ def run(self):
             _update_xlat_devtools()
         return install.install.run(self)
 
+if bdist_wheel:
+    class BDistWheelPythonnet(bdist_wheel.bdist_wheel):
+        user_options = bdist_wheel.bdist_wheel.user_options + [("xplat", None, None)]
 
-class BDistWheelPythonnet(bdist_wheel.bdist_wheel):
-    user_options = bdist_wheel.bdist_wheel.user_options + [("xplat", None, None)]
-
-    def initialize_options(self):
-        bdist_wheel.bdist_wheel.initialize_options(self)
-        self.xplat = None
+        def initialize_options(self):
+            bdist_wheel.bdist_wheel.initialize_options(self)
+            self.xplat = None
 
-    def finalize_options(self):
-        bdist_wheel.bdist_wheel.finalize_options(self)
+        def finalize_options(self):
+            bdist_wheel.bdist_wheel.finalize_options(self)
 
-    def run(self):
-        if self.xplat:
-            _update_xlat_devtools()
-        return bdist_wheel.bdist_wheel.run(self)
+        def run(self):
+            if self.xplat:
+                _update_xlat_devtools()
+            return bdist_wheel.bdist_wheel.run(self)
 
         ###############################################################################
 
@@ -616,29 +618,28 @@ def run(self):
 if setupdir:
     os.chdir(setupdir)
 
-setup_requires = []
-if not os.path.exists(_get_interop_filename()):
-    setup_requires.append("pycparser")
+cmdclass={
+    "install": InstallPythonnet,
+    "build_ext": BuildExtPythonnet,
+    "install_lib": InstallLibPythonnet,
+    "install_data": InstallDataPythonnet,
+}
+if bdist_wheel:
+    cmdclass["bdist_wheel"] = BDistWheelPythonnet
 
 setup(
     name="pythonnet",
-    version="2.4.0",
+    version="2.5.0",
     description=".Net and Mono integration for Python",
     url="https://pythonnet.github.io/",
     license="MIT",
-    author="The Python for .Net developers",
-    author_email="pythondotnet@python.org",
-    setup_requires=setup_requires,
+    author="The Contributors of the Python.NET Project",
+    author_email="pythonnet@python.org",
+    install_requires=["pycparser"],
     long_description=_get_long_description(),
     ext_modules=[Extension("clr", sources=list(_get_source_files()))],
     data_files=[("{install_platlib}", ["{build_lib}/Python.Runtime.dll"])],
-    cmdclass={
-        "install": InstallPythonnet,
-        "build_ext": BuildExtPythonnet,
-        "install_lib": InstallLibPythonnet,
-        "install_data": InstallDataPythonnet,
-        "bdist_wheel": BDistWheelPythonnet,
-    },
+    cmdclass=cmdclass,
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
@@ -647,10 +648,10 @@ def run(self):
         "Programming Language :: Python :: 2",
         "Programming Language :: Python :: 2.7",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.4",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
         "Operating System :: Microsoft :: Windows",
         "Operating System :: POSIX :: Linux",
         "Operating System :: MacOS :: MacOS X",
diff --git a/src/SharedAssemblyInfo.cs b/src/SharedAssemblyInfo.cs
index 669a43746..dd057b144 100644
--- a/src/SharedAssemblyInfo.cs
+++ b/src/SharedAssemblyInfo.cs
@@ -8,8 +8,8 @@
 // associated with an assembly.
 [assembly: AssemblyConfiguration("")]
 [assembly: AssemblyCompany("pythonnet")]
-[assembly: AssemblyProduct("Python for .NET")]
-[assembly: AssemblyCopyright("Copyright (c) 2006-2019 the contributors of the 'Python for .NET' project")]
+[assembly: AssemblyProduct("Python.NET")]
+[assembly: AssemblyCopyright("Copyright (c) 2006-2020 the contributors of the Python.NET project")]
 [assembly: AssemblyTrademark("")]
 
 [assembly: AssemblyCulture("")]
@@ -25,4 +25,4 @@
 // Version Information. Keeping it simple. May need to revisit for Nuget
 // See: https://codingforsmarties.wordpress.com/2016/01/21/how-to-version-assemblies-destined-for-nuget/
 // AssemblyVersion can only be numeric
-[assembly: AssemblyVersion("2.4.0")]
+[assembly: AssemblyVersion("2.5.0")]
diff --git a/src/clrmodule/ClrModule.cs b/src/clrmodule/ClrModule.cs
index 3bb3a533b..377be66d1 100644
--- a/src/clrmodule/ClrModule.cs
+++ b/src/clrmodule/ClrModule.cs
@@ -53,7 +53,7 @@ public static void initclr()
         {
 #if USE_PYTHON_RUNTIME_VERSION
             // Has no effect until SNK works. Keep updated anyways.
-            Version = new Version("2.4.0"),
+            Version = new Version("2.5.0"),
 #endif
             CultureInfo = CultureInfo.InvariantCulture
         };
diff --git a/src/clrmodule/clrmodule.15.csproj b/src/clrmodule/clrmodule.15.csproj
index 2585ffdd2..7fc9c2dda 100644
--- a/src/clrmodule/clrmodule.15.csproj
+++ b/src/clrmodule/clrmodule.15.csproj
@@ -9,7 +9,7 @@
     <RootNamespace>clrmodule</RootNamespace>
     <AssemblyName>clrmodule</AssemblyName>
     <PackageId>clrmodule</PackageId>
-    <VersionPrefix>2.4.0</VersionPrefix>
+    <VersionPrefix>2.5.0</VersionPrefix>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
     <GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
     <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
diff --git a/src/console/Console.15.csproj b/src/console/Console.15.csproj
index ec5008036..3c51caa31 100644
--- a/src/console/Console.15.csproj
+++ b/src/console/Console.15.csproj
@@ -8,7 +8,7 @@
     <AssemblyName>nPython</AssemblyName>
     <RootNamespace>Python.Runtime</RootNamespace>
     <PackageId>nPython</PackageId>
-    <VersionPrefix>2.4.0</VersionPrefix>
+    <VersionPrefix>2.5.0</VersionPrefix>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
     <GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
     <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
diff --git a/src/embed_tests/CodecGroups.cs b/src/embed_tests/CodecGroups.cs
new file mode 100644
index 000000000..5dd40210f
--- /dev/null
+++ b/src/embed_tests/CodecGroups.cs
@@ -0,0 +1,132 @@
+namespace Python.EmbeddingTest
+{
+    using System;
+    using System.Linq;
+    using NUnit.Framework;
+    using Python.Runtime;
+    using Python.Runtime.Codecs;
+
+    public class CodecGroups
+    {
+        [Test]
+        public void GetEncodersByType()
+        {
+            var encoder1 = new ObjectToEncoderInstanceEncoder<Uri>();
+            var encoder2 = new ObjectToEncoderInstanceEncoder<Uri>();
+            var group = new EncoderGroup {
+                new ObjectToEncoderInstanceEncoder<Tuple<int>>(),
+                encoder1,
+                encoder2,
+            };
+
+            var got = group.GetEncoders(typeof(Uri)).ToArray();
+            CollectionAssert.AreEqual(new[]{encoder1, encoder2}, got);
+        }
+
+        [Test]
+        public void CanEncode()
+        {
+            var group = new EncoderGroup {
+                new ObjectToEncoderInstanceEncoder<Tuple<int>>(),
+                new ObjectToEncoderInstanceEncoder<Uri>(),
+            };
+
+            Assert.IsTrue(group.CanEncode(typeof(Tuple<int>)));
+            Assert.IsTrue(group.CanEncode(typeof(Uri)));
+            Assert.IsFalse(group.CanEncode(typeof(string)));
+        }
+
+        [Test]
+        public void Encodes()
+        {
+            var encoder0 = new ObjectToEncoderInstanceEncoder<Tuple<int>>();
+            var encoder1 = new ObjectToEncoderInstanceEncoder<Uri>();
+            var encoder2 = new ObjectToEncoderInstanceEncoder<Uri>();
+            var group = new EncoderGroup {
+                encoder0,
+                encoder1,
+                encoder2,
+            };
+
+            var uri = group.TryEncode(new Uri("data:"));
+            var clrObject = (CLRObject)ManagedType.GetManagedObject(uri.Handle);
+            Assert.AreSame(encoder1, clrObject.inst);
+            Assert.AreNotSame(encoder2, clrObject.inst);
+
+            var tuple = group.TryEncode(Tuple.Create(1));
+            clrObject = (CLRObject)ManagedType.GetManagedObject(tuple.Handle);
+            Assert.AreSame(encoder0, clrObject.inst);
+        }
+
+        [Test]
+        public void GetDecodersByTypes()
+        {
+            var pyint = new PyInt(10).GetPythonType();
+            var pyfloat = new PyFloat(10).GetPythonType();
+            var pystr = new PyString("world").GetPythonType();
+            var decoder1 = new DecoderReturningPredefinedValue<long>(pyint, decodeResult: 42);
+            var decoder2 = new DecoderReturningPredefinedValue<string>(pyfloat, decodeResult: "atad:");
+            var group = new DecoderGroup {
+                decoder1,
+                decoder2,
+            };
+
+            var decoder = group.GetDecoder(pyfloat, typeof(string));
+            Assert.AreSame(decoder2, decoder);
+            decoder = group.GetDecoder(pystr, typeof(string));
+            Assert.IsNull(decoder);
+            decoder = group.GetDecoder(pyint, typeof(long));
+            Assert.AreSame(decoder1, decoder);
+        }
+        [Test]
+        public void CanDecode()
+        {
+            var pyint = new PyInt(10).GetPythonType();
+            var pyfloat = new PyFloat(10).GetPythonType();
+            var pystr = new PyString("world").GetPythonType();
+            var decoder1 = new DecoderReturningPredefinedValue<long>(pyint, decodeResult: 42);
+            var decoder2 = new DecoderReturningPredefinedValue<string>(pyfloat, decodeResult: "atad:");
+            var group = new DecoderGroup {
+                decoder1,
+                decoder2,
+            };
+
+            Assert.IsTrue(group.CanDecode(pyint, typeof(long)));
+            Assert.IsFalse(group.CanDecode(pyint, typeof(int)));
+            Assert.IsTrue(group.CanDecode(pyfloat, typeof(string)));
+            Assert.IsFalse(group.CanDecode(pystr, typeof(string)));
+        }
+
+        [Test]
+        public void Decodes()
+        {
+            var pyint = new PyInt(10).GetPythonType();
+            var pyfloat = new PyFloat(10).GetPythonType();
+            var decoder1 = new DecoderReturningPredefinedValue<long>(pyint, decodeResult: 42);
+            var decoder2 = new DecoderReturningPredefinedValue<string>(pyfloat, decodeResult: "atad:");
+            var group = new DecoderGroup {
+                decoder1,
+                decoder2,
+            };
+
+            Assert.IsTrue(group.TryDecode(new PyInt(10), out long longResult));
+            Assert.AreEqual(42, longResult);
+            Assert.IsTrue(group.TryDecode(new PyFloat(10), out string strResult));
+            Assert.AreSame("atad:", strResult);
+
+            Assert.IsFalse(group.TryDecode(new PyInt(10), out int _));
+        }
+
+        [SetUp]
+        public void SetUp()
+        {
+            PythonEngine.Initialize();
+        }
+
+        [TearDown]
+        public void Dispose()
+        {
+            PythonEngine.Shutdown();
+        }
+    }
+}
diff --git a/src/embed_tests/Codecs.cs b/src/embed_tests/Codecs.cs
new file mode 100644
index 000000000..18fcd32d1
--- /dev/null
+++ b/src/embed_tests/Codecs.cs
@@ -0,0 +1,123 @@
+namespace Python.EmbeddingTest {
+    using System;
+    using System.Collections.Generic;
+    using System.Text;
+    using NUnit.Framework;
+    using Python.Runtime;
+    using Python.Runtime.Codecs;
+
+    public class Codecs {
+        [SetUp]
+        public void SetUp() {
+            PythonEngine.Initialize();
+        }
+
+        [TearDown]
+        public void Dispose() {
+            PythonEngine.Shutdown();
+        }
+
+        [Test]
+        public void ConversionsGeneric() {
+            ConversionsGeneric<ValueTuple<int, string, object>, ValueTuple>();
+        }
+
+        static void ConversionsGeneric<T, TTuple>() {
+            TupleCodec<TTuple>.Register();
+            var tuple = Activator.CreateInstance(typeof(T), 42, "42", new object());
+            T restored = default;
+            using (Py.GIL())
+            using (var scope = Py.CreateScope()) {
+                void Accept(T value) => restored = value;
+                var accept = new Action<T>(Accept).ToPython();
+                scope.Set(nameof(tuple), tuple);
+                scope.Set(nameof(accept), accept);
+                scope.Exec($"{nameof(accept)}({nameof(tuple)})");
+                Assert.AreEqual(expected: tuple, actual: restored);
+            }
+        }
+
+        [Test]
+        public void ConversionsObject() {
+            ConversionsObject<ValueTuple<int, string, object>, ValueTuple>();
+        }
+        static void ConversionsObject<T, TTuple>() {
+            TupleCodec<TTuple>.Register();
+            var tuple = Activator.CreateInstance(typeof(T), 42, "42", new object());
+            T restored = default;
+            using (Py.GIL())
+            using (var scope = Py.CreateScope()) {
+                void Accept(object value) => restored = (T)value;
+                var accept = new Action<object>(Accept).ToPython();
+                scope.Set(nameof(tuple), tuple);
+                scope.Set(nameof(accept), accept);
+                scope.Exec($"{nameof(accept)}({nameof(tuple)})");
+                Assert.AreEqual(expected: tuple, actual: restored);
+            }
+        }
+
+        [Test]
+        public void TupleRoundtripObject() {
+            TupleRoundtripObject<ValueTuple<int, string, object>, ValueTuple>();
+        }
+        static void TupleRoundtripObject<T, TTuple>() {
+            var tuple = Activator.CreateInstance(typeof(T), 42, "42", new object());
+            using (Py.GIL()) {
+                var pyTuple = TupleCodec<TTuple>.Instance.TryEncode(tuple);
+                Assert.IsTrue(TupleCodec<TTuple>.Instance.TryDecode(pyTuple, out object restored));
+                Assert.AreEqual(expected: tuple, actual: restored);
+            }
+        }
+
+        [Test]
+        public void TupleRoundtripGeneric() {
+            TupleRoundtripGeneric<ValueTuple<int, string, object>, ValueTuple>();
+        }
+
+        static void TupleRoundtripGeneric<T, TTuple>() {
+            var tuple = Activator.CreateInstance(typeof(T), 42, "42", new object());
+            using (Py.GIL()) {
+                var pyTuple = TupleCodec<TTuple>.Instance.TryEncode(tuple);
+                Assert.IsTrue(TupleCodec<TTuple>.Instance.TryDecode(pyTuple, out T restored));
+                Assert.AreEqual(expected: tuple, actual: restored);
+            }
+        }
+    }
+
+    /// <summary>
+    /// "Decodes" only objects of exact type <typeparamref name="T"/>.
+    /// Result is just the raw proxy to the encoder instance itself.
+    /// </summary>
+    class ObjectToEncoderInstanceEncoder<T> : IPyObjectEncoder
+    {
+        public bool CanEncode(Type type) => type == typeof(T);
+        public PyObject TryEncode(object value) => PyObject.FromManagedObject(this);
+    }
+
+    /// <summary>
+    /// Decodes object of specified Python type to the predefined value <see cref="DecodeResult"/>
+    /// </summary>
+    /// <typeparam name="TTarget">Type of the <see cref="DecodeResult"/></typeparam>
+    class DecoderReturningPredefinedValue<TTarget> : IPyObjectDecoder
+    {
+        public PyObject TheOnlySupportedSourceType { get; }
+        public TTarget DecodeResult { get; }
+
+        public DecoderReturningPredefinedValue(PyObject objectType, TTarget decodeResult)
+        {
+            this.TheOnlySupportedSourceType = objectType;
+            this.DecodeResult = decodeResult;
+        }
+
+        public bool CanDecode(PyObject objectType, Type targetType)
+            => objectType.Handle == TheOnlySupportedSourceType.Handle
+               && targetType == typeof(TTarget);
+        public bool TryDecode<T>(PyObject pyObj, out T value)
+        {
+            if (typeof(T) != typeof(TTarget))
+                throw new ArgumentException(nameof(T));
+            value = (T)(object)DecodeResult;
+            return true;
+        }
+    }
+}
diff --git a/src/embed_tests/Python.EmbeddingTest.15.csproj b/src/embed_tests/Python.EmbeddingTest.15.csproj
index a741a589e..eb6957656 100644
--- a/src/embed_tests/Python.EmbeddingTest.15.csproj
+++ b/src/embed_tests/Python.EmbeddingTest.15.csproj
@@ -10,7 +10,7 @@
     <AssemblyName>Python.EmbeddingTest</AssemblyName>
     <RootNamespace>Python.EmbeddingTest</RootNamespace>
     <PackageId>Python.EmbeddingTest</PackageId>
-    <VersionPrefix>2.4.0</VersionPrefix>
+    <VersionPrefix>2.5.0</VersionPrefix>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
     <GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
     <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
@@ -23,7 +23,7 @@
     <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
     <PythonBuildDir Condition="'$(TargetFramework)'=='net40' AND '$(PythonBuildDir)' == ''">$(SolutionDir)\bin\</PythonBuildDir>
     <PublishDir Condition="'$(TargetFramework)'!='net40'">$(OutputPath)\$(TargetFramework)_publish</PublishDir>
-    <LangVersion>6</LangVersion>
+    <LangVersion>7.3</LangVersion>
     <ErrorReport>prompt</ErrorReport>
     <CustomDefineConstants Condition="'$(CustomDefineConstants)' == ''">$(PYTHONNET_DEFINE_CONSTANTS)</CustomDefineConstants>
     <BaseDefineConstants>XPLAT</BaseDefineConstants>
@@ -77,13 +77,18 @@
 
   
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="3.7.1" />
-    <PackageReference Include="NUnit.ConsoleRunner" Version="3.7.0" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.8.0" />
-    <PackageReference Include="NUnitLite" Version="3.7.2" />
+    <PackageReference Include="NonCopyableAnalyzer" Version="0.6.0" />
+    <PackageReference Include="System.ValueTuple" Version="4.5.0" />
+    <PackageReference Include="NUnit" Version="3.12.0" />
+    <PackageReference Include="NUnit.ConsoleRunner" Version="3.11.1" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="NUnitLite" Version="3.12.0" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp2.0'">
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\runtime\Python.Runtime.15.csproj" />
diff --git a/src/embed_tests/Python.EmbeddingTest.csproj b/src/embed_tests/Python.EmbeddingTest.csproj
index faa55fa27..5dee66e64 100644
--- a/src/embed_tests/Python.EmbeddingTest.csproj
+++ b/src/embed_tests/Python.EmbeddingTest.csproj
@@ -14,7 +14,7 @@
     <NoWarn>1591</NoWarn>
     <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
     <PythonBuildDir Condition=" '$(PythonBuildDir)' == '' ">$(SolutionDir)\bin\</PythonBuildDir>
-    <LangVersion>6</LangVersion>
+    <LangVersion>7.3</LangVersion>
     <RestorePackages>true</RestorePackages>
     <ErrorReport>prompt</ErrorReport>
   </PropertyGroup>
@@ -70,8 +70,11 @@
   </PropertyGroup>
   <ItemGroup>
     <Reference Include="Microsoft.CSharp" />
-    <Reference Include="nunit.framework, Version=3.7.1.0, Culture=neutral, PublicKeyToken=2638cd05610744eb, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\NUnit.3.7.1\lib\net40\nunit.framework.dll</HintPath>
+    <Reference Include="nunit.framework, Version=3.12.0.0, Culture=neutral, PublicKeyToken=2638cd05610744eb, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\NUnit.3.12.0\lib\net40\nunit.framework.dll</HintPath>
+    </Reference>
+    <Reference Include="System.ValueTuple, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\System.ValueTuple.4.5.0\lib\portable-net40+sl4+win8+wp8\System.ValueTuple.dll</HintPath>
     </Reference>
     <Reference Include="System" />
   </ItemGroup>
@@ -80,15 +83,19 @@
     <None Include="packages.config" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="CodecGroups.cs" />
+    <Compile Include="Codecs.cs" />
     <Compile Include="dynamic.cs" />
     <Compile Include="pyimport.cs" />
     <Compile Include="pyinitialize.cs" />
     <Compile Include="pyrunstring.cs" />
+    <Compile Include="References.cs" />
     <Compile Include="TestConverter.cs" />
     <Compile Include="TestCustomMarshal.cs" />
     <Compile Include="TestDomainReload.cs" />
     <Compile Include="TestExample.cs" />
     <Compile Include="TestFinalizer.cs" />
+    <Compile Include="TestInstanceWrapping.cs" />
     <Compile Include="TestPyAnsiString.cs" />
     <Compile Include="TestPyFloat.cs" />
     <Compile Include="TestPyInt.cs" />
@@ -126,4 +133,10 @@
     <Copy SourceFiles="$(TargetAssembly)" DestinationFolder="$(PythonBuildDir)" />
     <!--Copy SourceFiles="$(TargetAssemblyPdb)" Condition="Exists('$(TargetAssemblyPdb)')" DestinationFolder="$(PythonBuildDir)" /-->
   </Target>
+  <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
+    <PropertyGroup>
+      <ErrorText>This project references NuGet package(s) that are missing on this computer. Use NuGet Package Restore to download them.  For more information, see http://go.microsoft.com/fwlink/?LinkID=322105. The missing file is {0}.</ErrorText>
+    </PropertyGroup>
+    <Error Condition="!Exists('..\..\packages\NUnit.3.12.0\build\NUnit.props')" Text="$([System.String]::Format('$(ErrorText)', '..\..\packages\NUnit.3.12.0\build\NUnit.props'))" />
+  </Target>
 </Project>
diff --git a/src/embed_tests/References.cs b/src/embed_tests/References.cs
new file mode 100644
index 000000000..1d29e85c7
--- /dev/null
+++ b/src/embed_tests/References.cs
@@ -0,0 +1,55 @@
+namespace Python.EmbeddingTest
+{
+    using NUnit.Framework;
+    using Python.Runtime;
+
+    public class References
+    {
+        private Py.GILState _gs;
+
+        [SetUp]
+        public void SetUp()
+        {
+            _gs = Py.GIL();
+        }
+
+        [TearDown]
+        public void Dispose()
+        {
+            _gs.Dispose();
+        }
+
+        [Test]
+        public void MoveToPyObject_SetsNull()
+        {
+            var dict = new PyDict();
+            NewReference reference = Runtime.PyDict_Items(dict.Handle);
+            try
+            {
+                Assert.IsFalse(reference.IsNull());
+
+                using (reference.MoveToPyObject())
+                    Assert.IsTrue(reference.IsNull());
+            }
+            finally
+            {
+                reference.Dispose();
+            }
+        }
+
+        [Test]
+        public void CanBorrowFromNewReference()
+        {
+            var dict = new PyDict();
+            NewReference reference = Runtime.PyDict_Items(dict.Handle);
+            try
+            {
+                PythonException.ThrowIfIsNotZero(Runtime.PyList_Reverse(reference));
+            }
+            finally
+            {
+                reference.Dispose();
+            }
+        }
+    }
+}
diff --git a/src/embed_tests/TestCallbacks.cs b/src/embed_tests/TestCallbacks.cs
new file mode 100644
index 000000000..220b0a86a
--- /dev/null
+++ b/src/embed_tests/TestCallbacks.cs
@@ -0,0 +1,35 @@
+using System;
+
+using NUnit.Framework;
+using Python.Runtime;
+
+namespace Python.EmbeddingTest {
+    using Runtime = Python.Runtime.Runtime;
+
+    public class TestCallbacks {
+        [OneTimeSetUp]
+        public void SetUp() {
+            PythonEngine.Initialize();
+        }
+
+        [OneTimeTearDown]
+        public void Dispose() {
+            PythonEngine.Shutdown();
+        }
+
+        [Test]
+        public void TestNoOverloadException() {
+            int passed = 0;
+            var aFunctionThatCallsIntoPython = new Action<int>(value => passed = value);
+            using (Py.GIL()) {
+                dynamic callWith42 = PythonEngine.Eval("lambda f: f([42])");
+                var error =  Assert.Throws<PythonException>(() => callWith42(aFunctionThatCallsIntoPython.ToPython()));
+                Assert.AreEqual("TypeError", error.PythonTypeName);
+                string expectedArgTypes = Runtime.IsPython2
+                    ? "(<type 'list'>)"
+                    : "(<class 'list'>)";
+                StringAssert.EndsWith(expectedArgTypes, error.Message);
+            }
+        }
+    }
+}
diff --git a/src/embed_tests/TestConverter.cs b/src/embed_tests/TestConverter.cs
index caaec311b..40219973b 100644
--- a/src/embed_tests/TestConverter.cs
+++ b/src/embed_tests/TestConverter.cs
@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using NUnit.Framework;
 using Python.Runtime;
 
@@ -44,5 +46,26 @@ public void TestConvertDoubleToManaged(
             Assert.IsTrue(converted);
             Assert.IsTrue(((double) convertedValue).Equals(testValue));
         }
+
+        [Test]
+        public void RawListProxy()
+        {
+            var list = new List<string> {"hello", "world"};
+            var listProxy = PyObject.FromManagedObject(list);
+            var clrObject = (CLRObject)ManagedType.GetManagedObject(listProxy.Handle);
+            Assert.AreSame(list, clrObject.inst);
+        }
+
+        [Test]
+        public void RawPyObjectProxy()
+        {
+            var pyObject = "hello world!".ToPython();
+            var pyObjectProxy = PyObject.FromManagedObject(pyObject);
+            var clrObject = (CLRObject)ManagedType.GetManagedObject(pyObjectProxy.Handle);
+            Assert.AreSame(pyObject, clrObject.inst);
+
+            var proxiedHandle = pyObjectProxy.GetAttr("Handle").As<IntPtr>();
+            Assert.AreEqual(pyObject.Handle, proxiedHandle);
+        }
     }
 }
diff --git a/src/embed_tests/TestFinalizer.cs b/src/embed_tests/TestFinalizer.cs
index 53838f315..f82767af1 100644
--- a/src/embed_tests/TestFinalizer.cs
+++ b/src/embed_tests/TestFinalizer.cs
@@ -45,7 +45,7 @@ public void CollectBasicObject()
                 called = true;
             };
 
-            Assert.IsFalse(called);
+            Assert.IsFalse(called, "The event handler was called before it was installed");
             Finalizer.Instance.CollectOnce += handler;
 
             WeakReference shortWeak;
@@ -55,23 +55,35 @@ public void CollectBasicObject()
             }
             FullGCCollect();
             // The object has been resurrected
-            Assert.IsFalse(shortWeak.IsAlive);
-            Assert.IsTrue(longWeak.IsAlive);
+            Warn.If(
+                shortWeak.IsAlive,
+                "The referenced object is alive although it should have been collected",
+                shortWeak
+            );
+            Assert.IsTrue(
+                longWeak.IsAlive,
+                "The reference object is not alive although it should still be",
+                longWeak
+            );
 
             {
                 var garbage = Finalizer.Instance.GetCollectedObjects();
-                Assert.NotZero(garbage.Count);
-                Assert.IsTrue(garbage.Any(T => ReferenceEquals(T.Target, longWeak.Target)));
+                Assert.NotZero(garbage.Count, "There should still be garbage around");
+                Warn.Unless(
+                    garbage.Any(T => ReferenceEquals(T.Target, longWeak.Target)),
+                    $"The {nameof(longWeak)} reference doesn't show up in the garbage list",
+                    garbage
+                );
             }
             try
             {
-                Finalizer.Instance.Collect(forceDispose: false);
+                Finalizer.Instance.Collect();
             }
             finally
             {
                 Finalizer.Instance.CollectOnce -= handler;
             }
-            Assert.IsTrue(called);
+            Assert.IsTrue(called, "The event handler was not called during finalization");
             Assert.GreaterOrEqual(objectCount, 1);
         }
 
diff --git a/src/embed_tests/TestGILState.cs b/src/embed_tests/TestGILState.cs
new file mode 100644
index 000000000..bf6f02dc6
--- /dev/null
+++ b/src/embed_tests/TestGILState.cs
@@ -0,0 +1,33 @@
+namespace Python.EmbeddingTest
+{
+    using NUnit.Framework;
+    using Python.Runtime;
+
+    public class TestGILState
+    {
+        /// <summary>
+        /// Ensure, that calling <see cref="Py.GILState.Dispose"/> multiple times is safe
+        /// </summary>
+        [Test]
+        public void CanDisposeMultipleTimes()
+        {
+            using (var gilState = Py.GIL())
+            {
+                for(int i = 0; i < 50; i++)
+                    gilState.Dispose();
+            }
+        }
+
+        [OneTimeSetUp]
+        public void SetUp()
+        {
+            PythonEngine.Initialize();
+        }
+
+        [OneTimeTearDown]
+        public void Dispose()
+        {
+            PythonEngine.Shutdown();
+        }
+    }
+}
diff --git a/src/embed_tests/TestInstanceWrapping.cs b/src/embed_tests/TestInstanceWrapping.cs
new file mode 100644
index 000000000..8be207c00
--- /dev/null
+++ b/src/embed_tests/TestInstanceWrapping.cs
@@ -0,0 +1,58 @@
+using System;
+using System.Globalization;
+using NUnit.Framework;
+using Python.Runtime;
+
+namespace Python.EmbeddingTest
+{
+    public class TestInstanceWrapping
+    {
+        [OneTimeSetUp]
+        public void SetUp()
+        {
+            PythonEngine.Initialize();
+        }
+
+        [OneTimeTearDown]
+        public void Dispose()
+        {
+            PythonEngine.Shutdown();
+        }
+
+        // regression test for https://github.com/pythonnet/pythonnet/issues/811
+        [Test]
+        public void OverloadResolution_UnknownToObject()
+        {
+            var overloaded = new Overloaded();
+            using (Py.GIL())
+            {
+                var o = overloaded.ToPython();
+
+                dynamic callWithSelf = PythonEngine.Eval("lambda o: o.ObjOrClass(object())");
+                callWithSelf(o);
+                Assert.AreEqual(Overloaded.Object, overloaded.Value);
+            }
+        }
+
+        class Base {}
+        class Derived: Base { }
+
+        class Overloaded: Derived
+        {
+            public int Value { get; set; }
+            public void IntOrStr(int arg) => this.Value = arg;
+            public void IntOrStr(string arg) =>
+                this.Value = int.Parse(arg, NumberStyles.Integer, CultureInfo.InvariantCulture);
+
+            public const int Object = 1;
+            public const int ConcreteClass = 2;
+            public void ObjOrClass(object _) => this.Value = Object;
+            public void ObjOrClass(Overloaded _) => this.Value = ConcreteClass;
+
+            public const int Base = ConcreteClass + 1;
+            public const int Derived = Base + 1;
+            public void BaseOrDerived(Base _) => this.Value = Base;
+            public void BaseOrDerived(Derived _) => this.Value = Derived;
+        }
+    }
+}
diff --git a/src/embed_tests/TestPyObject.cs b/src/embed_tests/TestPyObject.cs
index 65ac20e9a..d4952d4a3 100644
--- a/src/embed_tests/TestPyObject.cs
+++ b/src/embed_tests/TestPyObject.cs
@@ -57,5 +57,11 @@ def add(self, x, y):
                 Assert.IsTrue(memberNames.Contains(expectedName), "Could not find member '{0}'.", expectedName);
             }
         }
+
+        [Test]
+        public void InvokeNull() {
+            var list = PythonEngine.Eval("list");
+            Assert.Throws<ArgumentNullException>(() => list.Invoke(new PyObject[] {null}));
+        }
     }
 }
diff --git a/src/embed_tests/TestPyScope.cs b/src/embed_tests/TestPyScope.cs
index 21c0d2b3f..701e698ec 100644
--- a/src/embed_tests/TestPyScope.cs
+++ b/src/embed_tests/TestPyScope.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Threading;
 using NUnit.Framework;
 using Python.Runtime;
 
@@ -337,13 +338,16 @@ public void TestThread()
                     //add function to the scope 
                     //can be call many times, more efficient than ast 
                     ps.Exec(
+                        "import threading\n" +
+                        "lock = threading.Lock()\n" +
                         "def update():\n" +
-                        "    global res, th_cnt\n" +
+                        "  global res, th_cnt\n" +
+                        "  with lock:\n" +
                         "    res += bb + 1\n" +
                         "    th_cnt += 1\n"
                     );
                 }
-                int th_cnt = 3;
+                int th_cnt = 100;
                 for (int i = 0; i < th_cnt; i++)
                 {
                     System.Threading.Thread th = new System.Threading.Thread(() =>
@@ -364,7 +368,7 @@ public void TestThread()
                     {
                         cnt = ps.Get<int>("th_cnt");
                     }
-                    System.Threading.Thread.Sleep(10);
+                    Thread.Yield();
                 }
                 using (Py.GIL())
                 {
diff --git a/src/embed_tests/TestPythonException.cs b/src/embed_tests/TestPythonException.cs
index 57a8d54af..000c32ca3 100644
--- a/src/embed_tests/TestPythonException.cs
+++ b/src/embed_tests/TestPythonException.cs
@@ -54,5 +54,41 @@ public void TestPythonErrorTypeName()
                 Assert.That(ex.PythonTypeName, Is.EqualTo("ModuleNotFoundError").Or.EqualTo("ImportError"));
             }
         }
+
+        [Test]
+        public void TestPythonExceptionFormat()
+        {
+            try
+            {
+                PythonEngine.Exec("raise ValueError('Error!')");
+                Assert.Fail("Exception should have been raised");
+            }
+            catch (PythonException ex)
+            {
+                Assert.That(ex.Format(), Does.Contain("Traceback").And.Contains("(most recent call last):").And.Contains("ValueError: Error!"));
+            }
+        }
+
+        [Test]
+        public void TestPythonExceptionFormatNoError()
+        {
+            var ex = new PythonException();
+            Assert.AreEqual(ex.StackTrace, ex.Format());
+        }
+
+        [Test]
+        public void TestPythonExceptionFormatNoTraceback()
+        {
+            try
+            {
+                var module = PythonEngine.ImportModule("really____unknown___module");
+                Assert.Fail("Unknown module should not be loaded");
+            }
+            catch (PythonException ex)
+            {
+                // ImportError/ModuleNotFoundError do not have a traceback when not running in a script 
+                Assert.AreEqual(ex.StackTrace, ex.Format());
+            }
+        }
     }
 }
diff --git a/src/embed_tests/TestRuntime.cs b/src/embed_tests/TestRuntime.cs
index ac1fa1ac0..4129d3df3 100644
--- a/src/embed_tests/TestRuntime.cs
+++ b/src/embed_tests/TestRuntime.cs
@@ -1,6 +1,8 @@
 using System;
+using System.Collections.Generic;
 using NUnit.Framework;
 using Python.Runtime;
+using Python.Runtime.Platform;
 
 namespace Python.EmbeddingTest
 {
@@ -26,11 +28,8 @@ public static void PlatformCache()
         {
             Runtime.Runtime.Initialize();
 
-            Assert.That(Runtime.Runtime.Machine, Is.Not.EqualTo(Runtime.Runtime.MachineType.Other));
-            Assert.That(!string.IsNullOrEmpty(Runtime.Runtime.MachineName));
-
-            Assert.That(Runtime.Runtime.OperatingSystem, Is.Not.EqualTo(Runtime.Runtime.OperatingSystemType.Other));
-            Assert.That(!string.IsNullOrEmpty(Runtime.Runtime.OperatingSystemName));
+            Assert.That(Runtime.Runtime.Machine, Is.Not.EqualTo(MachineType.Other));
+            Assert.That(Runtime.Runtime.OperatingSystem, Is.Not.EqualTo(OperatingSystemType.Other));
 
             // Don't shut down the runtime: if the python engine was initialized
             // but not shut down by another test, we'd end up in a bad state.
@@ -39,7 +38,7 @@ public static void PlatformCache()
         [Test]
         public static void Py_IsInitializedValue()
         {
-            Runtime.Runtime.Py_Finalize(); 
+            Runtime.Runtime.Py_Finalize();
             Assert.AreEqual(0, Runtime.Runtime.Py_IsInitialized());
             Runtime.Runtime.Py_Initialize();
             Assert.AreEqual(1, Runtime.Runtime.Py_IsInitialized());
@@ -109,9 +108,15 @@ public static void PyCheck_Iter_PyObject_IsIterable_ThreadingLock_Test()
             // Create an instance of threading.Lock, which is one of the very few types that does not have the
             // TypeFlags.HaveIter set in Python 2. This tests a different code path in PyObject_IsIterable and PyIter_Check.
             var threading = Runtime.Runtime.PyImport_ImportModule("threading");
+            Exceptions.ErrorCheck(threading);
             var threadingDict = Runtime.Runtime.PyModule_GetDict(threading);
+            Exceptions.ErrorCheck(threadingDict);
             var lockType = Runtime.Runtime.PyDict_GetItemString(threadingDict, "Lock");
+            if (lockType == IntPtr.Zero)
+                throw new KeyNotFoundException("class 'Lock' was not found in 'threading'");
+
             var lockInstance = Runtime.Runtime.PyObject_CallObject(lockType, Runtime.Runtime.PyTuple_New(0));
+            Exceptions.ErrorCheck(lockInstance);
 
             Assert.IsFalse(Runtime.Runtime.PyObject_IsIterable(lockInstance));
             Assert.IsFalse(Runtime.Runtime.PyIter_Check(lockInstance));
diff --git a/src/embed_tests/packages.config b/src/embed_tests/packages.config
index 8c175f441..590eaef8c 100644
--- a/src/embed_tests/packages.config
+++ b/src/embed_tests/packages.config
@@ -1,5 +1,6 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <packages>
-  <package id="NUnit" version="3.7.1" targetFramework="net40" />
-  <package id="NUnit.ConsoleRunner" version="3.7.0" targetFramework="net40" />
+  <package id="NUnit" version="3.12.0" targetFramework="net40" />
+  <package id="NUnit.ConsoleRunner" version="3.11.1" targetFramework="net40" />
+  <package id="System.ValueTuple" version="4.5.0" targetFramework="net40" />
 </packages>
diff --git a/src/embed_tests/pyimport.cs b/src/embed_tests/pyimport.cs
index acb3433de..6b2408745 100644
--- a/src/embed_tests/pyimport.cs
+++ b/src/embed_tests/pyimport.cs
@@ -39,7 +39,7 @@ public void SetUp()
 
             IntPtr str = Runtime.Runtime.PyString_FromString(testPath);
             IntPtr path = Runtime.Runtime.PySys_GetObject("path");
-            Runtime.Runtime.PyList_Append(path, str);
+            Runtime.Runtime.PyList_Append(new BorrowedReference(path), str);
         }
 
         [TearDown]
diff --git a/src/monoclr/pynetclr.h b/src/monoclr/pynetclr.h
index c5e181156..1863b1d43 100644
--- a/src/monoclr/pynetclr.h
+++ b/src/monoclr/pynetclr.h
@@ -7,12 +7,15 @@
 #include <mono/metadata/mono-config.h>
 #include <mono/metadata/debug-helpers.h>
 #include <mono/metadata/assembly.h>
-#include <glib.h>
 
 #define MONO_VERSION "v4.0.30319.1"
 #define MONO_DOMAIN "Python.Runtime"
 #define PR_ASSEMBLY "Python.Runtime.dll"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct
 {
     MonoDomain *domain;
@@ -27,7 +30,11 @@ typedef struct
 
 PyNet_Args *PyNet_Init(int);
 void PyNet_Finalize(PyNet_Args *);
-void main_thread_handler(gpointer user_data);
+void main_thread_handler(PyNet_Args *user_data);
 char *PyNet_ExceptionToString(MonoObject *);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // PYNET_CLR_H
diff --git a/src/monoclr/pynetinit.c b/src/monoclr/pynetinit.c
index 8b49ddae3..149d52296 100644
--- a/src/monoclr/pynetinit.c
+++ b/src/monoclr/pynetinit.c
@@ -19,6 +19,15 @@ PyNet_Args *PyNet_Init(int ext)
     pn_args->shutdown = NULL;
     pn_args->module = NULL;
 
+#ifdef __linux__
+    // Force preload libmono-2.0 as global. Without this, on some systems 
+    // symbols from libmono are not found by libmononative (which implements
+    // some of the System.* namespaces). Since the only happened on Linux so
+    // far, we hardcode the library name, load the symbols into the global
+    // namespace and leak the handle.
+    dlopen("libmono-2.0.so", RTLD_LAZY | RTLD_GLOBAL);
+#endif
+
     if (ext == 0)
     {
         pn_args->init_name = "Python.Runtime:Initialize()";
@@ -91,9 +100,9 @@ MonoMethod *getMethodFromClass(MonoClass *cls, char *name)
     return method;
 }
 
-void main_thread_handler(gpointer user_data)
+void main_thread_handler(PyNet_Args *user_data)
 {
-    PyNet_Args *pn_args = (PyNet_Args *)user_data;
+    PyNet_Args *pn_args = user_data;
     MonoMethod *init;
     MonoImage *pr_image;
     MonoClass *pythonengine;
@@ -122,7 +131,7 @@ void main_thread_handler(gpointer user_data)
             strcpy(new_ld_library_path, py_libdir);
             strcat(new_ld_library_path, ":");
             strcat(new_ld_library_path, ld_library_path);
-            setenv("LD_LIBRARY_PATH", py_libdir, 1);
+            setenv("LD_LIBRARY_PATH", new_ld_library_path, 1);
         }
     }
 
@@ -241,7 +250,7 @@ void main_thread_handler(gpointer user_data)
 // Get string from a Mono exception
 char *PyNet_ExceptionToString(MonoObject *e)
 {
-    MonoMethodDesc *mdesc = mono_method_desc_new(":ToString()", FALSE);
+    MonoMethodDesc *mdesc = mono_method_desc_new(":ToString()", 0 /*FALSE*/);
     MonoMethod *mmethod = mono_method_desc_search_in_class(mdesc, mono_get_object_class());
     mono_method_desc_free(mdesc);
 
diff --git a/src/perf_tests/BaselineComparisonBenchmarkBase.cs b/src/perf_tests/BaselineComparisonBenchmarkBase.cs
new file mode 100644
index 000000000..2388e3982
--- /dev/null
+++ b/src/perf_tests/BaselineComparisonBenchmarkBase.cs
@@ -0,0 +1,69 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+
+using Python.Runtime;
+
+namespace Python.PerformanceTests
+{
+    public class BaselineComparisonBenchmarkBase
+    {
+        public BaselineComparisonBenchmarkBase()
+        {
+            Console.WriteLine($"CWD: {Environment.CurrentDirectory}");
+            Console.WriteLine($"Using Python.Runtime from {typeof(PythonEngine).Assembly.Location} {typeof(PythonEngine).Assembly.GetName()}");
+
+            try {
+                PythonEngine.Initialize();
+                Console.WriteLine("Python Initialized");
+                if (PythonEngine.BeginAllowThreads() == IntPtr.Zero)
+                    throw new PythonException();
+                Console.WriteLine("Threading enabled");
+            }
+            catch (Exception e) {
+                Console.WriteLine(e);
+                throw;
+            }
+        }
+
+        static BaselineComparisonBenchmarkBase()
+        {
+            string pythonRuntimeDll = Environment.GetEnvironmentVariable(BaselineComparisonConfig.EnvironmentVariableName);
+            if (string.IsNullOrEmpty(pythonRuntimeDll))
+            {
+                throw new ArgumentException(
+                    "Required environment variable is missing",
+                    BaselineComparisonConfig.EnvironmentVariableName);
+            }
+
+            Console.WriteLine("Preloading " + pythonRuntimeDll);
+            Assembly.LoadFrom(pythonRuntimeDll);
+            foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies()) {
+                if (assembly.FullName.StartsWith("Python.Runtime"))
+                    Console.WriteLine(assembly.Location);
+                foreach(var dependency in assembly.GetReferencedAssemblies())
+                    if (dependency.FullName.Contains("Python.Runtime")) {
+                        Console.WriteLine($"{assembly} -> {dependency}");
+                    }
+            }
+
+            AppDomain.CurrentDomain.AssemblyResolve += CurrentDomainOnAssemblyResolve;
+        }
+
+        static Assembly CurrentDomainOnAssemblyResolve(object sender, ResolveEventArgs args) {
+            if (!args.Name.StartsWith("Python.Runtime"))
+                return null;
+
+            var preloaded = AppDomain.CurrentDomain.GetAssemblies()
+                .FirstOrDefault(a => a.GetName().Name == "Python.Runtime");
+            if (preloaded != null) return preloaded;
+
+            string pythonRuntimeDll = Environment.GetEnvironmentVariable(BaselineComparisonConfig.EnvironmentVariableName);
+            if (string.IsNullOrEmpty(pythonRuntimeDll))
+                return null;
+
+            return Assembly.LoadFrom(pythonRuntimeDll);
+        }
+    }
+}
diff --git a/src/perf_tests/BaselineComparisonConfig.cs b/src/perf_tests/BaselineComparisonConfig.cs
new file mode 100644
index 000000000..649bb56fd
--- /dev/null
+++ b/src/perf_tests/BaselineComparisonConfig.cs
@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Reflection;
+
+using BenchmarkDotNet.Configs;
+using BenchmarkDotNet.Jobs;
+using BenchmarkDotNet.Horology;
+
+namespace Python.PerformanceTests
+{
+    public class BaselineComparisonConfig : ManualConfig
+    {
+        public const string EnvironmentVariableName = "PythonRuntimeDLL";
+
+        public BaselineComparisonConfig()
+        {
+            this.Options |= ConfigOptions.DisableOptimizationsValidator;
+
+            string deploymentRoot = BenchmarkTests.DeploymentRoot;
+
+            var baseJob = Job.Default
+                .WithLaunchCount(1)
+                .WithWarmupCount(3)
+                .WithMaxIterationCount(100)
+                .WithIterationTime(TimeInterval.FromMilliseconds(100));
+            this.Add(baseJob
+                .WithId("baseline")
+                .WithEnvironmentVariable(EnvironmentVariableName,
+                    Path.Combine(deploymentRoot, "baseline", "Python.Runtime.dll"))
+                .WithBaseline(true));
+            this.Add(baseJob
+                .WithId("new")
+                .WithEnvironmentVariable(EnvironmentVariableName,
+                    Path.Combine(deploymentRoot, "new", "Python.Runtime.dll")));
+        }
+
+        static BaselineComparisonConfig() {
+            AppDomain.CurrentDomain.AssemblyResolve += CurrentDomainOnAssemblyResolve;
+        }
+
+        static Assembly CurrentDomainOnAssemblyResolve(object sender, ResolveEventArgs args) {
+            Console.WriteLine(args.Name);
+            if (!args.Name.StartsWith("Python.Runtime"))
+                return null;
+            string pythonRuntimeDll = Environment.GetEnvironmentVariable(EnvironmentVariableName);
+            if (string.IsNullOrEmpty(pythonRuntimeDll))
+                pythonRuntimeDll = Path.Combine(BenchmarkTests.DeploymentRoot, "baseline", "Python.Runtime.dll");
+            return Assembly.LoadFrom(pythonRuntimeDll);
+        }
+    }
+}
diff --git a/src/perf_tests/BenchmarkTests.cs b/src/perf_tests/BenchmarkTests.cs
new file mode 100644
index 000000000..6e0afca69
--- /dev/null
+++ b/src/perf_tests/BenchmarkTests.cs
@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Reflection;
+
+using BenchmarkDotNet.Reports;
+using BenchmarkDotNet.Running;
+using NUnit.Framework;
+
+namespace Python.PerformanceTests
+{
+    public class BenchmarkTests
+    {
+        Summary summary;
+
+        [OneTimeSetUp]
+        public void SetUp()
+        {
+            Environment.CurrentDirectory = Path.Combine(DeploymentRoot, "new");
+            this.summary = BenchmarkRunner.Run<PythonCallingNetBenchmark>();
+            Assert.IsNotEmpty(this.summary.Reports);
+            Assert.IsTrue(
+                condition: this.summary.Reports.All(r => r.Success),
+                message: "BenchmarkDotNet failed to execute or collect results of performance tests. See logs above.");
+        }
+
+        [Test]
+        public void ReadInt64Property()
+        {
+            double optimisticPerfRatio = GetOptimisticPerfRatio(this.summary.Reports);
+            AssertPerformanceIsBetterOrSame(optimisticPerfRatio, target: 0.57);
+        }
+
+        [Test]
+        public void WriteInt64Property()
+        {
+            double optimisticPerfRatio = GetOptimisticPerfRatio(this.summary.Reports);
+            AssertPerformanceIsBetterOrSame(optimisticPerfRatio, target: 0.57);
+        }
+
+        static double GetOptimisticPerfRatio(
+            IReadOnlyList<BenchmarkReport> reports,
+            [CallerMemberName] string methodName = null)
+        {
+            reports = reports.Where(r => r.BenchmarkCase.Descriptor.WorkloadMethod.Name == methodName).ToArray();
+            if (reports.Count == 0)
+                throw new ArgumentException(
+                    message: $"No reports found for {methodName}. "
+                             + "You have to match test method name to benchmark method name or "
+                             + "pass benchmark method name explicitly",
+                    paramName: nameof(methodName));
+
+            var baseline = reports.Single(r => r.BenchmarkCase.Job.ResolvedId == "baseline").ResultStatistics;
+            var @new = reports.Single(r => r.BenchmarkCase.Job.ResolvedId != "baseline").ResultStatistics;
+
+            double newTimeOptimistic = @new.Mean - (@new.StandardDeviation + baseline.StandardDeviation) * 0.5;
+
+            return newTimeOptimistic / baseline.Mean;
+        }
+
+        public static string DeploymentRoot => Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
+
+        public static void AssertPerformanceIsBetterOrSame(
+            double actual, double target,
+            double wiggleRoom = 1.1, [CallerMemberName] string testName = null) {
+            double threshold = target * wiggleRoom;
+            Assert.LessOrEqual(actual, threshold,
+                $"{testName}: {actual:F3} > {threshold:F3} (target: {target:F3})"
+                + ": perf result is higher than the failure threshold.");
+        }
+    }
+}
diff --git a/src/perf_tests/Python.PerformanceTests.csproj b/src/perf_tests/Python.PerformanceTests.csproj
new file mode 100644
index 000000000..25af89db0
--- /dev/null
+++ b/src/perf_tests/Python.PerformanceTests.csproj
@@ -0,0 +1,44 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net461</TargetFramework>
+    <Configurations>DebugMono;DebugMonoPY3;ReleaseMono;ReleaseMonoPY3;DebugWin;DebugWinPY3;ReleaseWin;ReleaseWinPY3</Configurations>
+    <OutputPath>bin\</OutputPath>
+
+    <IsPackable>false</IsPackable>
+
+    <Platforms>x64;x86</Platforms>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="BenchmarkDotNet" Version="0.12.0" />
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="nunit" Version="3.12.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
+    <PackageReference Include="pythonnet" Version="2.3.0" GeneratePathProperty="true">
+      <IncludeAssets>compile</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <Target Name="GetRuntimeLibBuildOutput" BeforeTargets="Build">
+    <MSBuild Projects="..\runtime\Python.Runtime.15.csproj" Properties="PYTHONNET_PY3_VERSION=PYTHON35;Configuration=$(Configuration);TargetFramework=net40;Python3Version=PYTHON35;OutputPath=bin\for_perf\">
+      <Output TaskParameter="TargetOutputs" ItemName="NewPythonRuntime" />
+    </MSBuild>
+  </Target>
+
+  <Target Name="CopyBaseline" AfterTargets="Build">
+    <Copy SourceFiles="$(Pkgpythonnet)\lib\net40\Python.Runtime.dll" DestinationFolder="$(OutDir)\baseline" />
+  </Target>
+
+  <Target Name="CopyNewBuild" AfterTargets="Build">
+    <Copy SourceFiles="@(NewPythonRuntime)" DestinationFolder="$(OutDir)\new" />
+  </Target>
+
+</Project>
diff --git a/src/perf_tests/PythonCallingNetBenchmark.cs b/src/perf_tests/PythonCallingNetBenchmark.cs
new file mode 100644
index 000000000..ef668a911
--- /dev/null
+++ b/src/perf_tests/PythonCallingNetBenchmark.cs
@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+using BenchmarkDotNet.Attributes;
+using Python.Runtime;
+
+namespace Python.PerformanceTests
+{
+    [Config(typeof(BaselineComparisonConfig))]
+    public class PythonCallingNetBenchmark: BaselineComparisonBenchmarkBase
+    {
+        [Benchmark]
+        public void ReadInt64Property()
+        {
+            using (Py.GIL())
+            {
+                var locals = new PyDict();
+                locals.SetItem("a", new NetObject().ToPython());
+                PythonEngine.Exec($@"
+s = 0
+for i in range(50000):
+  s += a.{nameof(NetObject.LongProperty)}
+", locals: locals.Handle);
+            }
+        }
+
+        [Benchmark]
+        public void WriteInt64Property() {
+            using (Py.GIL()) {
+                var locals = new PyDict();
+                locals.SetItem("a", new NetObject().ToPython());
+                PythonEngine.Exec($@"
+s = 0
+for i in range(50000):
+  a.{nameof(NetObject.LongProperty)} += i
+", locals: locals.Handle);
+            }
+        }
+    }
+
+    class NetObject
+    {
+        public long LongProperty { get; set; } = 42;
+    }
+}
diff --git a/src/runtime/BorrowedReference.cs b/src/runtime/BorrowedReference.cs
new file mode 100644
index 000000000..a3bf29056
--- /dev/null
+++ b/src/runtime/BorrowedReference.cs
@@ -0,0 +1,25 @@
+namespace Python.Runtime
+{
+    using System;
+    /// <summary>
+    /// Represents a reference to a Python object, that is being lent, and
+    /// can only be safely used until execution returns to the caller.
+    /// </summary>
+    readonly ref struct BorrowedReference
+    {
+        readonly IntPtr pointer;
+        public bool IsNull => this.pointer == IntPtr.Zero;
+
+        /// <summary>Gets a raw pointer to the Python object</summary>
+        public IntPtr DangerousGetAddress()
+            => this.IsNull ? throw new NullReferenceException() : this.pointer;
+
+        /// <summary>
+        /// Creates new instance of <see cref="BorrowedReference"/> from raw pointer. Unsafe.
+        /// </summary>
+        public BorrowedReference(IntPtr pointer)
+        {
+            this.pointer = pointer;
+        }
+    }
+}
diff --git a/src/runtime/Codecs/DecoderGroup.cs b/src/runtime/Codecs/DecoderGroup.cs
new file mode 100644
index 000000000..8a290d5d4
--- /dev/null
+++ b/src/runtime/Codecs/DecoderGroup.cs
@@ -0,0 +1,78 @@
+namespace Python.Runtime.Codecs
+{
+    using System;
+    using System.Collections;
+    using System.Collections.Generic;
+    using System.Linq;
+
+    /// <summary>
+    /// Represents a group of <see cref="IPyObjectDecoder"/>s. Useful to group them by priority.
+    /// </summary>
+    [Obsolete(Util.UnstableApiMessage)]
+    public sealed class DecoderGroup: IPyObjectDecoder, IEnumerable<IPyObjectDecoder>
+    {
+        readonly List<IPyObjectDecoder> decoders = new List<IPyObjectDecoder>();
+
+        /// <summary>
+        /// Add specified decoder to the group
+        /// </summary>
+        public void Add(IPyObjectDecoder item)
+        {
+            if (item is null) throw new ArgumentNullException(nameof(item));
+
+            this.decoders.Add(item);
+        }
+        /// <summary>
+        /// Remove all decoders from the group
+        /// </summary>
+        public void Clear() => this.decoders.Clear();
+
+        /// <inheritdoc />
+        public bool CanDecode(PyObject objectType, Type targetType)
+            => this.decoders.Any(decoder => decoder.CanDecode(objectType, targetType));
+        /// <inheritdoc />
+        public bool TryDecode<T>(PyObject pyObj, out T value)
+        {
+            if (pyObj is null) throw new ArgumentNullException(nameof(pyObj));
+
+            var decoder = this.GetDecoder(pyObj.GetPythonType(), typeof(T));
+            if (decoder is null)
+            {
+                value = default;
+                return false;
+            }
+            return decoder.TryDecode(pyObj, out value);
+        }
+
+        /// <inheritdoc />
+        public IEnumerator<IPyObjectDecoder> GetEnumerator() => this.decoders.GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => this.decoders.GetEnumerator();
+    }
+
+    [Obsolete(Util.UnstableApiMessage)]
+    public static class DecoderGroupExtensions
+    {
+        /// <summary>
+        /// Gets a concrete instance of <see cref="IPyObjectDecoder"/>
+        /// (potentially selecting one from a collection),
+        /// that can decode from <paramref name="objectType"/> to <paramref name="targetType"/>,
+        /// or <c>null</c> if a matching decoder can not be found.
+        /// </summary>
+        [Obsolete(Util.UnstableApiMessage)]
+        public static IPyObjectDecoder GetDecoder(
+            this IPyObjectDecoder decoder,
+            PyObject objectType, Type targetType)
+        {
+            if (decoder is null) throw new ArgumentNullException(nameof(decoder));
+
+            if (decoder is IEnumerable<IPyObjectDecoder> composite)
+            {
+                return composite
+                    .Select(nestedDecoder => nestedDecoder.GetDecoder(objectType, targetType))
+                    .FirstOrDefault(d => d != null);
+            }
+
+            return decoder.CanDecode(objectType, targetType) ? decoder : null;
+        }
+    }
+}
diff --git a/src/runtime/Codecs/EncoderGroup.cs b/src/runtime/Codecs/EncoderGroup.cs
new file mode 100644
index 000000000..a5708c0bb
--- /dev/null
+++ b/src/runtime/Codecs/EncoderGroup.cs
@@ -0,0 +1,79 @@
+namespace Python.Runtime.Codecs
+{
+    using System;
+    using System.Collections;
+    using System.Collections.Generic;
+    using System.Linq;
+
+    /// <summary>
+    /// Represents a group of <see cref="IPyObjectDecoder"/>s. Useful to group them by priority.
+    /// </summary>
+    [Obsolete(Util.UnstableApiMessage)]
+    public sealed class EncoderGroup: IPyObjectEncoder, IEnumerable<IPyObjectEncoder>
+    {
+        readonly List<IPyObjectEncoder> encoders = new List<IPyObjectEncoder>();
+
+        /// <summary>
+        /// Add specified encoder to the group
+        /// </summary>
+        public void Add(IPyObjectEncoder item)
+        {
+            if (item is null) throw new ArgumentNullException(nameof(item));
+            this.encoders.Add(item);
+        }
+        /// <summary>
+        /// Remove all encoders from the group
+        /// </summary>
+        public void Clear() => this.encoders.Clear();
+
+        /// <inheritdoc />
+        public bool CanEncode(Type type) => this.encoders.Any(encoder => encoder.CanEncode(type));
+        /// <inheritdoc />
+        public PyObject TryEncode(object value)
+        {
+            if (value is null) throw new ArgumentNullException(nameof(value));
+
+            foreach (var encoder in this.GetEncoders(value.GetType()))
+            {
+                var result = encoder.TryEncode(value);
+                if (result != null)
+                {
+                    return result;
+                }
+            }
+
+            return null;
+        }
+
+        /// <inheritdoc />
+        public IEnumerator<IPyObjectEncoder> GetEnumerator() => this.encoders.GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => this.encoders.GetEnumerator();
+    }
+
+    [Obsolete(Util.UnstableApiMessage)]
+    public static class EncoderGroupExtensions
+    {
+        /// <summary>
+        /// Gets specific instances of <see cref="IPyObjectEncoder"/>
+        /// (potentially selecting one from a collection),
+        /// that can encode the specified <paramref name="type"/>.
+        /// </summary>
+        [Obsolete(Util.UnstableApiMessage)]
+        public static IEnumerable<IPyObjectEncoder> GetEncoders(this IPyObjectEncoder decoder, Type type)
+        {
+            if (decoder is null) throw new ArgumentNullException(nameof(decoder));
+
+            if (decoder is IEnumerable<IPyObjectEncoder> composite)
+            {
+                foreach (var nestedEncoder in composite)
+                foreach (var match in nestedEncoder.GetEncoders(type))
+                {
+                    yield return match;
+                }
+            } else if (decoder.CanEncode(type))
+            {
+                yield return decoder;
+            }
+        }
+    }
+}
diff --git a/src/runtime/Codecs/RawProxyEncoder.cs b/src/runtime/Codecs/RawProxyEncoder.cs
new file mode 100644
index 000000000..a1b6c52b3
--- /dev/null
+++ b/src/runtime/Codecs/RawProxyEncoder.cs
@@ -0,0 +1,21 @@
+using System;
+
+namespace Python.Runtime.Codecs
+{
+    /// <summary>
+    /// A .NET object encoder, that returns raw proxies (e.g. no conversion to Python types).
+    /// <para>You must inherit from this class and override <see cref="CanEncode"/>.</para>
+    /// </summary>
+    [Obsolete(Util.UnstableApiMessage)]
+    public class RawProxyEncoder: IPyObjectEncoder
+    {
+        public PyObject TryEncode(object value)
+        {
+            if (value is null) throw new ArgumentNullException(nameof(value));
+
+            return PyObject.FromManagedObject(value);
+        }
+
+        public virtual bool CanEncode(Type type) => false;
+    }
+}
diff --git a/src/runtime/Codecs/TupleCodecs.cs b/src/runtime/Codecs/TupleCodecs.cs
new file mode 100644
index 000000000..a9ae33fe0
--- /dev/null
+++ b/src/runtime/Codecs/TupleCodecs.cs
@@ -0,0 +1,135 @@
+namespace Python.Runtime.Codecs
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Reflection;
+
+    [Obsolete(Util.UnstableApiMessage)]
+    public sealed class TupleCodec<TTuple> : IPyObjectEncoder, IPyObjectDecoder
+    {
+        TupleCodec() { }
+        public static TupleCodec<TTuple> Instance { get; } = new TupleCodec<TTuple>();
+
+        public bool CanEncode(Type type)
+        {
+            if (type == typeof(object) || type == typeof(TTuple)) return true;
+            return type.Namespace == typeof(TTuple).Namespace
+                   // generic versions of tuples are named Tuple`TYPE_ARG_COUNT
+                   && type.Name.StartsWith(typeof(TTuple).Name + '`');
+        }
+
+        public PyObject TryEncode(object value)
+        {
+            if (value == null) return null;
+
+            var tupleType = value.GetType();
+            if (tupleType == typeof(object)) return null;
+            if (!this.CanEncode(tupleType)) return null;
+            if (tupleType == typeof(TTuple)) return new PyTuple();
+
+            long fieldCount = tupleType.GetGenericArguments().Length;
+            var tuple = Runtime.PyTuple_New(fieldCount);
+            Exceptions.ErrorCheck(tuple);
+            int fieldIndex = 0;
+            foreach (FieldInfo field in tupleType.GetFields())
+            {
+                var item = field.GetValue(value);
+                IntPtr pyItem = Converter.ToPython(item);
+                Runtime.PyTuple_SetItem(tuple, fieldIndex, pyItem);
+                fieldIndex++;
+            }
+            return new PyTuple(tuple);
+        }
+
+        public bool CanDecode(PyObject objectType, Type targetType)
+            => objectType.Handle == Runtime.PyTupleType && this.CanEncode(targetType);
+
+        public bool TryDecode<T>(PyObject pyObj, out T value)
+        {
+            if (pyObj == null) throw new ArgumentNullException(nameof(pyObj));
+
+            value = default;
+
+            if (!Runtime.PyTuple_Check(pyObj.Handle)) return false;
+
+            if (typeof(T) == typeof(object))
+            {
+                bool converted = Decode(pyObj, out object result);
+                if (converted)
+                {
+                    value = (T)result;
+                    return true;
+                }
+
+                return false;
+            }
+
+            var itemTypes = typeof(T).GetGenericArguments();
+            long itemCount = Runtime.PyTuple_Size(pyObj.Handle);
+            if (itemTypes.Length != itemCount) return false;
+
+            if (itemCount == 0)
+            {
+                value = (T)EmptyTuple;
+                return true;
+            }
+
+            var elements = new object[itemCount];
+            for (int itemIndex = 0; itemIndex < itemTypes.Length; itemIndex++)
+            {
+                IntPtr pyItem = Runtime.PyTuple_GetItem(pyObj.Handle, itemIndex);
+                if (!Converter.ToManaged(pyItem, itemTypes[itemIndex], out elements[itemIndex], setError: false))
+                {
+                    Exceptions.Clear();
+                    return false;
+                }
+            }
+            var factory = tupleCreate[itemCount].MakeGenericMethod(itemTypes);
+            value = (T)factory.Invoke(null, elements);
+            return true;
+        }
+
+        static bool Decode(PyObject tuple, out object value)
+        {
+            long itemCount = Runtime.PyTuple_Size(tuple.Handle);
+            if (itemCount == 0)
+            {
+                value = EmptyTuple;
+                return true;
+            }
+            var elements = new object[itemCount];
+            var itemTypes = new Type[itemCount];
+            value = null;
+            for (int itemIndex = 0; itemIndex < elements.Length; itemIndex++)
+            {
+                var pyItem = Runtime.PyTuple_GetItem(tuple.Handle, itemIndex);
+                if (!Converter.ToManaged(pyItem, typeof(object), out elements[itemIndex], setError: false))
+                {
+                    Exceptions.Clear();
+                    return false;
+                }
+
+                itemTypes[itemIndex] = elements[itemIndex]?.GetType() ?? typeof(object);
+            }
+
+            var factory = tupleCreate[itemCount].MakeGenericMethod(itemTypes);
+            value = factory.Invoke(null, elements);
+            return true;
+        }
+
+        static readonly MethodInfo[] tupleCreate =
+            typeof(TTuple).GetMethods(BindingFlags.Public | BindingFlags.Static)
+                .Where(m => m.Name == nameof(Tuple.Create))
+                .OrderBy(m => m.GetParameters().Length)
+                .ToArray();
+
+        static readonly object EmptyTuple = tupleCreate[0].Invoke(null, parameters: new object[0]);
+
+        public static void Register()
+        {
+            PyObjectConversions.RegisterEncoder(Instance);
+            PyObjectConversions.RegisterDecoder(Instance);
+        }
+    }
+}
diff --git a/src/runtime/NewReference.cs b/src/runtime/NewReference.cs
new file mode 100644
index 000000000..6e66232d0
--- /dev/null
+++ b/src/runtime/NewReference.cs
@@ -0,0 +1,70 @@
+namespace Python.Runtime
+{
+    using System;
+    using System.Diagnostics.Contracts;
+
+    /// <summary>
+    /// Represents a reference to a Python object, that is tracked by Python's reference counting.
+    /// </summary>
+    [NonCopyable]
+    ref struct NewReference
+    {
+        IntPtr pointer;
+
+        [Pure]
+        public static implicit operator BorrowedReference(in NewReference reference)
+            => new BorrowedReference(reference.pointer);
+
+        /// <summary>
+        /// Returns <see cref="PyObject"/> wrapper around this reference, which now owns
+        /// the pointer. Sets the original reference to <c>null</c>, as it no longer owns it.
+        /// </summary>
+        public PyObject MoveToPyObject()
+        {
+            if (this.IsNull()) throw new NullReferenceException();
+
+            var result = new PyObject(this.pointer);
+            this.pointer = IntPtr.Zero;
+            return result;
+        }
+        /// <summary>
+        /// Removes this reference to a Python object, and sets it to <c>null</c>.
+        /// </summary>
+        public void Dispose()
+        {
+            if (!this.IsNull())
+                Runtime.XDecref(this.pointer);
+            this.pointer = IntPtr.Zero;
+        }
+
+        /// <summary>
+        /// Creates <see cref="NewReference"/> from a raw pointer
+        /// </summary>
+        [Pure]
+        public static NewReference DangerousFromPointer(IntPtr pointer)
+            => new NewReference {pointer = pointer};
+
+        [Pure]
+        internal static IntPtr DangerousGetAddress(in NewReference reference)
+            => IsNull(reference) ? throw new NullReferenceException() : reference.pointer;
+        [Pure]
+        internal static bool IsNull(in NewReference reference)
+            => reference.pointer == IntPtr.Zero;
+    }
+
+    /// <summary>
+    /// These members can not be directly in <see cref="NewReference"/> type,
+    /// because <c>this</c> is always passed by value, which we need to avoid.
+    /// (note <code>this in NewReference</code> vs the usual <code>this NewReference</code>)
+    /// </summary>
+    static class NewReferenceExtensions
+    {
+        /// <summary>Gets a raw pointer to the Python object</summary>
+        [Pure]
+        public static IntPtr DangerousGetAddress(this in NewReference reference)
+            => NewReference.DangerousGetAddress(reference);
+        [Pure]
+        public static bool IsNull(this in NewReference reference)
+            => NewReference.IsNull(reference);
+    }
+}
diff --git a/src/runtime/NonCopyableAttribute.cs b/src/runtime/NonCopyableAttribute.cs
new file mode 100644
index 000000000..63d36ab42
--- /dev/null
+++ b/src/runtime/NonCopyableAttribute.cs
@@ -0,0 +1,6 @@
+namespace Python.Runtime
+{
+    using  System;
+    [AttributeUsage(AttributeTargets.Struct)]
+    class NonCopyableAttribute : Attribute { }
+}
diff --git a/src/runtime/Properties/AssemblyInfo.cs b/src/runtime/Properties/AssemblyInfo.cs
index a5d33c7ab..cab9df30b 100644
--- a/src/runtime/Properties/AssemblyInfo.cs
+++ b/src/runtime/Properties/AssemblyInfo.cs
@@ -4,7 +4,7 @@
 // General Information about an assembly is controlled through the following
 // set of attributes. Change these attribute values to modify the information
 // associated with an assembly.
-[assembly: AssemblyTitle("Python for .NET")]
+[assembly: AssemblyTitle("Python.NET")]
 [assembly: AssemblyDescription("")]
 [assembly: AssemblyDefaultAlias("Python.Runtime.dll")]
 
diff --git a/src/runtime/PyExportAttribute.cs b/src/runtime/PyExportAttribute.cs
new file mode 100644
index 000000000..52a8be15d
--- /dev/null
+++ b/src/runtime/PyExportAttribute.cs
@@ -0,0 +1,16 @@
+namespace Python.Runtime {
+    using System;
+
+    /// <summary>
+    /// Controls visibility to Python for public .NET type or an entire assembly
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Enum
+                    | AttributeTargets.Interface | AttributeTargets.Struct | AttributeTargets.Assembly,
+        AllowMultiple = false,
+        Inherited = false)]
+    public class PyExportAttribute : Attribute
+    {
+        internal readonly bool Export;
+        public PyExportAttribute(bool export) { this.Export = export; }
+    }
+}
diff --git a/src/runtime/Python.Runtime.15.csproj b/src/runtime/Python.Runtime.15.csproj
index 29177b78c..d753a5dff 100644
--- a/src/runtime/Python.Runtime.15.csproj
+++ b/src/runtime/Python.Runtime.15.csproj
@@ -7,14 +7,21 @@
     <AssetTargetFallback Condition="'$(TargetFramework)'=='net40' AND $(Configuration.Contains('Mono'))">net45</AssetTargetFallback>
     <RootNamespace>Python.Runtime</RootNamespace>
     <AssemblyName>Python.Runtime</AssemblyName>
-    <PackageId>Python.Runtime</PackageId>
-    <VersionPrefix>2.4.0</VersionPrefix>
-    <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
-    <GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
-    <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
-    <GenerateAssemblyProductAttribute>false</GenerateAssemblyProductAttribute>
-    <GenerateAssemblyTitleAttribute>false</GenerateAssemblyTitleAttribute>
-    <GenerateAssemblyDescriptionAttribute>false</GenerateAssemblyDescriptionAttribute>
+    <PackageId>pythonnet</PackageId>
+    <VersionPrefix>2.5.0</VersionPrefix>
+    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+    <Title>Python.NET</Title>
+    <Copyright>Copyright (c) 2006-2020 the contributors of the Python.NET project</Copyright>
+    <Description>Python and CLR (.NET and Mono) cross-platform language interop</Description>
+    <Authors>pythonnet</Authors>
+    <PackageLicenseUrl>https://github.com/pythonnet/pythonnet/blob/master/LICENSE</PackageLicenseUrl>
+    <RepositoryUrl>https://github.com/pythonnet/pythonnet</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+    <!--<PackageReleaseNotes>https://github.com/pythonnet/pythonnet/releases/tag/v2.4.0</PackageReleaseNotes>-->
+    <PackageTags>python interop dynamic dlr Mono pinvoke</PackageTags>
+    <PackageIconUrl>https://raw.githubusercontent.com/pythonnet/pythonnet/master/src/console/python-clear.ico</PackageIconUrl>
+    <PackageProjectUrl>https://pythonnet.github.io/</PackageProjectUrl>
     <OutputPath>bin\</OutputPath>
     <AppendTargetFrameworkToOutputPath Condition="'$(TargetFramework)'=='net40'">false</AppendTargetFrameworkToOutputPath>
     <DocumentationFile Condition="'$(TargetFramework)'=='net40'">$(OutputPath)\$(AssemblyName).xml</DocumentationFile>
@@ -23,7 +30,7 @@
     <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
     <PythonBuildDir Condition="'$(PythonBuildDir)' == ''">$(SolutionDir)\bin\</PythonBuildDir>
     <PublishDir Condition="'$(TargetFramework)'!='net40'">$(PythonBuildDir)\$(TargetFramework)\</PublishDir>
-    <LangVersion>6</LangVersion>
+    <LangVersion>7.3</LangVersion>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
     <AssemblyOriginatorKeyFile>..\pythonnet.snk</AssemblyOriginatorKeyFile>
     <CustomDefineConstants Condition="'$(CustomDefineConstants)' == ''">$(PYTHONNET_DEFINE_CONSTANTS)</CustomDefineConstants>
@@ -35,7 +42,7 @@
     <Python2Version>$(PYTHONNET_PY2_VERSION)</Python2Version>
     <Python2Version Condition="'$(Python2Version)'==''">PYTHON27</Python2Version>
     <Python3Version>$(PYTHONNET_PY3_VERSION)</Python3Version>
-    <Python3Version Condition="'$(Python3Version)'==''">PYTHON37</Python3Version>
+    <Python3Version Condition="'$(Python3Version)'==''">PYTHON38</Python3Version>
     <PythonWinDefineConstants>$(PYTHONNET_WIN_DEFINE_CONSTANTS)</PythonWinDefineConstants>
     <PythonWinDefineConstants Condition="'$(PythonWinDefineConstants)'==''">UCS2</PythonWinDefineConstants>
     <PythonMonoDefineConstants>$(PYTHONNET_MONO_DEFINE_CONSTANTS)</PythonMonoDefineConstants>
@@ -122,6 +129,13 @@
     <PackageReference Include="Microsoft.TargetingPack.NETFramework.v4.5" Version="1.0.1" ExcludeAssets="All" PrivateAssets="All" />
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageReference Include="NonCopyableAnalyzer" Version="0.6.0">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
   <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
 
   <PropertyGroup>
diff --git a/src/runtime/Python.Runtime.csproj b/src/runtime/Python.Runtime.csproj
index 19f776c77..75f5e2fab 100644
--- a/src/runtime/Python.Runtime.csproj
+++ b/src/runtime/Python.Runtime.csproj
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
   <PropertyGroup>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
@@ -15,18 +15,18 @@
     <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
     <PythonBuildDir Condition=" '$(PythonBuildDir)' == '' ">$(SolutionDir)\bin\</PythonBuildDir>
     <AppDesignerFolder>Properties</AppDesignerFolder>
-    <LangVersion>6</LangVersion>
+    <LangVersion>7.3</LangVersion>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <SignAssembly>false</SignAssembly>
     <AssemblyOriginatorKeyFile>..\pythonnet.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <!--We can relax binding to platform because code references no any platform dependent assemblies-->
   <!--This will allows to use any build of this assebly as a compile ref assebly-->
-  <!--<PropertyGroup Condition=" '$(Platform)' == 'x86'">
-    <PlatformTarget>x86</PlatformTarget>
-  </PropertyGroup>
-  <PropertyGroup Condition=" '$(Platform)' == 'x64'">
-    <PlatformTarget>x64</PlatformTarget>
+  <!--<PropertyGroup Condition=" '$(Platform)' == 'x86'">
+    <PlatformTarget>x86</PlatformTarget>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Platform)' == 'x64'">
+    <PlatformTarget>x64</PlatformTarget>
   </PropertyGroup>-->
   <PropertyGroup Condition=" '$(Configuration)' == 'ReleaseMono'">
     <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON2;PYTHON27;UCS4</DefineConstants>
@@ -34,7 +34,7 @@
     <DebugType>pdbonly</DebugType>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'ReleaseMonoPY3'">
-    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON37;UCS4</DefineConstants>
+    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON38;UCS4</DefineConstants>
     <Optimize>true</Optimize>
     <DebugType>pdbonly</DebugType>
   </PropertyGroup>
@@ -46,7 +46,7 @@
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'DebugMonoPY3'">
     <DebugSymbols>true</DebugSymbols>
-    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON37;UCS4;TRACE;DEBUG</DefineConstants>
+    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON38;UCS4;TRACE;DEBUG</DefineConstants>
     <Optimize>false</Optimize>
     <DebugType>full</DebugType>
   </PropertyGroup>
@@ -56,7 +56,7 @@
     <DebugType>pdbonly</DebugType>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'ReleaseWinPY3'">
-    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON37;UCS2</DefineConstants>
+    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON38;UCS2</DefineConstants>
     <Optimize>true</Optimize>
     <DebugType>pdbonly</DebugType>
   </PropertyGroup>
@@ -68,7 +68,7 @@
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'DebugWinPY3'">
     <DebugSymbols>true</DebugSymbols>
-    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON37;UCS2;TRACE;DEBUG</DefineConstants>
+    <DefineConstants Condition="'$(DefineConstants)' == ''">PYTHON3;PYTHON38;UCS2;TRACE;DEBUG</DefineConstants>
     <Optimize>false</Optimize>
     <DebugType>full</DebugType>
   </PropertyGroup>
@@ -76,6 +76,11 @@
     <Reference Include="System" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="Codecs\EncoderGroup.cs" />
+    <Compile Include="Codecs\DecoderGroup.cs" />
+    <Compile Include="Codecs\RawProxyEncoder.cs" />
+    <Compile Include="Codecs\TupleCodecs.cs" />
+    <Compile Include="converterextensions.cs" />
     <Compile Include="finalizer.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="..\SharedAssemblyInfo.cs">
@@ -83,6 +88,7 @@
     </Compile>
     <Compile Include="arrayobject.cs" />
     <Compile Include="assemblymanager.cs" />
+    <Compile Include="BorrowedReference.cs" />
     <Compile Include="classderived.cs" />
     <Compile Include="classbase.cs" />
     <Compile Include="classmanager.cs" />
@@ -119,10 +125,13 @@
     <Compile Include="moduleobject.cs" />
     <Compile Include="modulepropertyobject.cs" />
     <Compile Include="nativecall.cs" />
+    <Compile Include="NewReference.cs" />
+    <Compile Include="NonCopyableAttribute.cs" />
     <Compile Include="overload.cs" />
     <Compile Include="propertyobject.cs" />
     <Compile Include="pyansistring.cs" />
     <Compile Include="pydict.cs" />
+    <Compile Include="PyExportAttribute.cs" />
     <Compile Include="pyfloat.cs" />
     <Compile Include="pyint.cs" />
     <Compile Include="pyiter.cs" />
@@ -136,10 +145,15 @@
     <Compile Include="pythonengine.cs" />
     <Compile Include="pythonexception.cs" />
     <Compile Include="pytuple.cs" />
+    <Compile Include="ReferenceExtensions.cs" />
     <Compile Include="runtime.cs" />
     <Compile Include="typemanager.cs" />
     <Compile Include="typemethod.cs" />
     <Compile Include="Util.cs" />
+    <Compile Include="platform\Types.cs" />
+    <Compile Include="platform\LibraryLoader.cs" />
+    <Compile Include="polyfill\ReflectionPolifills.cs" />
+    <Compile Include="slots\mp_length.cs" />
   </ItemGroup>
   <ItemGroup Condition=" '$(PythonInteropFile)' != '' ">
     <Compile Include="$(PythonInteropFile)" />
@@ -149,7 +163,8 @@
     <Compile Include="interop34.cs" />
     <Compile Include="interop35.cs" />
     <Compile Include="interop36.cs" />
-    <Compile Include="interop37.cs" />
+    <Compile Include="interop37.cs" />
+    <Compile Include="interop38.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="..\pythonnet.snk" />
@@ -168,4 +183,4 @@
     <Copy SourceFiles="$(TargetAssembly)" DestinationFolder="$(PythonBuildDir)" />
     <!--Copy SourceFiles="$(TargetAssemblyPdb)" Condition="Exists('$(TargetAssemblyPdb)')" DestinationFolder="$(PythonBuildDir)" /-->
   </Target>
-</Project>
+</Project>
\ No newline at end of file
diff --git a/src/runtime/ReferenceExtensions.cs b/src/runtime/ReferenceExtensions.cs
new file mode 100644
index 000000000..8fa2731b7
--- /dev/null
+++ b/src/runtime/ReferenceExtensions.cs
@@ -0,0 +1,20 @@
+namespace Python.Runtime
+{
+    using System.Diagnostics.Contracts;
+
+    static class ReferenceExtensions
+    {
+        /// <summary>
+        /// Checks if the reference points to Python object <c>None</c>.
+        /// </summary>
+        [Pure]
+        public static bool IsNone(this in NewReference reference)
+            => reference.DangerousGetAddress() == Runtime.PyNone;
+        /// <summary>
+        /// Checks if the reference points to Python object <c>None</c>.
+        /// </summary>
+        [Pure]
+        public static bool IsNone(this BorrowedReference reference)
+            => reference.DangerousGetAddress() == Runtime.PyNone;
+    }
+}
diff --git a/src/runtime/Util.cs b/src/runtime/Util.cs
index dc5f78608..eb21cddbb 100644
--- a/src/runtime/Util.cs
+++ b/src/runtime/Util.cs
@@ -3,8 +3,11 @@
 
 namespace Python.Runtime
 {
-    internal class Util
+    internal static class Util
     {
+        internal const string UnstableApiMessage =
+            "This API is unstable, and might be changed or removed in the next minor release";
+
         internal static Int64 ReadCLong(IntPtr tp, int offset)
         {
             // On Windows, a C long is always 32 bits.
@@ -29,5 +32,12 @@ internal static void WriteCLong(IntPtr type, int offset, Int64 flags)
                 Marshal.WriteInt64(type, offset, flags);
             }
         }
+
+        /// <summary>
+        /// Null-coalesce: if <paramref name="primary"/> parameter is not
+        /// <see cref="IntPtr.Zero"/>, return it. Otherwise return <paramref name="fallback"/>.
+        /// </summary>
+        internal static IntPtr Coalesce(this IntPtr primary, IntPtr fallback)
+            => primary == IntPtr.Zero ? fallback : primary;
     }
-}
\ No newline at end of file
+}
diff --git a/src/runtime/arrayobject.cs b/src/runtime/arrayobject.cs
index c37295704..1ef318473 100644
--- a/src/runtime/arrayobject.cs
+++ b/src/runtime/arrayobject.cs
@@ -244,16 +244,5 @@ public static int sq_contains(IntPtr ob, IntPtr v)
 
             return 0;
         }
-
-
-        /// <summary>
-        /// Implements __len__ for array types.
-        /// </summary>
-        public static int mp_length(IntPtr ob)
-        {
-            var self = (CLRObject)GetManagedObject(ob);
-            var items = self.inst as Array;
-            return items.Length;
-        }
     }
 }
diff --git a/src/runtime/assemblymanager.cs b/src/runtime/assemblymanager.cs
index 3085bb639..46909a370 100644
--- a/src/runtime/assemblymanager.cs
+++ b/src/runtime/assemblymanager.cs
@@ -145,7 +145,7 @@ internal static void UpdatePath()
                 probed.Clear();
                 for (var i = 0; i < count; i++)
                 {
-                    IntPtr item = Runtime.PyList_GetItem(list, i);
+                    BorrowedReference item = Runtime.PyList_GetItem(list, i);
                     string path = Runtime.GetManagedString(item);
                     if (path != null)
                     {
@@ -346,6 +346,10 @@ public static bool LoadImplicit(string name, bool warn = true)
         /// </summary>
         internal static void ScanAssembly(Assembly assembly)
         {
+            if (assembly.GetCustomAttribute<PyExportAttribute>()?.Export == false)
+            {
+                return;
+            }
             // A couple of things we want to do here: first, we want to
             // gather a list of all of the namespaces contributed to by
             // the assembly.
@@ -452,12 +456,13 @@ public static List<string> GetNames(string nsname)
         /// looking in the currently loaded assemblies for the named
         /// type. Returns null if the named type cannot be found.
         /// </summary>
+        [Obsolete("Use LookupTypes and handle name conflicts")]
         public static Type LookupType(string qname)
         {
             foreach (Assembly assembly in assemblies)
             {
                 Type type = assembly.GetType(qname);
-                if (type != null)
+                if (type != null && IsExported(type))
                 {
                     return type;
                 }
@@ -465,25 +470,33 @@ public static Type LookupType(string qname)
             return null;
         }
 
+        /// <summary>
+        /// Returns the <see cref="Type"/> objects for the given qualified name,
+        /// looking in the currently loaded assemblies for the named
+        /// type.
+        /// </summary>
+        public static IEnumerable<Type> LookupTypes(string qualifiedName)
+            => assemblies.Select(assembly => assembly.GetType(qualifiedName)).Where(type => type != null && IsExported(type));
+
         internal static Type[] GetTypes(Assembly a)
         {
             if (a.IsDynamic)
             {
                 try
                 {
-                    return a.GetTypes();
+                    return a.GetTypes().Where(IsExported).ToArray();
                 }
                 catch (ReflectionTypeLoadException exc)
                 {
                     // Return all types that were successfully loaded
-                    return exc.Types.Where(x => x != null).ToArray();
+                    return exc.Types.Where(x => x != null && IsExported(x)).ToArray();
                 }
             }
             else
             {
                 try
                 {
-                    return a.GetExportedTypes();
+                    return a.GetExportedTypes().Where(IsExported).ToArray();
                 }
                 catch (FileNotFoundException)
                 {
@@ -491,5 +504,7 @@ internal static Type[] GetTypes(Assembly a)
                 }
             }
         }
+
+        static bool IsExported(Type type) => type.GetCustomAttribute<PyExportAttribute>()?.Export != false;
     }
-}
\ No newline at end of file
+}
diff --git a/src/runtime/classbase.cs b/src/runtime/classbase.cs
index 5846fa82a..144bac9d3 100644
--- a/src/runtime/classbase.cs
+++ b/src/runtime/classbase.cs
@@ -246,6 +246,44 @@ public static IntPtr tp_str(IntPtr ob)
             }
         }
 
+        public static IntPtr tp_repr(IntPtr ob)
+        {
+            var co = GetManagedObject(ob) as CLRObject;
+            if (co == null)
+            {
+                return Exceptions.RaiseTypeError("invalid object");
+            }
+            try
+            {
+                //if __repr__ is defined, use it
+                var instType = co.inst.GetType();
+                System.Reflection.MethodInfo methodInfo = instType.GetMethod("__repr__");
+                if (methodInfo != null && methodInfo.IsPublic)
+                {
+                    var reprString = methodInfo.Invoke(co.inst, null) as string;
+                    return Runtime.PyString_FromString(reprString);
+                }
+
+                //otherwise use the standard object.__repr__(inst)
+                IntPtr args = Runtime.PyTuple_New(1);
+                Runtime.PyTuple_SetItem(args, 0, ob);
+                IntPtr reprFunc = Runtime.PyObject_GetAttrString(Runtime.PyBaseObjectType, "__repr__");
+                var output =  Runtime.PyObject_Call(reprFunc, args, IntPtr.Zero);
+                Runtime.XDecref(args);
+                Runtime.XDecref(reprFunc);
+                return output;
+            }
+            catch (Exception e)
+            {
+                if (e.InnerException != null)
+                {
+                    e = e.InnerException;
+                }
+                Exceptions.SetError(e);
+                return IntPtr.Zero;
+            }
+        }
+
 
         /// <summary>
         /// Standard dealloc implementation for instances of reflected types.
@@ -253,7 +291,7 @@ public static IntPtr tp_str(IntPtr ob)
         public static void tp_dealloc(IntPtr ob)
         {
             ManagedType self = GetManagedObject(ob);
-            IntPtr dict = Marshal.ReadIntPtr(ob, ObjectOffset.DictOffset(ob));
+            IntPtr dict = Marshal.ReadIntPtr(ob, ObjectOffset.TypeDictOffset(self.tpHandle));
             if (dict != IntPtr.Zero)
             {
                 Runtime.XDecref(dict);
diff --git a/src/runtime/classderived.cs b/src/runtime/classderived.cs
index ec3734ea5..af16b1359 100644
--- a/src/runtime/classderived.cs
+++ b/src/runtime/classderived.cs
@@ -877,7 +877,7 @@ public static void Finalize(IPythonDerivedType obj)
                         // the C# object is being destroyed which must mean there are no more
                         // references to the Python object as well so now we can dealloc the
                         // python object.
-                        IntPtr dict = Marshal.ReadIntPtr(self.pyHandle, ObjectOffset.DictOffset(self.pyHandle));
+                        IntPtr dict = Marshal.ReadIntPtr(self.pyHandle, ObjectOffset.TypeDictOffset(self.tpHandle));
                         if (dict != IntPtr.Zero)
                         {
                             Runtime.XDecref(dict);
diff --git a/src/runtime/clrobject.cs b/src/runtime/clrobject.cs
index 502677655..5c7ad7891 100644
--- a/src/runtime/clrobject.cs
+++ b/src/runtime/clrobject.cs
@@ -14,11 +14,11 @@ internal CLRObject(object ob, IntPtr tp)
             long flags = Util.ReadCLong(tp, TypeOffset.tp_flags);
             if ((flags & TypeFlags.Subclass) != 0)
             {
-                IntPtr dict = Marshal.ReadIntPtr(py, ObjectOffset.DictOffset(tp));
+                IntPtr dict = Marshal.ReadIntPtr(py, ObjectOffset.TypeDictOffset(tp));
                 if (dict == IntPtr.Zero)
                 {
                     dict = Runtime.PyDict_New();
-                    Marshal.WriteIntPtr(py, ObjectOffset.DictOffset(tp), dict);
+                    Marshal.WriteIntPtr(py, ObjectOffset.TypeDictOffset(tp), dict);
                 }
             }
 
@@ -35,13 +35,13 @@ internal CLRObject(object ob, IntPtr tp)
         }
 
 
-        internal static CLRObject GetInstance(object ob, IntPtr pyType)
+        static CLRObject GetInstance(object ob, IntPtr pyType)
         {
             return new CLRObject(ob, pyType);
         }
 
 
-        internal static CLRObject GetInstance(object ob)
+        static CLRObject GetInstance(object ob)
         {
             ClassBase cc = ClassManager.GetClass(ob.GetType());
             return GetInstance(ob, cc.tpHandle);
diff --git a/src/runtime/constructorbinder.cs b/src/runtime/constructorbinder.cs
index 1fc541920..0f9806c0e 100644
--- a/src/runtime/constructorbinder.cs
+++ b/src/runtime/constructorbinder.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Reflection;
+using System.Text;
 
 namespace Python.Runtime
 {
@@ -93,7 +94,15 @@ internal object InvokeRaw(IntPtr inst, IntPtr args, IntPtr kw, MethodBase info)
 
                 if (binding == null)
                 {
-                    Exceptions.SetError(Exceptions.TypeError, "no constructor matches given arguments");
+                    var errorMessage = new StringBuilder("No constructor matches given arguments");
+                    if (info != null && info.IsConstructor && info.DeclaringType != null)
+                    {
+                        errorMessage.Append(" for ").Append(info.DeclaringType.Name);
+                    }
+
+                    errorMessage.Append(": ");
+                    AppendArgumentTypes(to: errorMessage, args);
+                    Exceptions.SetError(Exceptions.TypeError, errorMessage.ToString());
                     return null;
                 }
             }
diff --git a/src/runtime/converter.cs b/src/runtime/converter.cs
index 11c67bf82..3add8aba0 100644
--- a/src/runtime/converter.cs
+++ b/src/runtime/converter.cs
@@ -135,8 +135,17 @@ internal static IntPtr ToPython(object value, Type type)
                 return result;
             }
 
-			if (value is IList && !(value is INotifyPropertyChanged) && value.GetType().IsGenericType)
-			{
+            if (Type.GetTypeCode(type) == TypeCode.Object && value.GetType() != typeof(object)) {
+                var encoded = PyObjectConversions.TryEncode(value, type);
+                if (encoded != null) {
+                    result = encoded.Handle;
+                    Runtime.XIncref(result);
+                    return result;
+                }
+            }
+
+            if (value is IList && !(value is INotifyPropertyChanged) && value.GetType().IsGenericType)
+            {
                 using (var resultlist = new PyList())
                 {
                     foreach (object o in (IEnumerable)value)
@@ -377,17 +386,21 @@ internal static bool ToManagedValue(IntPtr value, Type obType,
                     return ToPrimitive(value, doubleType, out result, setError);
                 }
 
-                if (Runtime.PySequence_Check(value))
+                // give custom codecs a chance to take over conversion of sequences
+                IntPtr pyType = Runtime.PyObject_TYPE(value);
+                if (PyObjectConversions.TryDecode(value, pyType, obType, out result))
                 {
-                    return ToArray(value, typeof(object[]), out result, setError);
+                    return true;
                 }
 
-                if (setError)
+                if (Runtime.PySequence_Check(value))
                 {
-                    Exceptions.SetError(Exceptions.TypeError, "value cannot be converted to Object");
+                    return ToArray(value, typeof(object[]), out result, setError);
                 }
 
-                return false;
+                Runtime.XIncref(value); // PyObject() assumes ownership
+                result = new PyObject(value);
+                return true;
             }
 
             // Conversion to 'Type' is done using the same mappings as above for objects.
@@ -437,9 +450,21 @@ internal static bool ToManagedValue(IntPtr value, Type obType,
                 return false;
             }
 
+            TypeCode typeCode = Type.GetTypeCode(obType);
+            if (typeCode == TypeCode.Object)
+            {
+                IntPtr pyType = Runtime.PyObject_TYPE(value);
+                if (PyObjectConversions.TryDecode(value, pyType, obType, out result))
+                {
+                    return true;
+                }
+            }
+
             return ToPrimitive(value, obType, out result, setError);
         }
 
+        internal delegate bool TryConvertFromPythonDelegate(IntPtr pyObj, out object result);
+
         /// <summary>
         /// Convert a Python value to an instance of a primitive managed type.
         /// </summary>
@@ -728,7 +753,20 @@ private static bool ToPrimitive(IntPtr value, Type obType, out object result, bo
                         }
                         goto type_error;
                     }
-                    uint ui = (uint)Runtime.PyLong_AsUnsignedLong(op);
+                    
+                    uint ui;
+                    try 
+                    {
+                        ui = Convert.ToUInt32(Runtime.PyLong_AsUnsignedLong(op));
+                    } catch (OverflowException)
+                    {
+                        // Probably wasn't an overflow in python but was in C# (e.g. if cpython
+                        // longs are 64 bit then 0xFFFFFFFF + 1 will not overflow in 
+                        // PyLong_AsUnsignedLong)
+                        Runtime.XDecref(op);
+                        goto overflow;
+                    }
+                    
 
                     if (Exceptions.ErrorOccurred())
                     {
@@ -837,17 +875,20 @@ private static void SetConversionError(IntPtr value, Type target)
 
         /// <summary>
         /// Convert a Python value to a correctly typed managed array instance.
-        /// The Python value must support the Python sequence protocol and the
+        /// The Python value must support the Python iterator protocol or and the
         /// items in the sequence must be convertible to the target array type.
         /// </summary>
         private static bool ToArray(IntPtr value, Type obType, out object result, bool setError)
         {
             Type elementType = obType.GetElementType();
-            var size = Runtime.PySequence_Size(value);
             result = null;
 
-            if (size < 0)
-            {
+            bool IsSeqObj = Runtime.PySequence_Check(value);
+            var len = IsSeqObj ? Runtime.PySequence_Size(value) : -1;
+
+            IntPtr IterObject = Runtime.PyObject_GetIter(value);
+
+            if(IterObject==IntPtr.Zero) {
                 if (setError)
                 {
                     SetConversionError(value, obType);
@@ -855,21 +896,17 @@ private static bool ToArray(IntPtr value, Type obType, out object result, bool s
                 return false;
             }
 
-            Array items = Array.CreateInstance(elementType, size);
+            Array items;
+
+            var listType = typeof(List<>);
+            var constructedListType = listType.MakeGenericType(elementType);
+            IList list = IsSeqObj ? (IList) Activator.CreateInstance(constructedListType, new Object[] {(int) len}) : 
+                                        (IList) Activator.CreateInstance(constructedListType);
+            IntPtr item;
 
-            // XXX - is there a better way to unwrap this if it is a real array?
-            for (var i = 0; i < size; i++)
+            while ((item = Runtime.PyIter_Next(IterObject)) != IntPtr.Zero)
             {
                 object obj = null;
-                IntPtr item = Runtime.PySequence_GetItem(value, i);
-                if (item == IntPtr.Zero)
-                {
-                    if (setError)
-                    {
-                        SetConversionError(value, obType);
-                        return false;
-                    }
-                }
 
                 if (!Converter.ToManaged(item, elementType, out obj, true))
                 {
@@ -877,10 +914,14 @@ private static bool ToArray(IntPtr value, Type obType, out object result, bool s
                     return false;
                 }
 
-                items.SetValue(obj, i);
+                list.Add(obj);
                 Runtime.XDecref(item);
             }
+            Runtime.XDecref(IterObject);
 
+            items = Array.CreateInstance(elementType, list.Count);
+            list.CopyTo(items, 0);
+            
             result = items;
             return true;
         }
diff --git a/src/runtime/converterextensions.cs b/src/runtime/converterextensions.cs
new file mode 100644
index 000000000..667fc6f00
--- /dev/null
+++ b/src/runtime/converterextensions.cs
@@ -0,0 +1,192 @@
+namespace Python.Runtime
+{
+    using System;
+    using System.Collections.Concurrent;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Reflection;
+    using Python.Runtime.Codecs;
+
+    /// <summary>
+    /// Defines <see cref="PyObject"/> conversion to CLR types (unmarshalling)
+    /// </summary>
+    [Obsolete(Util.UnstableApiMessage)]
+    public interface IPyObjectDecoder
+    {
+        /// <summary>
+        /// Checks if this decoder can decode from <paramref name="objectType"/> to <paramref name="targetType"/>
+        /// </summary>
+        bool CanDecode(PyObject objectType, Type targetType);
+        /// <summary>
+        /// Attempts do decode <paramref name="pyObj"/> into a variable of specified type
+        /// </summary>
+        /// <typeparam name="T">CLR type to decode into</typeparam>
+        /// <param name="pyObj">Object to decode</param>
+        /// <param name="value">The variable, that will receive decoding result</param>
+        /// <returns></returns>
+        bool TryDecode<T>(PyObject pyObj, out T value);
+    }
+
+    /// <summary>
+    /// Defines conversion from CLR objects into Python objects (e.g. <see cref="PyObject"/>) (marshalling)
+    /// </summary>
+    [Obsolete(Util.UnstableApiMessage)]
+    public interface IPyObjectEncoder
+    {
+        /// <summary>
+        /// Checks if encoder can encode CLR objects of specified type
+        /// </summary>
+        bool CanEncode(Type type);
+        /// <summary>
+        /// Attempts to encode CLR object <paramref name="value"/> into Python object
+        /// </summary>
+        PyObject TryEncode(object value);
+    }
+
+    /// <summary>
+    /// This class allows to register additional marshalling codecs.
+    /// <para>Python.NET will pick suitable encoder/decoder registered first</para>
+    /// </summary>
+    [Obsolete(Util.UnstableApiMessage)]
+    public static class PyObjectConversions
+    {
+        static readonly DecoderGroup decoders = new DecoderGroup();
+        static readonly EncoderGroup encoders = new EncoderGroup();
+
+        /// <summary>
+        /// Registers specified encoder (marshaller)
+        /// <para>Python.NET will pick suitable encoder/decoder registered first</para>
+        /// </summary>
+        public static void RegisterEncoder(IPyObjectEncoder encoder)
+        {
+            if (encoder == null) throw new ArgumentNullException(nameof(encoder));
+
+            lock (encoders)
+            {
+                encoders.Add(encoder);
+            }
+        }
+
+        /// <summary>
+        /// Registers specified decoder (unmarshaller)
+        /// <para>Python.NET will pick suitable encoder/decoder registered first</para>
+        /// </summary>
+        public static void RegisterDecoder(IPyObjectDecoder decoder)
+        {
+            if (decoder == null) throw new ArgumentNullException(nameof(decoder));
+
+            lock (decoders)
+            {
+                decoders.Add(decoder);
+            }
+        }
+
+        #region Encoding
+        internal static PyObject TryEncode(object obj, Type type)
+        {
+            if (obj == null) throw new ArgumentNullException(nameof(obj));
+            if (type == null) throw new ArgumentNullException(nameof(type));
+
+            foreach (var encoder in clrToPython.GetOrAdd(type, GetEncoders))
+            {
+                var result = encoder.TryEncode(obj);
+                if (result != null) return result;
+            }
+
+            return null;
+        }
+
+        static readonly ConcurrentDictionary<Type, IPyObjectEncoder[]>
+            clrToPython = new ConcurrentDictionary<Type, IPyObjectEncoder[]>();
+        static IPyObjectEncoder[] GetEncoders(Type type)
+        {
+            lock (encoders)
+            {
+                return encoders.GetEncoders(type).ToArray();
+            }
+        }
+        #endregion
+
+        #region Decoding
+        static readonly ConcurrentDictionary<TypePair, Converter.TryConvertFromPythonDelegate>
+            pythonToClr = new ConcurrentDictionary<TypePair, Converter.TryConvertFromPythonDelegate>();
+        internal static bool TryDecode(IntPtr pyHandle, IntPtr pyType, Type targetType, out object result)
+        {
+            if (pyHandle == IntPtr.Zero) throw new ArgumentNullException(nameof(pyHandle));
+            if (pyType == IntPtr.Zero) throw new ArgumentNullException(nameof(pyType));
+            if (targetType == null) throw new ArgumentNullException(nameof(targetType));
+
+            var decoder = pythonToClr.GetOrAdd(new TypePair(pyType, targetType), pair => GetDecoder(pair.PyType, pair.ClrType));
+            result = null;
+            if (decoder == null) return false;
+            return decoder.Invoke(pyHandle, out result);
+        }
+
+        static Converter.TryConvertFromPythonDelegate GetDecoder(IntPtr sourceType, Type targetType)
+        {
+            IPyObjectDecoder decoder;
+            using (var pyType = new PyObject(Runtime.SelfIncRef(sourceType)))
+            {
+                lock (decoders)
+                {
+                    decoder = decoders.GetDecoder(pyType, targetType);
+                    if (decoder == null) return null;
+                }
+            }
+
+            var decode = genericDecode.MakeGenericMethod(targetType);
+
+            bool TryDecode(IntPtr pyHandle, out object result)
+            {
+                var pyObj = new PyObject(Runtime.SelfIncRef(pyHandle));
+                var @params = new object[] { pyObj, null };
+                bool success = (bool)decode.Invoke(decoder, @params);
+                if (!success)
+                {
+                    pyObj.Dispose();
+                }
+
+                result = @params[1];
+                return success;
+            }
+
+            return TryDecode;
+        }
+
+        static readonly MethodInfo genericDecode = typeof(IPyObjectDecoder).GetMethod(nameof(IPyObjectDecoder.TryDecode));
+
+        #endregion
+
+        internal static void Reset()
+        {
+            lock (encoders)
+                lock (decoders)
+                {
+                    clrToPython.Clear();
+                    pythonToClr.Clear();
+                    encoders.Clear();
+                    decoders.Clear();
+                }
+        }
+
+        struct TypePair : IEquatable<TypePair>
+        {
+            internal readonly IntPtr PyType;
+            internal readonly Type ClrType;
+
+            public TypePair(IntPtr pyType, Type clrType)
+            {
+                this.PyType = pyType;
+                this.ClrType = clrType;
+            }
+
+            public override int GetHashCode()
+                => this.ClrType.GetHashCode() ^ this.PyType.GetHashCode();
+
+            public bool Equals(TypePair other)
+                => this.PyType == other.PyType && this.ClrType == other.ClrType;
+
+            public override bool Equals(object obj) => obj is TypePair other && this.Equals(other);
+        }
+    }
+}
diff --git a/src/runtime/exceptions.cs b/src/runtime/exceptions.cs
index 8bed0abfd..31c367eb2 100644
--- a/src/runtime/exceptions.cs
+++ b/src/runtime/exceptions.cs
@@ -36,6 +36,29 @@ internal static Exception ToException(IntPtr ob)
             return e;
         }
 
+        /// <summary>
+        /// Exception __repr__ implementation
+        /// </summary>
+        public new static IntPtr tp_repr(IntPtr ob)
+        {
+            Exception e = ToException(ob);
+            if (e == null)
+            {
+                return Exceptions.RaiseTypeError("invalid object");
+            }
+            string name = e.GetType().Name;
+            string message;
+            if (e.Message != String.Empty)
+            {
+                message = String.Format("{0}('{1}')", name, e.Message);
+            }
+            else
+            {
+                message = String.Format("{0}()", name);
+            }
+            return Runtime.PyUnicode_FromString(message);
+        }
+
         /// <summary>
         /// Exception __str__ implementation
         /// </summary>
diff --git a/src/runtime/extensiontype.cs b/src/runtime/extensiontype.cs
index 693a46f42..6585180c1 100644
--- a/src/runtime/extensiontype.cs
+++ b/src/runtime/extensiontype.cs
@@ -38,6 +38,7 @@ public ExtensionType()
 
             Runtime.PyObject_GC_UnTrack(py);
 
+            // Steals a ref to tpHandle.
             tpHandle = tp;
             pyHandle = py;
             gcHandle = gc;
@@ -50,7 +51,7 @@ public ExtensionType()
         public static void FinalizeObject(ManagedType self)
         {
             Runtime.PyObject_GC_Del(self.pyHandle);
-            Runtime.XDecref(self.tpHandle);
+            // Not necessary for decref of `tpHandle`.
             self.gcHandle.Free();
         }
 
diff --git a/src/runtime/finalizer.cs b/src/runtime/finalizer.cs
index dd5c0b4dd..ba562cc26 100644
--- a/src/runtime/finalizer.cs
+++ b/src/runtime/finalizer.cs
@@ -28,9 +28,7 @@ public class ErrorArgs : EventArgs
         public bool Enable { get; set; }
 
         private ConcurrentQueue<IPyDisposable> _objQueue = new ConcurrentQueue<IPyDisposable>();
-        private bool _pending = false;
-        private readonly object _collectingLock = new object();
-        private Task _finalizerTask;
+        private int _throttled;
 
         #region FINALIZER_CHECK
 
@@ -75,19 +73,16 @@ private Finalizer()
             Threshold = 200;
         }
 
-        public void Collect(bool forceDispose = true)
+        [Obsolete("forceDispose parameter is unused. All objects are disposed regardless.")]
+        public void Collect(bool forceDispose) => this.DisposeAll();
+        public void Collect() => this.DisposeAll();
+
+        internal void ThrottledCollect()
         {
-            if (Instance._finalizerTask != null
-                && !Instance._finalizerTask.IsCompleted)
-            {
-                var ts = PythonEngine.BeginAllowThreads();
-                Instance._finalizerTask.Wait();
-                PythonEngine.EndAllowThreads(ts);
-            }
-            else if (forceDispose)
-            {
-                Instance.DisposeAll();
-            }
+            _throttled = unchecked(this._throttled + 1);
+            if (!Enable || _throttled < Threshold) return;
+            _throttled = 0;
+            this.Collect();
         }
 
         public List<WeakReference> GetCollectedObjects()
@@ -101,62 +96,18 @@ internal void AddFinalizedObject(IPyDisposable obj)
             {
                 return;
             }
-            if (Runtime.Py_IsInitialized() == 0)
-            {
-                // XXX: Memory will leak if a PyObject finalized after Python shutdown,
-                // for avoiding that case, user should call GC.Collect manual before shutdown.
-                return;
-            }
+
 #if FINALIZER_CHECK
             lock (_queueLock)
 #endif
             {
-                _objQueue.Enqueue(obj);
-            }
-            GC.ReRegisterForFinalize(obj);
-            if (!_pending && _objQueue.Count >= Threshold)
-            {
-                AddPendingCollect();
+                this._objQueue.Enqueue(obj);
             }
         }
 
         internal static void Shutdown()
         {
-            if (Runtime.Py_IsInitialized() == 0)
-            {
-                Instance._objQueue = new ConcurrentQueue<IPyDisposable>();
-                return;
-            }
-            Instance.Collect(forceDispose: true);
-        }
-
-        private void AddPendingCollect()
-        {
-            if(Monitor.TryEnter(_collectingLock))
-            {
-                try
-                {
-                    if (!_pending)
-                    {
-                        _pending = true;
-                        // should already be complete but just in case
-                        _finalizerTask?.Wait();
-
-                        _finalizerTask = Task.Factory.StartNew(() =>
-                        {
-                            using (Py.GIL())
-                            {
-                                Instance.DisposeAll();
-                                _pending = false;
-                            }
-                        });
-                    }
-                }
-                finally
-                {
-                    Monitor.Exit(_collectingLock);
-                }
-            }
+            Instance.DisposeAll();
         }
 
         private void DisposeAll()
@@ -178,12 +129,18 @@ private void DisposeAll()
                     try
                     {
                         obj.Dispose();
-                        Runtime.CheckExceptionOccurred();
                     }
                     catch (Exception e)
                     {
-                        // We should not bother the main thread
-                        ErrorHandler?.Invoke(this, new ErrorArgs()
+                        var handler = ErrorHandler;
+                        if (handler is null)
+                        {
+                            throw new FinalizationException(
+                                "Python object finalization failed",
+                                disposable: obj, innerException: e);
+                        }
+
+                        handler.Invoke(this, new ErrorArgs()
                         {
                             Error = e
                         });
@@ -267,4 +224,15 @@ private void ValidateRefCount()
         }
 #endif
     }
+
+    public class FinalizationException : Exception
+    {
+        public IPyDisposable Disposable { get; }
+
+        public FinalizationException(string message, IPyDisposable disposable, Exception innerException)
+            : base(message, innerException)
+        {
+            this.Disposable = disposable ?? throw new ArgumentNullException(nameof(disposable));
+        }
+    }
 }
diff --git a/src/runtime/importhook.cs b/src/runtime/importhook.cs
index 7e4a208f5..aa3bbab6d 100644
--- a/src/runtime/importhook.cs
+++ b/src/runtime/importhook.cs
@@ -23,28 +23,63 @@ internal static void InitializeModuleDef()
                 module_def = ModuleDefOffset.AllocModuleDef("clr");
             }
         }
+
+        internal static void ReleaseModuleDef()
+        {
+            if (module_def == IntPtr.Zero)
+            {
+                return;
+            }
+            ModuleDefOffset.FreeModuleDef(module_def);
+            module_def = IntPtr.Zero;
+        }
 #endif
 
         /// <summary>
-        /// Initialization performed on startup of the Python runtime.
+        /// Initialize just the __import__ hook itself.
         /// </summary>
-        internal static void Initialize()
+        static void InitImport()
         {
-            // Initialize the Python <--> CLR module hook. We replace the
-            // built-in Python __import__ with our own. This isn't ideal,
-            // but it provides the most "Pythonic" way of dealing with CLR
-            // modules (Python doesn't provide a way to emulate packages).
-            IntPtr dict = Runtime.PyImport_GetModuleDict();
+            // We replace the built-in Python __import__ with our own: first
+            // look in CLR modules, then if we don't find any call the default
+            // Python __import__.
+            IntPtr builtins = Runtime.GetBuiltins();
+            py_import = Runtime.PyObject_GetAttrString(builtins, "__import__");
+            PythonException.ThrowIfIsNull(py_import);
 
-            IntPtr mod = Runtime.IsPython3
-                ? Runtime.PyImport_ImportModule("builtins")
-                : Runtime.PyDict_GetItemString(dict, "__builtin__");
-
-            py_import = Runtime.PyObject_GetAttrString(mod, "__import__");
             hook = new MethodWrapper(typeof(ImportHook), "__import__", "TernaryFunc");
-            Runtime.PyObject_SetAttrString(mod, "__import__", hook.ptr);
-            Runtime.XDecref(hook.ptr);
+            int res = Runtime.PyObject_SetAttrString(builtins, "__import__", hook.ptr);
+            PythonException.ThrowIfIsNotZero(res);
+
+            Runtime.XDecref(builtins);
+        }
+
+        /// <summary>
+        /// Restore the __import__ hook.
+        /// </summary>
+        static void RestoreImport()
+        {
+            IntPtr builtins = Runtime.GetBuiltins();
+
+            int res = Runtime.PyObject_SetAttrString(builtins, "__import__", py_import);
+            PythonException.ThrowIfIsNotZero(res);
+            Runtime.XDecref(py_import);
+            py_import = IntPtr.Zero;
 
+            hook.Release();
+            hook = null;
+
+            Runtime.XDecref(builtins);
+        }
+
+        /// <summary>
+        /// Initialization performed on startup of the Python runtime.
+        /// </summary>
+        internal static void Initialize()
+        {
+            InitImport();
+
+            // Initialize the clr module and tell Python about it.
             root = new CLRModule();
 
 #if PYTHON3
@@ -62,6 +97,7 @@ internal static void Initialize()
             Runtime.XIncref(root.pyHandle); // we are using the module two times
             py_clr_module = root.pyHandle; // Alias handle for PY2/PY3
 #endif
+            IntPtr dict = Runtime.PyImport_GetModuleDict();
             Runtime.PyDict_SetItemString(dict, "CLR", py_clr_module);
             Runtime.PyDict_SetItemString(dict, "clr", py_clr_module);
         }
@@ -72,12 +108,26 @@ internal static void Initialize()
         /// </summary>
         internal static void Shutdown()
         {
-            if (Runtime.Py_IsInitialized() != 0)
+            if (Runtime.Py_IsInitialized() == 0)
+            {
+                return;
+            }
+
+            RestoreImport();
+
+            bool shouldFreeDef = Runtime.Refcount(py_clr_module) == 1;
+            Runtime.XDecref(py_clr_module);
+            py_clr_module = IntPtr.Zero;
+#if PYTHON3
+            if (shouldFreeDef)
             {
-                Runtime.XDecref(py_clr_module);
-                Runtime.XDecref(root.pyHandle);
-                Runtime.XDecref(py_import);
+                ReleaseModuleDef();
             }
+#endif
+
+            Runtime.XDecref(root.pyHandle);
+            root = null;
+            CLRModule.Reset();
         }
 
         /// <summary>
diff --git a/src/runtime/interop.cs b/src/runtime/interop.cs
index 4ae4b61e0..039feddc7 100644
--- a/src/runtime/interop.cs
+++ b/src/runtime/interop.cs
@@ -1,9 +1,11 @@
 using System;
 using System.Collections;
-using System.Collections.Specialized;
+using System.Collections.Generic;
+using System.Diagnostics;
 using System.Runtime.InteropServices;
 using System.Reflection;
 using System.Text;
+using System.Collections.Generic;
 
 namespace Python.Runtime
 {
@@ -67,11 +69,47 @@ public ModulePropertyAttribute()
         }
     }
 
+    internal static class ManagedDataOffsets
+    {
+        static ManagedDataOffsets()
+        {
+            FieldInfo[] fi = typeof(ManagedDataOffsets).GetFields(BindingFlags.Static | BindingFlags.Public);
+            for (int i = 0; i < fi.Length; i++)
+            {
+                fi[i].SetValue(null, -(i * IntPtr.Size) - IntPtr.Size);
+            }
 
-    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
-    internal class ObjectOffset
+            size = fi.Length * IntPtr.Size;
+        }
+
+        public static readonly int ob_data;
+        public static readonly int ob_dict;
+
+        private static int BaseOffset(IntPtr type)
+        {
+            Debug.Assert(type != IntPtr.Zero);
+            int typeSize = Marshal.ReadInt32(type, TypeOffset.tp_basicsize);
+            Debug.Assert(typeSize > 0 && typeSize <= ExceptionOffset.Size());
+            return typeSize;
+        }
+        public static int DataOffset(IntPtr type)
+        {
+            return BaseOffset(type) + ob_data;
+        }
+
+        public static int DictOffset(IntPtr type)
+        {
+            return BaseOffset(type) + ob_dict;
+        }
+
+        public static int Size { get { return size; } }
+
+        private static readonly int size;
+    }
+
+    internal static class OriginalObjectOffsets
     {
-        static ObjectOffset()
+        static OriginalObjectOffsets()
         {
             int size = IntPtr.Size;
             var n = 0; // Py_TRACE_REFS add two pointers to PyObject_HEAD
@@ -82,42 +120,58 @@ static ObjectOffset()
 #endif
             ob_refcnt = (n + 0) * size;
             ob_type = (n + 1) * size;
-            ob_dict = (n + 2) * size;
-            ob_data = (n + 3) * size;
         }
 
-        public static int magic(IntPtr ob)
+        public static int Size { get { return size; } }
+
+        private static readonly int size =
+#if PYTHON_WITH_PYDEBUG
+            4 * IntPtr.Size;
+#else
+            2 * IntPtr.Size;
+#endif
+
+#if PYTHON_WITH_PYDEBUG
+        public static int _ob_next;
+        public static int _ob_prev;
+#endif
+        public static int ob_refcnt;
+        public static int ob_type;
+    }
+
+    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
+    internal class ObjectOffset
+    {
+        static ObjectOffset()
+        {
+#if PYTHON_WITH_PYDEBUG
+            _ob_next = OriginalObjectOffsets._ob_next;
+            _ob_prev = OriginalObjectOffsets._ob_prev;
+#endif
+            ob_refcnt = OriginalObjectOffsets.ob_refcnt;
+            ob_type = OriginalObjectOffsets.ob_type;
+
+            size = OriginalObjectOffsets.Size + ManagedDataOffsets.Size;
+        }
+
+        public static int magic(IntPtr type)
         {
-            if ((Runtime.PyObject_TypeCheck(ob, Exceptions.BaseException) ||
-                 (Runtime.PyType_Check(ob) && Runtime.PyType_IsSubtype(ob, Exceptions.BaseException))))
-            {
-                return ExceptionOffset.ob_data;
-            }
-            return ob_data;
+            return ManagedDataOffsets.DataOffset(type);
         }
 
-        public static int DictOffset(IntPtr ob)
+        public static int TypeDictOffset(IntPtr type)
         {
-            if ((Runtime.PyObject_TypeCheck(ob, Exceptions.BaseException) ||
-                 (Runtime.PyType_Check(ob) && Runtime.PyType_IsSubtype(ob, Exceptions.BaseException))))
-            {
-                return ExceptionOffset.ob_dict;
-            }
-            return ob_dict;
+            return ManagedDataOffsets.DictOffset(type);
         }
 
-        public static int Size(IntPtr ob)
+        public static int Size(IntPtr pyType)
         {
-            if ((Runtime.PyObject_TypeCheck(ob, Exceptions.BaseException) ||
-                 (Runtime.PyType_Check(ob) && Runtime.PyType_IsSubtype(ob, Exceptions.BaseException))))
+            if (IsException(pyType))
             {
                 return ExceptionOffset.Size();
             }
-#if PYTHON_WITH_PYDEBUG
-            return 6 * IntPtr.Size;
-#else
-            return 4 * IntPtr.Size;
-#endif
+
+            return size;
         }
 
 #if PYTHON_WITH_PYDEBUG
@@ -126,8 +180,15 @@ public static int Size(IntPtr ob)
 #endif
         public static int ob_refcnt;
         public static int ob_type;
-        private static int ob_dict;
-        private static int ob_data;
+        private static readonly int size;
+
+        private static bool IsException(IntPtr pyObject)
+        {
+            var type = Runtime.PyObject_TYPE(pyObject);
+            return Runtime.PyType_IsSameAsOrSubtype(type, ofType: Exceptions.BaseException)
+                || Runtime.PyType_IsSameAsOrSubtype(type, ofType: Runtime.PyTypeType)
+                && Runtime.PyType_IsSubtype(pyObject, Exceptions.BaseException);
+        }
     }
 
     [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
@@ -136,19 +197,17 @@ internal class ExceptionOffset
         static ExceptionOffset()
         {
             Type type = typeof(ExceptionOffset);
-            FieldInfo[] fi = type.GetFields();
-            int size = IntPtr.Size;
+            FieldInfo[] fi = type.GetFields(BindingFlags.Static | BindingFlags.Public);
             for (int i = 0; i < fi.Length; i++)
             {
-                fi[i].SetValue(null, (i * size) + ObjectOffset.ob_type + size);
+                fi[i].SetValue(null, (i * IntPtr.Size) + OriginalObjectOffsets.Size);
             }
-        }
 
-        public static int Size()
-        {
-            return ob_data + IntPtr.Size;
+            size = fi.Length * IntPtr.Size + OriginalObjectOffsets.Size + ManagedDataOffsets.Size;
         }
 
+        public static int Size() { return size; }
+
         // PyException_HEAD
         // (start after PyObject_HEAD)
         public static int dict = 0;
@@ -162,9 +221,7 @@ public static int Size()
         public static int suppress_context = 0;
 #endif
 
-        // extra c# data
-        public static int ob_dict;
-        public static int ob_data;
+        private static readonly int size;
     }
 
 
@@ -227,7 +284,7 @@ public static IntPtr AllocModuleDef(string modulename)
             byte[] ascii = Encoding.ASCII.GetBytes(modulename);
             int size = name + ascii.Length + 1;
             IntPtr ptr = Marshal.AllocHGlobal(size);
-            for (int i = 0; i < m_free; i += IntPtr.Size)
+            for (int i = 0; i <= m_free; i += IntPtr.Size)
                 Marshal.WriteIntPtr(ptr, i, IntPtr.Zero);
             Marshal.Copy(ascii, 0, (IntPtr)(ptr + name), ascii.Length);
             Marshal.WriteIntPtr(ptr, m_name, (IntPtr)(ptr + name));
@@ -334,7 +391,7 @@ internal class TypeFlags
 
     internal class Interop
     {
-        private static ArrayList keepAlive;
+        private static List<ThunkInfo> keepAlive;
         private static Hashtable pmap;
 
         static Interop()
@@ -351,8 +408,7 @@ static Interop()
                 p[item.Name] = item;
             }
 
-            keepAlive = new ArrayList();
-            Marshal.AllocHGlobal(IntPtr.Size);
+            keepAlive = new List<ThunkInfo>();
             pmap = new Hashtable();
 
             pmap["tp_dealloc"] = p["DestructorFunc"];
@@ -449,7 +505,7 @@ internal static Type GetPrototype(string name)
             return pmap[name] as Type;
         }
 
-        internal static IntPtr GetThunk(MethodInfo method, string funcType = null)
+        internal static ThunkInfo GetThunk(MethodInfo method, string funcType = null)
         {
             Type dt;
             if (funcType != null)
@@ -457,18 +513,15 @@ internal static IntPtr GetThunk(MethodInfo method, string funcType = null)
             else
                 dt = GetPrototype(method.Name);
 
-            if (dt != null)
+            if (dt == null)
             {
-                IntPtr tmp = Marshal.AllocHGlobal(IntPtr.Size);
-                Delegate d = Delegate.CreateDelegate(dt, method);
-                Thunk cb = new Thunk(d);
-                Marshal.StructureToPtr(cb, tmp, false);
-                IntPtr fp = Marshal.ReadIntPtr(tmp, 0);
-                Marshal.FreeHGlobal(tmp);
-                keepAlive.Add(d);
-                return fp;
+                return ThunkInfo.Empty;
             }
-            return IntPtr.Zero;
+            Delegate d = Delegate.CreateDelegate(dt, method);
+            var info = new ThunkInfo(d);
+            // TODO: remove keepAlive when #958 merged, let the lifecycle of ThunkInfo transfer to caller.
+            keepAlive.Add(info);
+            return info;
         }
 
         [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
@@ -522,4 +575,22 @@ public Thunk(Delegate d)
             fn = d;
         }
     }
+
+    internal class ThunkInfo
+    {
+        public readonly Delegate Target;
+        public readonly IntPtr Address;
+
+        public static readonly ThunkInfo Empty = new ThunkInfo(null);
+
+        public ThunkInfo(Delegate target)
+        {
+            if (target == null)
+            {
+                return;
+            }
+            Target = target;
+            Address = Marshal.GetFunctionPointerForDelegate(target);
+        }
+    }
 }
diff --git a/src/runtime/interop38.cs b/src/runtime/interop38.cs
new file mode 100644
index 000000000..9126bca6a
--- /dev/null
+++ b/src/runtime/interop38.cs
@@ -0,0 +1,152 @@
+
+// Auto-generated by geninterop.py.
+// DO NOT MODIFY BY HAND.
+
+
+#if PYTHON38
+using System;
+using System.Collections;
+using System.Collections.Specialized;
+using System.Runtime.InteropServices;
+using System.Reflection;
+using System.Text;
+
+namespace Python.Runtime
+{
+    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
+    internal class TypeOffset
+    {
+        static TypeOffset()
+        {
+            Type type = typeof(TypeOffset);
+            FieldInfo[] fi = type.GetFields();
+            int size = IntPtr.Size;
+            for (int i = 0; i < fi.Length; i++)
+            {
+                fi[i].SetValue(null, i * size);
+            }
+        }
+
+        public static int magic()
+        {
+            return ob_size;
+        }
+
+        // Auto-generated from PyHeapTypeObject in Python.h
+        public static int ob_refcnt = 0;
+        public static int ob_type = 0;
+        public static int ob_size = 0;
+        public static int tp_name = 0;
+        public static int tp_basicsize = 0;
+        public static int tp_itemsize = 0;
+        public static int tp_dealloc = 0;
+        public static int tp_vectorcall_offset = 0;
+        public static int tp_getattr = 0;
+        public static int tp_setattr = 0;
+        public static int tp_as_async = 0;
+        public static int tp_repr = 0;
+        public static int tp_as_number = 0;
+        public static int tp_as_sequence = 0;
+        public static int tp_as_mapping = 0;
+        public static int tp_hash = 0;
+        public static int tp_call = 0;
+        public static int tp_str = 0;
+        public static int tp_getattro = 0;
+        public static int tp_setattro = 0;
+        public static int tp_as_buffer = 0;
+        public static int tp_flags = 0;
+        public static int tp_doc = 0;
+        public static int tp_traverse = 0;
+        public static int tp_clear = 0;
+        public static int tp_richcompare = 0;
+        public static int tp_weaklistoffset = 0;
+        public static int tp_iter = 0;
+        public static int tp_iternext = 0;
+        public static int tp_methods = 0;
+        public static int tp_members = 0;
+        public static int tp_getset = 0;
+        public static int tp_base = 0;
+        public static int tp_dict = 0;
+        public static int tp_descr_get = 0;
+        public static int tp_descr_set = 0;
+        public static int tp_dictoffset = 0;
+        public static int tp_init = 0;
+        public static int tp_alloc = 0;
+        public static int tp_new = 0;
+        public static int tp_free = 0;
+        public static int tp_is_gc = 0;
+        public static int tp_bases = 0;
+        public static int tp_mro = 0;
+        public static int tp_cache = 0;
+        public static int tp_subclasses = 0;
+        public static int tp_weaklist = 0;
+        public static int tp_del = 0;
+        public static int tp_version_tag = 0;
+        public static int tp_finalize = 0;
+        public static int tp_vectorcall = 0;
+        public static int tp_print = 0;
+        public static int am_await = 0;
+        public static int am_aiter = 0;
+        public static int am_anext = 0;
+        public static int nb_add = 0;
+        public static int nb_subtract = 0;
+        public static int nb_multiply = 0;
+        public static int nb_remainder = 0;
+        public static int nb_divmod = 0;
+        public static int nb_power = 0;
+        public static int nb_negative = 0;
+        public static int nb_positive = 0;
+        public static int nb_absolute = 0;
+        public static int nb_bool = 0;
+        public static int nb_invert = 0;
+        public static int nb_lshift = 0;
+        public static int nb_rshift = 0;
+        public static int nb_and = 0;
+        public static int nb_xor = 0;
+        public static int nb_or = 0;
+        public static int nb_int = 0;
+        public static int nb_reserved = 0;
+        public static int nb_float = 0;
+        public static int nb_inplace_add = 0;
+        public static int nb_inplace_subtract = 0;
+        public static int nb_inplace_multiply = 0;
+        public static int nb_inplace_remainder = 0;
+        public static int nb_inplace_power = 0;
+        public static int nb_inplace_lshift = 0;
+        public static int nb_inplace_rshift = 0;
+        public static int nb_inplace_and = 0;
+        public static int nb_inplace_xor = 0;
+        public static int nb_inplace_or = 0;
+        public static int nb_floor_divide = 0;
+        public static int nb_true_divide = 0;
+        public static int nb_inplace_floor_divide = 0;
+        public static int nb_inplace_true_divide = 0;
+        public static int nb_index = 0;
+        public static int nb_matrix_multiply = 0;
+        public static int nb_inplace_matrix_multiply = 0;
+        public static int mp_length = 0;
+        public static int mp_subscript = 0;
+        public static int mp_ass_subscript = 0;
+        public static int sq_length = 0;
+        public static int sq_concat = 0;
+        public static int sq_repeat = 0;
+        public static int sq_item = 0;
+        public static int was_sq_slice = 0;
+        public static int sq_ass_item = 0;
+        public static int was_sq_ass_slice = 0;
+        public static int sq_contains = 0;
+        public static int sq_inplace_concat = 0;
+        public static int sq_inplace_repeat = 0;
+        public static int bf_getbuffer = 0;
+        public static int bf_releasebuffer = 0;
+        public static int name = 0;
+        public static int ht_slots = 0;
+        public static int qualname = 0;
+        public static int ht_cached_keys = 0;
+
+        /* here are optional user slots, followed by the members. */
+        public static int members = 0;
+    }
+}
+
+#endif
diff --git a/src/runtime/managedtype.cs b/src/runtime/managedtype.cs
index 3191da949..23f5898d1 100644
--- a/src/runtime/managedtype.cs
+++ b/src/runtime/managedtype.cs
@@ -33,7 +33,7 @@ internal static ManagedType GetManagedObject(IntPtr ob)
                 {
                     IntPtr op = tp == ob
                         ? Marshal.ReadIntPtr(tp, TypeOffset.magic())
-                        : Marshal.ReadIntPtr(ob, ObjectOffset.magic(ob));
+                        : Marshal.ReadIntPtr(ob, ObjectOffset.magic(tp));
                     if (op == IntPtr.Zero)
                     {
                         return null;
diff --git a/src/runtime/metatype.cs b/src/runtime/metatype.cs
index 8853c2d5e..5af2e1a7e 100644
--- a/src/runtime/metatype.cs
+++ b/src/runtime/metatype.cs
@@ -266,6 +266,7 @@ private static IntPtr DoInstanceCheck(IntPtr tp, IntPtr args, bool checkType)
                 return Runtime.PyFalse;
             }
 
+            Runtime.XIncref(args);
             using (var argsObj = new PyList(args))
             {
                 if (argsObj.Length() != 1)
diff --git a/src/runtime/methodbinder.cs b/src/runtime/methodbinder.cs
index 7471d5d7c..b74f21754 100644
--- a/src/runtime/methodbinder.cs
+++ b/src/runtime/methodbinder.cs
@@ -1,6 +1,9 @@
 using System;
 using System.Collections;
 using System.Reflection;
+using System.Text;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace Python.Runtime
 {
@@ -279,6 +282,22 @@ internal Binding Bind(IntPtr inst, IntPtr args, IntPtr kw, MethodBase info, Meth
         {
             // loop to find match, return invoker w/ or /wo error
             MethodBase[] _methods = null;
+
+            var kwargDict = new Dictionary<string, IntPtr>();
+            if (kw != IntPtr.Zero)
+            {
+                var pynkwargs = (int)Runtime.PyDict_Size(kw);
+                IntPtr keylist = Runtime.PyDict_Keys(kw);
+                IntPtr valueList = Runtime.PyDict_Values(kw);
+                for (int i = 0; i < pynkwargs; ++i)
+                {
+                    var keyStr = Runtime.GetManagedString(Runtime.PyList_GetItem(keylist, i));
+                    kwargDict[keyStr] = Runtime.PyList_GetItem(valueList, i).DangerousGetAddress();
+                }
+                Runtime.XDecref(keylist);
+                Runtime.XDecref(valueList);
+            }
+
             var pynargs = (int)Runtime.PyTuple_Size(args);
             var isGeneric = false;
             if (info != null)
@@ -302,11 +321,12 @@ internal Binding Bind(IntPtr inst, IntPtr args, IntPtr kw, MethodBase info, Meth
                 ArrayList defaultArgList;
                 bool paramsArray;
 
-                if (!MatchesArgumentCount(pynargs, pi, out paramsArray, out defaultArgList)) {
+                if (!MatchesArgumentCount(pynargs, pi, kwargDict, out paramsArray, out defaultArgList))
+                {
                     continue;
                 }
                 var outs = 0;
-                var margs = TryConvertArguments(pi, paramsArray, args, pynargs, defaultArgList,
+                var margs = TryConvertArguments(pi, paramsArray, args, pynargs, kwargDict, defaultArgList,
                     needsResolution: _methods.Length > 1,
                     outs: out outs);
 
@@ -349,20 +369,57 @@ internal Binding Bind(IntPtr inst, IntPtr args, IntPtr kw, MethodBase info, Meth
             return null;
         }
 
+        static IntPtr HandleParamsArray(IntPtr args, int arrayStart, int pyArgCount, out bool isNewReference)
+        {
+            isNewReference = false;
+            IntPtr op;
+            // for a params method, we may have a sequence or single/multiple items
+            // here we look to see if the item at the paramIndex is there or not
+            // and then if it is a sequence itself.
+            if ((pyArgCount - arrayStart) == 1)
+            {
+                // we only have one argument left, so we need to check it
+                // to see if it is a sequence or a single item
+                IntPtr item = Runtime.PyTuple_GetItem(args, arrayStart);
+                if (!Runtime.PyString_Check(item) && Runtime.PySequence_Check(item))
+                {
+                    // it's a sequence (and not a string), so we use it as the op
+                    op = item;
+                }
+                else
+                {
+                    isNewReference = true;
+                    op = Runtime.PyTuple_GetSlice(args, arrayStart, pyArgCount);
+                    if (item != IntPtr.Zero)
+                    {
+                        Runtime.XDecref(item);
+                    }
+                }
+            }
+            else
+            {
+                isNewReference = true;
+                op = Runtime.PyTuple_GetSlice(args, arrayStart, pyArgCount);
+            }
+            return op;
+        }
+
         /// <summary>
-        /// Attempts to convert Python argument tuple into an array of managed objects,
-        /// that can be passed to a method.
+        /// Attempts to convert Python positional argument tuple and keyword argument table
+        /// into an array of managed objects, that can be passed to a method.
         /// </summary>
         /// <param name="pi">Information about expected parameters</param>
         /// <param name="paramsArray"><c>true</c>, if the last parameter is a params array.</param>
         /// <param name="args">A pointer to the Python argument tuple</param>
         /// <param name="pyArgCount">Number of arguments, passed by Python</param>
+        /// <param name="kwargDict">Dictionary of keyword argument name to python object pointer</param>
         /// <param name="defaultArgList">A list of default values for omitted parameters</param>
         /// <param name="needsResolution"><c>true</c>, if overloading resolution is required</param>
         /// <param name="outs">Returns number of output parameters</param>
         /// <returns>An array of .NET arguments, that can be passed to a method.</returns>
         static object[] TryConvertArguments(ParameterInfo[] pi, bool paramsArray,
             IntPtr args, int pyArgCount,
+            Dictionary<string, IntPtr> kwargDict,
             ArrayList defaultArgList,
             bool needsResolution,
             out int outs)
@@ -373,7 +430,11 @@ static object[] TryConvertArguments(ParameterInfo[] pi, bool paramsArray,
 
             for (int paramIndex = 0; paramIndex < pi.Length; paramIndex++)
             {
-                if (paramIndex >= pyArgCount)
+                var parameter = pi[paramIndex];
+                bool hasNamedParam = kwargDict.ContainsKey(parameter.Name);
+                bool isNewReference = false;
+
+                if (paramIndex >= pyArgCount && !(hasNamedParam || (paramsArray && paramIndex == arrayStart)))
                 {
                     if (defaultArgList != null)
                     {
@@ -383,12 +444,22 @@ static object[] TryConvertArguments(ParameterInfo[] pi, bool paramsArray,
                     continue;
                 }
 
-                var parameter = pi[paramIndex];
-                IntPtr op = (arrayStart == paramIndex)
-                    // map remaining Python arguments to a tuple since
-                    // the managed function accepts it - hopefully :]
-                    ? Runtime.PyTuple_GetSlice(args, arrayStart, pyArgCount)
-                    : Runtime.PyTuple_GetItem(args, paramIndex);
+                IntPtr op;
+                if (hasNamedParam)
+                {
+                    op = kwargDict[parameter.Name];
+                }
+                else
+                {
+                    if(arrayStart == paramIndex)
+                    {
+                        op = HandleParamsArray(args, arrayStart, pyArgCount, out isNewReference);                                                                 
+                    }
+                    else
+                    {
+                        op = Runtime.PyTuple_GetItem(args, paramIndex);
+                    }
+                }
 
                 bool isOut;
                 if (!TryConvertArgument(op, parameter.ParameterType, needsResolution, out margs[paramIndex], out isOut))
@@ -396,7 +467,7 @@ static object[] TryConvertArguments(ParameterInfo[] pi, bool paramsArray,
                     return null;
                 }
 
-                if (arrayStart == paramIndex)
+                if (isNewReference)
                 {
                     // TODO: is this a bug? Should this happen even if the conversion fails?
                     // GetSlice() creates a new reference but GetItem()
@@ -504,29 +575,49 @@ static Type TryComputeClrArgumentType(Type parameterType, IntPtr argument, bool
             return clrtype;
         }
 
-        static bool MatchesArgumentCount(int argumentCount, ParameterInfo[] parameters,
+        static bool MatchesArgumentCount(int positionalArgumentCount, ParameterInfo[] parameters,
+            Dictionary<string, IntPtr> kwargDict,
             out bool paramsArray,
             out ArrayList defaultArgList)
         {
             defaultArgList = null;
             var match = false;
-            paramsArray = false;
+            paramsArray = parameters.Length > 0 ? Attribute.IsDefined(parameters[parameters.Length - 1], typeof(ParamArrayAttribute)) : false;
 
-            if (argumentCount == parameters.Length)
+            if (positionalArgumentCount == parameters.Length)
             {
                 match = true;
-            } else if (argumentCount < parameters.Length)
+            }
+            else if (positionalArgumentCount < parameters.Length)
             {
+                // every parameter past 'positionalArgumentCount' must have either
+                // a corresponding keyword argument or a default parameter
                 match = true;
                 defaultArgList = new ArrayList();
-                for (var v = argumentCount; v < parameters.Length; v++) {
-                    if (parameters[v].DefaultValue == DBNull.Value) {
+                for (var v = positionalArgumentCount; v < parameters.Length; v++)
+                {
+                    if (kwargDict.ContainsKey(parameters[v].Name))
+                    {
+                        // we have a keyword argument for this parameter,
+                        // no need to check for a default parameter, but put a null
+                        // placeholder in defaultArgList
+                        defaultArgList.Add(null);
+                    }
+                    else if (parameters[v].IsOptional)
+                    {
+                        // IsOptional will be true if the parameter has a default value,
+                        // or if the parameter has the [Optional] attribute specified.
+                        // The GetDefaultValue() extension method will return the value
+                        // to be passed in as the parameter value
+                        defaultArgList.Add(parameters[v].GetDefaultValue());
+                    }
+                    else if(!paramsArray)
+                    {
                         match = false;
-                    } else {
-                        defaultArgList.Add(parameters[v].DefaultValue);
                     }
                 }
-            } else if (argumentCount > parameters.Length && parameters.Length > 0 &&
+            }
+            else if (positionalArgumentCount > parameters.Length && parameters.Length > 0 &&
                        Attribute.IsDefined(parameters[parameters.Length - 1], typeof(ParamArrayAttribute)))
             {
                 // This is a `foo(params object[] bar)` style method
@@ -547,6 +638,40 @@ internal virtual IntPtr Invoke(IntPtr inst, IntPtr args, IntPtr kw, MethodBase i
             return Invoke(inst, args, kw, info, null);
         }
 
+        protected static void AppendArgumentTypes(StringBuilder to, IntPtr args)
+        {
+            long argCount = Runtime.PyTuple_Size(args);
+            to.Append("(");
+            for (long argIndex = 0; argIndex < argCount; argIndex++)
+            {
+                var arg = Runtime.PyTuple_GetItem(args, argIndex);
+                if (arg != IntPtr.Zero)
+                {
+                    var type = Runtime.PyObject_Type(arg);
+                    if (type != IntPtr.Zero)
+                    {
+                        try
+                        {
+                            var description = Runtime.PyObject_Unicode(type);
+                            if (description != IntPtr.Zero)
+                            {
+                                to.Append(Runtime.GetManagedString(description));
+                                Runtime.XDecref(description);
+                            }
+                        }
+                        finally
+                        {
+                            Runtime.XDecref(type);
+                        }
+                    }
+                }
+
+                if (argIndex + 1 < argCount)
+                    to.Append(", ");
+            }
+            to.Append(')');
+        }
+
         internal virtual IntPtr Invoke(IntPtr inst, IntPtr args, IntPtr kw, MethodBase info, MethodInfo[] methodinfo)
         {
             Binding binding = Bind(inst, args, kw, info, methodinfo);
@@ -555,12 +680,15 @@ internal virtual IntPtr Invoke(IntPtr inst, IntPtr args, IntPtr kw, MethodBase i
 
             if (binding == null)
             {
-                var value = "No method matches given arguments";
+                var value = new StringBuilder("No method matches given arguments");
                 if (methodinfo != null && methodinfo.Length > 0)
                 {
-                    value += $" for {methodinfo[0].Name}";
+                    value.Append($" for {methodinfo[0].Name}");
                 }
-                Exceptions.SetError(Exceptions.TypeError, value);
+
+                value.Append(": ");
+                AppendArgumentTypes(to: value, args);
+                Exceptions.SetError(Exceptions.TypeError, value.ToString());
                 return IntPtr.Zero;
             }
 
@@ -697,4 +825,33 @@ internal Binding(MethodBase info, object inst, object[] args, int outs)
             this.outs = outs;
         }
     }
+
+
+    static internal class ParameterInfoExtensions
+    {
+        public static object GetDefaultValue(this ParameterInfo parameterInfo)
+        {
+            // parameterInfo.HasDefaultValue is preferable but doesn't exist in .NET 4.0
+            bool hasDefaultValue = (parameterInfo.Attributes & ParameterAttributes.HasDefault) ==
+                ParameterAttributes.HasDefault;
+
+            if (hasDefaultValue)
+            {
+                return parameterInfo.DefaultValue;
+            }
+            else
+            {
+                // [OptionalAttribute] was specified for the parameter.
+                // See https://stackoverflow.com/questions/3416216/optionalattribute-parameters-default-value
+                // for rules on determining the value to pass to the parameter
+                var type = parameterInfo.ParameterType;
+                if (type == typeof(object))
+                    return Type.Missing;
+                else if (type.IsValueType)
+                    return Activator.CreateInstance(type);
+                else
+                    return null;
+            }
+        }
+    }
 }
diff --git a/src/runtime/methodwrapper.cs b/src/runtime/methodwrapper.cs
index 2f3ce3ef2..bc7500dab 100644
--- a/src/runtime/methodwrapper.cs
+++ b/src/runtime/methodwrapper.cs
@@ -12,18 +12,21 @@ internal class MethodWrapper
     {
         public IntPtr mdef;
         public IntPtr ptr;
+        private bool _disposed = false;
+
+        private ThunkInfo _thunk;
 
         public MethodWrapper(Type type, string name, string funcType = null)
         {
             // Turn the managed method into a function pointer
 
-            IntPtr fp = Interop.GetThunk(type.GetMethod(name), funcType);
+            _thunk = Interop.GetThunk(type.GetMethod(name), funcType);
 
             // Allocate and initialize a PyMethodDef structure to represent
             // the managed method, then create a PyCFunction.
 
             mdef = Runtime.PyMem_Malloc(4 * IntPtr.Size);
-            TypeManager.WriteMethodDef(mdef, name, fp, 0x0003);
+            TypeManager.WriteMethodDef(mdef, name, _thunk.Address, 0x0003);
             ptr = Runtime.PyCFunction_NewEx(mdef, IntPtr.Zero, IntPtr.Zero);
         }
 
@@ -31,5 +34,21 @@ public IntPtr Call(IntPtr args, IntPtr kw)
         {
             return Runtime.PyCFunction_Call(ptr, args, kw);
         }
+
+        public void Release()
+        {
+            if (_disposed)
+            {
+                return;
+            }
+            _disposed = true;
+            bool freeDef = Runtime.Refcount(ptr) == 1;
+            Runtime.XDecref(ptr);
+            if (freeDef && mdef != IntPtr.Zero)
+            {
+                Runtime.PyMem_Free(mdef);
+                mdef = IntPtr.Zero;
+            }
+        }
     }
 }
diff --git a/src/runtime/moduleobject.cs b/src/runtime/moduleobject.cs
index 7a45c6c81..15e4feee8 100644
--- a/src/runtime/moduleobject.cs
+++ b/src/runtime/moduleobject.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.IO;
 using System.Reflection;
 using System.Runtime.InteropServices;
@@ -53,7 +54,7 @@ public ModuleObject(string name)
             Runtime.XDecref(pyfilename);
             Runtime.XDecref(pydocstring);
 
-            Marshal.WriteIntPtr(pyHandle, ObjectOffset.DictOffset(pyHandle), dict);
+            Marshal.WriteIntPtr(pyHandle, ObjectOffset.TypeDictOffset(tpHandle), dict);
 
             InitializeModuleMembers();
         }
@@ -105,13 +106,9 @@ public ManagedType GetAttribute(string name, bool guess)
             // Look for a type in the current namespace. Note that this
             // includes types, delegates, enums, interfaces and structs.
             // Only public namespace members are exposed to Python.
-            type = AssemblyManager.LookupType(qname);
+            type = AssemblyManager.LookupTypes(qname).FirstOrDefault(t => t.IsPublic);
             if (type != null)
             {
-                if (!type.IsPublic)
-                {
-                    return null;
-                }
                 c = ClassManager.GetClass(type);
                 StoreAttribute(name, c);
                 return c;
@@ -131,13 +128,9 @@ public ManagedType GetAttribute(string name, bool guess)
                     return m;
                 }
 
-                type = AssemblyManager.LookupType(qname);
+                type = AssemblyManager.LookupTypes(qname).FirstOrDefault(t => t.IsPublic);
                 if (type != null)
                 {
-                    if (!type.IsPublic)
-                    {
-                        return null;
-                    }
                     c = ClassManager.GetClass(type);
                     StoreAttribute(name, c);
                     return c;
@@ -280,7 +273,18 @@ public static IntPtr tp_getattro(IntPtr ob, IntPtr key)
                 return self.dict;
             }
 
-            ManagedType attr = self.GetAttribute(name, true);
+            ManagedType attr = null;
+
+            try
+            {
+                attr = self.GetAttribute(name, true);
+            }
+            catch (Exception e)
+            {
+                Exceptions.SetError(e);
+                return IntPtr.Zero;
+            }
+            
 
             if (attr == null)
             {
@@ -316,6 +320,11 @@ internal class CLRModule : ModuleObject
         internal static bool _SuppressDocs = false;
         internal static bool _SuppressOverloads = false;
 
+        static CLRModule()
+        {
+            Reset();
+        }
+
         public CLRModule() : base("clr")
         {
             _namespace = string.Empty;
diff --git a/src/runtime/nativecall.cs b/src/runtime/nativecall.cs
index b5bf25dd7..4a7bf05c8 100644
--- a/src/runtime/nativecall.cs
+++ b/src/runtime/nativecall.cs
@@ -32,19 +32,21 @@ internal class NativeCall
 
         public static void Void_Call_1(IntPtr fp, IntPtr a1)
         {
-            ((Void_1_Delegate)Marshal.GetDelegateForFunctionPointer(fp, typeof(Void_1_Delegate)))(a1);
+            var d = Marshal.GetDelegateForFunctionPointer<Interop.DestructorFunc>(fp);
+            d(a1);
         }
 
         public static IntPtr Call_3(IntPtr fp, IntPtr a1, IntPtr a2, IntPtr a3)
         {
-            var d = (Interop.TernaryFunc)Marshal.GetDelegateForFunctionPointer(fp, typeof(Interop.TernaryFunc));
+            var d = Marshal.GetDelegateForFunctionPointer<Interop.TernaryFunc>(fp);
             return d(a1, a2, a3);
         }
 
 
         public static int Int_Call_3(IntPtr fp, IntPtr a1, IntPtr a2, IntPtr a3)
         {
-            return ((Int_3_Delegate)Marshal.GetDelegateForFunctionPointer(fp, typeof(Int_3_Delegate)))(a1, a2, a3);
+            var d = Marshal.GetDelegateForFunctionPointer<Interop.ObjObjArgFunc>(fp);
+            return d(a1, a2, a3);
         }
 #else
         private static AssemblyBuilder aBuilder;
diff --git a/src/runtime/platform/LibraryLoader.cs b/src/runtime/platform/LibraryLoader.cs
new file mode 100644
index 000000000..a6d88cd19
--- /dev/null
+++ b/src/runtime/platform/LibraryLoader.cs
@@ -0,0 +1,208 @@
+using System;
+using System.ComponentModel;
+using System.Runtime.InteropServices;
+
+namespace Python.Runtime.Platform
+{
+    interface ILibraryLoader
+    {
+        IntPtr Load(string dllToLoad);
+
+        IntPtr GetFunction(IntPtr hModule, string procedureName);
+
+        void Free(IntPtr hModule);
+    }
+
+    static class LibraryLoader
+    {
+        public static ILibraryLoader Get(OperatingSystemType os)
+        {
+            switch (os)
+            {
+                case OperatingSystemType.Windows:
+                    return new WindowsLoader();
+                case OperatingSystemType.Darwin:
+                    return new DarwinLoader();
+                case OperatingSystemType.Linux:
+                    return new LinuxLoader();
+                default:
+                    throw new PlatformNotSupportedException($"This operating system ({os}) is not supported");
+            }
+        }
+    }
+
+    class LinuxLoader : ILibraryLoader
+    {
+        private static int RTLD_NOW = 0x2;
+        private static int RTLD_GLOBAL = 0x100;
+        private static IntPtr RTLD_DEFAULT = IntPtr.Zero;
+        private const string NativeDll = "libdl.so";
+
+        public IntPtr Load(string dllToLoad)
+        {
+            var filename = $"lib{dllToLoad}.so";
+            ClearError();
+            var res = dlopen(filename, RTLD_NOW | RTLD_GLOBAL);
+            if (res == IntPtr.Zero)
+            {
+                var err = GetError();
+                throw new DllNotFoundException($"Could not load {filename} with flags RTLD_NOW | RTLD_GLOBAL: {err}");
+            }
+
+            return res;
+        }
+
+        public void Free(IntPtr handle)
+        {
+            dlclose(handle);
+        }
+
+        public IntPtr GetFunction(IntPtr dllHandle, string name)
+        {
+            // look in the exe if dllHandle is NULL
+            if (dllHandle == IntPtr.Zero)
+            {
+                dllHandle = RTLD_DEFAULT;
+            }
+
+            ClearError();
+            IntPtr res = dlsym(dllHandle, name);
+            if (res == IntPtr.Zero)
+            {
+                var err = GetError();
+                throw new MissingMethodException($"Failed to load symbol {name}: {err}");
+            }
+            return res;
+        }
+
+        void ClearError()
+        {
+            dlerror();
+        }
+
+        string GetError()
+        {
+            var res = dlerror();
+            if (res != IntPtr.Zero)
+                return Marshal.PtrToStringAnsi(res);
+            else
+                return null;
+        }
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
+        public static extern IntPtr dlopen(string fileName, int flags);
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
+        private static extern IntPtr dlsym(IntPtr handle, string symbol);
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl)]
+        private static extern int dlclose(IntPtr handle);
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl)]
+        private static extern IntPtr dlerror();
+    }
+
+    class DarwinLoader : ILibraryLoader
+    {
+        private static int RTLD_NOW = 0x2;
+        private static int RTLD_GLOBAL = 0x8;
+        private const string NativeDll = "/usr/lib/libSystem.dylib";
+        private static IntPtr RTLD_DEFAULT = new IntPtr(-2);
+
+        public IntPtr Load(string dllToLoad)
+        {
+            var filename = $"lib{dllToLoad}.dylib";
+            ClearError();
+            var res = dlopen(filename, RTLD_NOW | RTLD_GLOBAL);
+            if (res == IntPtr.Zero)
+            {
+                var err = GetError();
+                throw new DllNotFoundException($"Could not load {filename} with flags RTLD_NOW | RTLD_GLOBAL: {err}");
+            }
+
+            return res;
+        }
+
+        public void Free(IntPtr handle)
+        {
+            dlclose(handle);
+        }
+
+        public IntPtr GetFunction(IntPtr dllHandle, string name)
+        {
+            // look in the exe if dllHandle is NULL
+            if (dllHandle == IntPtr.Zero)
+            {
+                dllHandle = RTLD_DEFAULT;
+            }
+
+            ClearError();
+            IntPtr res = dlsym(dllHandle, name);
+            if (res == IntPtr.Zero)
+            {
+                var err = GetError();
+                throw new MissingMethodException($"Failed to load symbol {name}: {err}");
+            }
+            return res;
+        }
+
+        void ClearError()
+        {
+            dlerror();
+        }
+
+        string GetError()
+        {
+            var res = dlerror();
+            if (res != IntPtr.Zero)
+                return Marshal.PtrToStringAnsi(res);
+            else
+                return null;
+        }
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
+        public static extern IntPtr dlopen(String fileName, int flags);
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
+        private static extern IntPtr dlsym(IntPtr handle, String symbol);
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl)]
+        private static extern int dlclose(IntPtr handle);
+
+        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl)]
+        private static extern IntPtr dlerror();
+    }
+
+    class WindowsLoader : ILibraryLoader
+    {
+        private const string NativeDll = "kernel32.dll";
+
+
+        public IntPtr Load(string dllToLoad)
+        {
+            var res = WindowsLoader.LoadLibrary(dllToLoad);
+            if (res == IntPtr.Zero)
+                throw new DllNotFoundException($"Could not load {dllToLoad}", new Win32Exception());
+            return res;
+        }
+
+        public IntPtr GetFunction(IntPtr hModule, string procedureName)
+        {
+            var res = WindowsLoader.GetProcAddress(hModule, procedureName);
+            if (res == IntPtr.Zero)
+                throw new MissingMethodException($"Failed to load symbol {procedureName}", new Win32Exception());
+            return res;
+        }
+
+        public void Free(IntPtr hModule) => WindowsLoader.FreeLibrary(hModule);
+
+        [DllImport(NativeDll, SetLastError = true)]
+        static extern IntPtr LoadLibrary(string dllToLoad);
+
+        [DllImport(NativeDll, SetLastError = true)]
+        static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);
+
+        [DllImport(NativeDll)]
+        static extern bool FreeLibrary(IntPtr hModule);
+    }
+}
diff --git a/src/runtime/platform/Types.cs b/src/runtime/platform/Types.cs
new file mode 100644
index 000000000..62be0e421
--- /dev/null
+++ b/src/runtime/platform/Types.cs
@@ -0,0 +1,23 @@
+namespace Python.Runtime.Platform
+{
+    public enum MachineType
+    {
+        i386,
+        x86_64,
+        armv7l,
+        armv8,
+        aarch64,
+        Other
+    };
+
+    /// <summary>
+    /// Operating system type as reported by Python.
+    /// </summary>
+    public enum OperatingSystemType
+    {
+        Windows,
+        Darwin,
+        Linux,
+        Other
+    }
+}
diff --git a/src/runtime/polyfill/ReflectionPolifills.cs b/src/runtime/polyfill/ReflectionPolifills.cs
index a7e9c879a..b9ce78d63 100644
--- a/src/runtime/polyfill/ReflectionPolifills.cs
+++ b/src/runtime/polyfill/ReflectionPolifills.cs
@@ -1,12 +1,14 @@
 using System;
+using System.Linq;
 using System.Reflection;
 using System.Reflection.Emit;
 
 namespace Python.Runtime
 {
-#if NETSTANDARD
+    [Obsolete("This API is for internal use only")]
     public static class ReflectionPolifills
     {
+#if NETSTANDARD
         public static AssemblyBuilder DefineDynamicAssembly(this AppDomain appDomain, AssemblyName assemblyName, AssemblyBuilderAccess assemblyBuilderAccess)
         {
             return AssemblyBuilder.DefineDynamicAssembly(assemblyName, assemblyBuilderAccess);
@@ -16,6 +18,19 @@ public static Type CreateType(this TypeBuilder typeBuilder)
         {
             return typeBuilder.GetTypeInfo().GetType();
         }
-    }
 #endif
+        public static T GetCustomAttribute<T>(this Type type) where T: Attribute
+        {
+            return type.GetCustomAttributes(typeof(T), inherit: false)
+                .Cast<T>()
+                .SingleOrDefault();
+        }
+
+        public static T GetCustomAttribute<T>(this Assembly assembly) where T: Attribute
+        {
+            return assembly.GetCustomAttributes(typeof(T), inherit: false)
+                .Cast<T>()
+                .SingleOrDefault();
+        }
+    }
 }
diff --git a/src/runtime/pydict.cs b/src/runtime/pydict.cs
index 7237d1990..7ff7a83c8 100644
--- a/src/runtime/pydict.cs
+++ b/src/runtime/pydict.cs
@@ -139,12 +139,20 @@ public PyObject Values()
         /// </remarks>
         public PyObject Items()
         {
-            IntPtr items = Runtime.PyDict_Items(obj);
-            if (items == IntPtr.Zero)
+            var items = Runtime.PyDict_Items(this.obj);
+            try
             {
-                throw new PythonException();
+                if (items.IsNull())
+                {
+                    throw new PythonException();
+                }
+
+                return items.MoveToPyObject();
+            }
+            finally
+            {
+                items.Dispose();
             }
-            return new PyObject(items);
         }
 
 
diff --git a/src/runtime/pyint.cs b/src/runtime/pyint.cs
index f6911d9d7..217cf7e20 100644
--- a/src/runtime/pyint.cs
+++ b/src/runtime/pyint.cs
@@ -62,7 +62,7 @@ public PyInt(int value)
         /// Creates a new Python int from a uint32 value.
         /// </remarks>
         [CLSCompliant(false)]
-        public PyInt(uint value) : base(IntPtr.Zero)
+        public PyInt(uint value)
         {
             obj = Runtime.PyInt_FromInt64(value);
             Runtime.CheckExceptionOccurred();
@@ -75,7 +75,7 @@ public PyInt(uint value) : base(IntPtr.Zero)
         /// <remarks>
         /// Creates a new Python int from an int64 value.
         /// </remarks>
-        public PyInt(long value) : base(IntPtr.Zero)
+        public PyInt(long value)
         {
             obj = Runtime.PyInt_FromInt64(value);
             Runtime.CheckExceptionOccurred();
@@ -89,7 +89,7 @@ public PyInt(long value) : base(IntPtr.Zero)
         /// Creates a new Python int from a uint64 value.
         /// </remarks>
         [CLSCompliant(false)]
-        public PyInt(ulong value) : base(IntPtr.Zero)
+        public PyInt(ulong value)
         {
             obj = Runtime.PyInt_FromInt64((long)value);
             Runtime.CheckExceptionOccurred();
diff --git a/src/runtime/pylist.cs b/src/runtime/pylist.cs
index b22d9d51f..347cc3000 100644
--- a/src/runtime/pylist.cs
+++ b/src/runtime/pylist.cs
@@ -120,7 +120,7 @@ public static PyList AsList(PyObject value)
         /// </remarks>
         public void Append(PyObject item)
         {
-            int r = Runtime.PyList_Append(obj, item.obj);
+            int r = Runtime.PyList_Append(this.Reference, item.obj);
             if (r < 0)
             {
                 throw new PythonException();
@@ -135,7 +135,7 @@ public void Append(PyObject item)
         /// </remarks>
         public void Insert(int index, PyObject item)
         {
-            int r = Runtime.PyList_Insert(obj, index, item.obj);
+            int r = Runtime.PyList_Insert(this.Reference, index, item.obj);
             if (r < 0)
             {
                 throw new PythonException();
@@ -151,7 +151,7 @@ public void Insert(int index, PyObject item)
         /// </remarks>
         public void Reverse()
         {
-            int r = Runtime.PyList_Reverse(obj);
+            int r = Runtime.PyList_Reverse(this.Reference);
             if (r < 0)
             {
                 throw new PythonException();
@@ -167,7 +167,7 @@ public void Reverse()
         /// </remarks>
         public void Sort()
         {
-            int r = Runtime.PyList_Sort(obj);
+            int r = Runtime.PyList_Sort(this.Reference);
             if (r < 0)
             {
                 throw new PythonException();
diff --git a/src/runtime/pyobject.cs b/src/runtime/pyobject.cs
index c86504802..699ebf873 100644
--- a/src/runtime/pyobject.cs
+++ b/src/runtime/pyobject.cs
@@ -3,6 +3,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Dynamic;
+using System.Linq;
 using System.Linq.Expressions;
 
 namespace Python.Runtime
@@ -29,8 +30,8 @@ public class PyObject : DynamicObject, IEnumerable, IPyDisposable
 #endif  
 
         protected internal IntPtr obj = IntPtr.Zero;
-        private bool disposed = false;
-        private bool _finalized = false;
+
+        internal BorrowedReference Reference => new BorrowedReference(obj);
 
         /// <summary>
         /// PyObject Constructor
@@ -43,7 +44,26 @@ public class PyObject : DynamicObject, IEnumerable, IPyDisposable
         /// </remarks>
         public PyObject(IntPtr ptr)
         {
+            if (ptr == IntPtr.Zero) throw new ArgumentNullException(nameof(ptr));
+
             obj = ptr;
+            Finalizer.Instance.ThrottledCollect();
+#if TRACE_ALLOC
+            Traceback = new StackTrace(1);
+#endif
+        }
+
+        /// <summary>
+        /// Creates new <see cref="PyObject"/> pointing to the same object as
+        /// the <paramref name="reference"/>. Increments refcount, allowing <see cref="PyObject"/>
+        /// to have ownership over its own reference.
+        /// </summary>
+        internal PyObject(BorrowedReference reference)
+        {
+            if (reference.IsNull) throw new ArgumentNullException(nameof(reference));
+
+            obj = Runtime.SelfIncRef(reference.DangerousGetAddress());
+            Finalizer.Instance.ThrottledCollect();
 #if TRACE_ALLOC
             Traceback = new StackTrace(1);
 #endif
@@ -51,9 +71,10 @@ public PyObject(IntPtr ptr)
 
         // Protected default constructor to allow subclasses to manage
         // initialization in different ways as appropriate.
-
+        [Obsolete("Please, always use PyObject(*Reference)")]
         protected PyObject()
         {
+            Finalizer.Instance.ThrottledCollect();
 #if TRACE_ALLOC
             Traceback = new StackTrace(1);
 #endif
@@ -67,12 +88,6 @@ protected PyObject()
             {
                 return;
             }
-            if (_finalized || disposed)
-            {
-                return;
-            }
-            // Prevent a infinity loop by calling GC.WaitForPendingFinalizers
-            _finalized = true;
             Finalizer.Instance.AddFinalizedObject(this);
         }
 
@@ -91,7 +106,8 @@ public IntPtr Handle
 
 
         /// <summary>
-        /// FromManagedObject Method
+        /// Gets raw Python proxy for this object (bypasses all conversions,
+        /// except <c>null</c> &lt;==&gt; <c>None</c>)
         /// </summary>
         /// <remarks>
         /// Given an arbitrary managed object, return a Python instance that
@@ -162,17 +178,41 @@ public T As<T>()
         /// </remarks>
         protected virtual void Dispose(bool disposing)
         {
-            if (!disposed)
+            if (this.obj == IntPtr.Zero)
             {
-                if (Runtime.Py_IsInitialized() > 0 && !Runtime.IsFinalizing)
+                return;
+            }
+
+            if (Runtime.Py_IsInitialized() == 0)
+                throw new InvalidOperationException("Python runtime must be initialized");
+
+            if (!Runtime.IsFinalizing)
+            {
+                long refcount = Runtime.Refcount(this.obj);
+                Debug.Assert(refcount > 0, "Object refcount is 0 or less");
+
+                if (refcount == 1)
                 {
-                    IntPtr gs = PythonEngine.AcquireLock();
-                    Runtime.XDecref(obj);
-                    obj = IntPtr.Zero;
-                    PythonEngine.ReleaseLock(gs);
+                    Runtime.PyErr_Fetch(out var errType, out var errVal, out var traceback);
+
+                    try
+                    {
+                        Runtime.XDecref(this.obj);
+                        Runtime.CheckExceptionOccurred();
+                    }
+                    finally
+                    {
+                        // Python requires finalizers to preserve exception:
+                        // https://docs.python.org/3/extending/newtypes.html#finalization-and-de-allocation
+                        Runtime.PyErr_Restore(errType, errVal, traceback);
+                    }
+                }
+                else
+                {
+                    Runtime.XDecref(this.obj);
                 }
-                disposed = true;
             }
+            this.obj = IntPtr.Zero;
         }
 
         public void Dispose()
@@ -209,6 +249,8 @@ public PyObject GetPythonType()
         /// </remarks>
         public bool TypeCheck(PyObject typeOrClass)
         {
+            if (typeOrClass == null) throw new ArgumentNullException(nameof(typeOrClass));
+
             return Runtime.PyObject_TypeCheck(obj, typeOrClass.obj);
         }
 
@@ -221,6 +263,8 @@ public bool TypeCheck(PyObject typeOrClass)
         /// </remarks>
         public bool HasAttr(string name)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             return Runtime.PyObject_HasAttrString(obj, name) != 0;
         }
 
@@ -234,6 +278,8 @@ public bool HasAttr(string name)
         /// </remarks>
         public bool HasAttr(PyObject name)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             return Runtime.PyObject_HasAttr(obj, name.obj) != 0;
         }
 
@@ -247,6 +293,8 @@ public bool HasAttr(PyObject name)
         /// </remarks>
         public PyObject GetAttr(string name)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             IntPtr op = Runtime.PyObject_GetAttrString(obj, name);
             if (op == IntPtr.Zero)
             {
@@ -257,7 +305,7 @@ public PyObject GetAttr(string name)
 
 
         /// <summary>
-        /// GetAttr Method
+        /// GetAttr Method. Returns fallback value if getting attribute fails for any reason.
         /// </summary>
         /// <remarks>
         /// Returns the named attribute of the Python object, or the given
@@ -265,6 +313,8 @@ public PyObject GetAttr(string name)
         /// </remarks>
         public PyObject GetAttr(string name, PyObject _default)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             IntPtr op = Runtime.PyObject_GetAttrString(obj, name);
             if (op == IntPtr.Zero)
             {
@@ -285,6 +335,8 @@ public PyObject GetAttr(string name, PyObject _default)
         /// </remarks>
         public PyObject GetAttr(PyObject name)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             IntPtr op = Runtime.PyObject_GetAttr(obj, name.obj);
             if (op == IntPtr.Zero)
             {
@@ -304,6 +356,8 @@ public PyObject GetAttr(PyObject name)
         /// </remarks>
         public PyObject GetAttr(PyObject name, PyObject _default)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             IntPtr op = Runtime.PyObject_GetAttr(obj, name.obj);
             if (op == IntPtr.Zero)
             {
@@ -323,6 +377,9 @@ public PyObject GetAttr(PyObject name, PyObject _default)
         /// </remarks>
         public void SetAttr(string name, PyObject value)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (value == null) throw new ArgumentNullException(nameof(value));
+
             int r = Runtime.PyObject_SetAttrString(obj, name, value.obj);
             if (r < 0)
             {
@@ -341,6 +398,9 @@ public void SetAttr(string name, PyObject value)
         /// </remarks>
         public void SetAttr(PyObject name, PyObject value)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (value == null) throw new ArgumentNullException(nameof(value));
+
             int r = Runtime.PyObject_SetAttr(obj, name.obj, value.obj);
             if (r < 0)
             {
@@ -358,6 +418,8 @@ public void SetAttr(PyObject name, PyObject value)
         /// </remarks>
         public void DelAttr(string name)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             int r = Runtime.PyObject_SetAttrString(obj, name, IntPtr.Zero);
             if (r < 0)
             {
@@ -376,6 +438,8 @@ public void DelAttr(string name)
         /// </remarks>
         public void DelAttr(PyObject name)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+
             int r = Runtime.PyObject_SetAttr(obj, name.obj, IntPtr.Zero);
             if (r < 0)
             {
@@ -394,6 +458,8 @@ public void DelAttr(PyObject name)
         /// </remarks>
         public virtual PyObject GetItem(PyObject key)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+
             IntPtr op = Runtime.PyObject_GetItem(obj, key.obj);
             if (op == IntPtr.Zero)
             {
@@ -413,6 +479,8 @@ public virtual PyObject GetItem(PyObject key)
         /// </remarks>
         public virtual PyObject GetItem(string key)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+
             using (var pyKey = new PyString(key))
             {
                 return GetItem(pyKey);
@@ -447,6 +515,9 @@ public virtual PyObject GetItem(int index)
         /// </remarks>
         public virtual void SetItem(PyObject key, PyObject value)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+            if (value == null) throw new ArgumentNullException(nameof(value));
+
             int r = Runtime.PyObject_SetItem(obj, key.obj, value.obj);
             if (r < 0)
             {
@@ -465,6 +536,9 @@ public virtual void SetItem(PyObject key, PyObject value)
         /// </remarks>
         public virtual void SetItem(string key, PyObject value)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+            if (value == null) throw new ArgumentNullException(nameof(value));
+
             using (var pyKey = new PyString(key))
             {
                 SetItem(pyKey, value);
@@ -482,6 +556,8 @@ public virtual void SetItem(string key, PyObject value)
         /// </remarks>
         public virtual void SetItem(int index, PyObject value)
         {
+            if (value == null) throw new ArgumentNullException(nameof(value));
+
             using (var pyindex = new PyInt(index))
             {
                 SetItem(pyindex, value);
@@ -499,6 +575,8 @@ public virtual void SetItem(int index, PyObject value)
         /// </remarks>
         public virtual void DelItem(PyObject key)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+
             int r = Runtime.PyObject_DelItem(obj, key.obj);
             if (r < 0)
             {
@@ -517,6 +595,8 @@ public virtual void DelItem(PyObject key)
         /// </remarks>
         public virtual void DelItem(string key)
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+
             using (var pyKey = new PyString(key))
             {
                 DelItem(pyKey);
@@ -639,10 +719,13 @@ public IEnumerator GetEnumerator()
         /// </summary>
         /// <remarks>
         /// Invoke the callable object with the given arguments, passed as a
-        /// PyObject[]. A PythonException is raised if the invokation fails.
+        /// PyObject[]. A PythonException is raised if the invocation fails.
         /// </remarks>
         public PyObject Invoke(params PyObject[] args)
         {
+            if (args == null) throw new ArgumentNullException(nameof(args));
+            if (args.Contains(null)) throw new ArgumentNullException();
+
             var t = new PyTuple(args);
             IntPtr r = Runtime.PyObject_Call(obj, t.obj, IntPtr.Zero);
             t.Dispose();
@@ -659,10 +742,12 @@ public PyObject Invoke(params PyObject[] args)
         /// </summary>
         /// <remarks>
         /// Invoke the callable object with the given arguments, passed as a
-        /// Python tuple. A PythonException is raised if the invokation fails.
+        /// Python tuple. A PythonException is raised if the invocation fails.
         /// </remarks>
         public PyObject Invoke(PyTuple args)
         {
+            if (args == null) throw new ArgumentNullException(nameof(args));
+
             IntPtr r = Runtime.PyObject_Call(obj, args.obj, IntPtr.Zero);
             if (r == IntPtr.Zero)
             {
@@ -677,12 +762,15 @@ public PyObject Invoke(PyTuple args)
         /// </summary>
         /// <remarks>
         /// Invoke the callable object with the given positional and keyword
-        /// arguments. A PythonException is raised if the invokation fails.
+        /// arguments. A PythonException is raised if the invocation fails.
         /// </remarks>
         public PyObject Invoke(PyObject[] args, PyDict kw)
         {
+            if (args == null) throw new ArgumentNullException(nameof(args));
+            if (args.Contains(null)) throw new ArgumentNullException();
+
             var t = new PyTuple(args);
-            IntPtr r = Runtime.PyObject_Call(obj, t.obj, kw != null ? kw.obj : IntPtr.Zero);
+            IntPtr r = Runtime.PyObject_Call(obj, t.obj, kw?.obj ?? IntPtr.Zero);
             t.Dispose();
             if (r == IntPtr.Zero)
             {
@@ -697,11 +785,13 @@ public PyObject Invoke(PyObject[] args, PyDict kw)
         /// </summary>
         /// <remarks>
         /// Invoke the callable object with the given positional and keyword
-        /// arguments. A PythonException is raised if the invokation fails.
+        /// arguments. A PythonException is raised if the invocation fails.
         /// </remarks>
         public PyObject Invoke(PyTuple args, PyDict kw)
         {
-            IntPtr r = Runtime.PyObject_Call(obj, args.obj, kw != null ? kw.obj : IntPtr.Zero);
+            if (args == null) throw new ArgumentNullException(nameof(args));
+
+            IntPtr r = Runtime.PyObject_Call(obj, args.obj, kw?.obj ?? IntPtr.Zero);
             if (r == IntPtr.Zero)
             {
                 throw new PythonException();
@@ -715,10 +805,14 @@ public PyObject Invoke(PyTuple args, PyDict kw)
         /// </summary>
         /// <remarks>
         /// Invoke the named method of the object with the given arguments.
-        /// A PythonException is raised if the invokation is unsuccessful.
+        /// A PythonException is raised if the invocation is unsuccessful.
         /// </remarks>
         public PyObject InvokeMethod(string name, params PyObject[] args)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (args == null) throw new ArgumentNullException(nameof(args));
+            if (args.Contains(null)) throw new ArgumentNullException();
+
             PyObject method = GetAttr(name);
             PyObject result = method.Invoke(args);
             method.Dispose();
@@ -731,10 +825,51 @@ public PyObject InvokeMethod(string name, params PyObject[] args)
         /// </summary>
         /// <remarks>
         /// Invoke the named method of the object with the given arguments.
-        /// A PythonException is raised if the invokation is unsuccessful.
+        /// A PythonException is raised if the invocation is unsuccessful.
         /// </remarks>
         public PyObject InvokeMethod(string name, PyTuple args)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (args == null) throw new ArgumentNullException(nameof(args));
+
+            PyObject method = GetAttr(name);
+            PyObject result = method.Invoke(args);
+            method.Dispose();
+            return result;
+        }
+
+        /// <summary>
+        /// InvokeMethod Method
+        /// </summary>
+        /// <remarks>
+        /// Invoke the named method of the object with the given arguments.
+        /// A PythonException is raised if the invocation is unsuccessful.
+        /// </remarks>
+        public PyObject InvokeMethod(PyObject name, params PyObject[] args)
+        {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (args == null) throw new ArgumentNullException(nameof(args));
+            if (args.Contains(null)) throw new ArgumentNullException();
+
+            PyObject method = GetAttr(name);
+            PyObject result = method.Invoke(args);
+            method.Dispose();
+            return result;
+        }
+
+
+        /// <summary>
+        /// InvokeMethod Method
+        /// </summary>
+        /// <remarks>
+        /// Invoke the named method of the object with the given arguments.
+        /// A PythonException is raised if the invocation is unsuccessful.
+        /// </remarks>
+        public PyObject InvokeMethod(PyObject name, PyTuple args)
+        {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (args == null) throw new ArgumentNullException(nameof(args));
+
             PyObject method = GetAttr(name);
             PyObject result = method.Invoke(args);
             method.Dispose();
@@ -748,10 +883,14 @@ public PyObject InvokeMethod(string name, PyTuple args)
         /// <remarks>
         /// Invoke the named method of the object with the given arguments
         /// and keyword arguments. Keyword args are passed as a PyDict object.
-        /// A PythonException is raised if the invokation is unsuccessful.
+        /// A PythonException is raised if the invocation is unsuccessful.
         /// </remarks>
         public PyObject InvokeMethod(string name, PyObject[] args, PyDict kw)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (args == null) throw new ArgumentNullException(nameof(args));
+            if (args.Contains(null)) throw new ArgumentNullException();
+
             PyObject method = GetAttr(name);
             PyObject result = method.Invoke(args, kw);
             method.Dispose();
@@ -765,10 +904,13 @@ public PyObject InvokeMethod(string name, PyObject[] args, PyDict kw)
         /// <remarks>
         /// Invoke the named method of the object with the given arguments
         /// and keyword arguments. Keyword args are passed as a PyDict object.
-        /// A PythonException is raised if the invokation is unsuccessful.
+        /// A PythonException is raised if the invocation is unsuccessful.
         /// </remarks>
         public PyObject InvokeMethod(string name, PyTuple args, PyDict kw)
         {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (args == null) throw new ArgumentNullException(nameof(args));
+
             PyObject method = GetAttr(name);
             PyObject result = method.Invoke(args, kw);
             method.Dispose();
@@ -785,6 +927,8 @@ public PyObject InvokeMethod(string name, PyTuple args, PyDict kw)
         /// </remarks>
         public bool IsInstance(PyObject typeOrClass)
         {
+            if (typeOrClass == null) throw new ArgumentNullException(nameof(typeOrClass));
+
             int r = Runtime.PyObject_IsInstance(obj, typeOrClass.obj);
             if (r < 0)
             {
@@ -804,6 +948,8 @@ public bool IsInstance(PyObject typeOrClass)
         /// </remarks>
         public bool IsSubclass(PyObject typeOrClass)
         {
+            if (typeOrClass == null) throw new ArgumentNullException(nameof(typeOrClass));
+
             int r = Runtime.PyObject_IsSubclass(obj, typeOrClass.obj);
             if (r < 0)
             {
@@ -852,6 +998,10 @@ public bool IsTrue()
             return Runtime.PyObject_IsTrue(obj) != 0;
         }
 
+        /// <summary>
+        /// Return true if the object is None
+        /// </summary>
+        public bool IsNone() => CheckNone(this) == null;
 
         /// <summary>
         /// Dir Method
diff --git a/src/runtime/pyscope.cs b/src/runtime/pyscope.cs
index 4008ce29a..8738824f5 100644
--- a/src/runtime/pyscope.cs
+++ b/src/runtime/pyscope.cs
@@ -278,11 +278,19 @@ public PyObject Eval(string code, PyDict locals = null)
             Check();
             IntPtr _locals = locals == null ? variables : locals.obj;
             var flag = (IntPtr)Runtime.Py_eval_input;
-            IntPtr ptr = Runtime.PyRun_String(
+
+            NewReference reference = Runtime.PyRun_String(
                 code, flag, variables, _locals
             );
-            Runtime.CheckExceptionOccurred();
-            return new PyObject(ptr);
+            try
+            {
+                Runtime.CheckExceptionOccurred();
+                return reference.MoveToPyObject();
+            }
+            finally
+            {
+                reference.Dispose();
+            }
         }
 
         /// <summary>
@@ -316,15 +324,22 @@ public void Exec(string code, PyDict locals = null)
         private void Exec(string code, IntPtr _globals, IntPtr _locals)
         {
             var flag = (IntPtr)Runtime.Py_file_input;
-            IntPtr ptr = Runtime.PyRun_String(
+            NewReference reference = Runtime.PyRun_String(
                 code, flag, _globals, _locals
             );
-            Runtime.CheckExceptionOccurred();
-            if (ptr != Runtime.PyNone)
+
+            try
             {
-                throw new PythonException();
+                Runtime.CheckExceptionOccurred();
+                if (!reference.IsNone())
+                {
+                    throw new PythonException();
+                }
+            }
+            finally
+            {
+                reference.Dispose();
             }
-            Runtime.XDecref(ptr);
         }
 
         /// <summary>
diff --git a/src/runtime/pythonengine.cs b/src/runtime/pythonengine.cs
index c1b663d22..11fc88cd4 100644
--- a/src/runtime/pythonengine.cs
+++ b/src/runtime/pythonengine.cs
@@ -80,6 +80,7 @@ public static string PythonHome
             }
             set
             {
+                // this value is null in the beginning
                 Marshal.FreeHGlobal(_pythonHome);
                 _pythonHome = UcsMarshaler.Py3UnicodePy2StringtoPtr(value);
                 Runtime.Py_SetPythonHome(_pythonHome);
@@ -130,6 +131,16 @@ public static string Compiler
             get { return Marshal.PtrToStringAnsi(Runtime.Py_GetCompiler()); }
         }
 
+        /// <summary>
+        /// Set the NoSiteFlag to disable loading the site module.
+        /// Must be called before Initialize.
+        /// https://docs.python.org/3/c-api/init.html#c.Py_NoSiteFlag
+        /// </summary>
+        public static void SetNoSiteFlag()
+        {
+            Runtime.SetNoSiteFlag();
+        }
+
         public static int RunSimpleString(string code)
         {
             return Runtime.PyRun_SimpleString(code);
@@ -318,6 +329,8 @@ public static void Shutdown()
 
                 ExecuteShutdownHandlers();
 
+                PyObjectConversions.Reset();
+
                 initialized = false;
             }
         }
@@ -493,7 +506,7 @@ public static PyObject ReloadModule(PyObject module)
         /// </remarks>
         public static PyObject ModuleFromString(string name, string code)
         {
-            IntPtr c = Runtime.Py_CompileString(code, "none", (IntPtr)257);
+            IntPtr c = Runtime.Py_CompileString(code, "none", (int)RunFlagType.File);
             Runtime.CheckExceptionOccurred();
             IntPtr m = Runtime.PyImport_ExecCodeModule(name, c);
             Runtime.CheckExceptionOccurred();
@@ -502,7 +515,7 @@ public static PyObject ModuleFromString(string name, string code)
 
         public static PyObject Compile(string code, string filename = "", RunFlagType mode = RunFlagType.File)
         {
-            var flag = (IntPtr)mode;
+            var flag = (int)mode;
             IntPtr ptr = Runtime.Py_CompileString(code, filename, flag);
             Runtime.CheckExceptionOccurred();
             return new PyObject(ptr);
@@ -531,12 +544,13 @@ public static PyObject Eval(string code, IntPtr? globals = null, IntPtr? locals
         /// </remarks>
         public static void Exec(string code, IntPtr? globals = null, IntPtr? locals = null)
         {
-            PyObject result = RunString(code, globals, locals, RunFlagType.File);
-            if (result.obj != Runtime.PyNone)
+            using (PyObject result = RunString(code, globals, locals, RunFlagType.File))
             {
-                throw new PythonException();
+                if (result.obj != Runtime.PyNone)
+                {
+                    throw new PythonException();
+                }
             }
-            result.Dispose();
         }
 
 
@@ -582,13 +596,20 @@ internal static PyObject RunString(string code, IntPtr? globals, IntPtr? locals,
 
             try
             {
-                IntPtr result = Runtime.PyRun_String(
+                NewReference result = Runtime.PyRun_String(
                     code, (IntPtr)flag, globals.Value, locals.Value
                 );
 
-                Runtime.CheckExceptionOccurred();
+                try
+                {
+                    Runtime.CheckExceptionOccurred();
 
-                return new PyObject(result);
+                    return result.MoveToPyObject();
+                }
+                finally
+                {
+                    result.Dispose();
+                }
             }
             finally
             {
@@ -600,7 +621,7 @@ internal static PyObject RunString(string code, IntPtr? globals, IntPtr? locals,
         }
     }
 
-    public enum RunFlagType
+    public enum RunFlagType : int
     {
         Single = 256,
         File = 257, /* Py_file_input */
@@ -633,7 +654,8 @@ public static PyScope CreateScope(string name)
         
         public class GILState : IDisposable
         {
-            private IntPtr state;
+            private readonly IntPtr state;
+            private bool isDisposed;
 
             internal GILState()
             {
@@ -642,8 +664,11 @@ internal GILState()
 
             public void Dispose()
             {
+                if (this.isDisposed) return;
+
                 PythonEngine.ReleaseLock(state);
                 GC.SuppressFinalize(this);
+                this.isDisposed = true;
             }
 
             ~GILState()
@@ -744,11 +769,14 @@ public static void With(PyObject obj, Action<dynamic> Body)
             catch (PythonException e)
             {
                 ex = e;
-                type = ex.PyType;
-                val = ex.PyValue;
-                traceBack = ex.PyTB;
+                type = ex.PyType.Coalesce(type);
+                val = ex.PyValue.Coalesce(val);
+                traceBack = ex.PyTB.Coalesce(traceBack);
             }
 
+            Runtime.XIncref(type);
+            Runtime.XIncref(val);
+            Runtime.XIncref(traceBack);
             var exitResult = obj.InvokeMethod("__exit__", new PyObject(type), new PyObject(val), new PyObject(traceBack));
 
             if (ex != null && !exitResult.IsTrue()) throw ex;
diff --git a/src/runtime/pythonexception.cs b/src/runtime/pythonexception.cs
index 295a63b3d..8efdccc91 100644
--- a/src/runtime/pythonexception.cs
+++ b/src/runtime/pythonexception.cs
@@ -1,4 +1,6 @@
 using System;
+using System.Runtime.CompilerServices;
+using System.Text;
 
 namespace Python.Runtime
 {
@@ -20,7 +22,7 @@ public class PythonException : System.Exception, IPyDisposable
         public PythonException()
         {
             IntPtr gs = PythonEngine.AcquireLock();
-            Runtime.PyErr_Fetch(ref _pyType, ref _pyValue, ref _pyTB);
+            Runtime.PyErr_Fetch(out _pyType, out _pyValue, out _pyTB);
             if (_pyType != IntPtr.Zero && _pyValue != IntPtr.Zero)
             {
                 string type;
@@ -144,6 +146,47 @@ public string PythonTypeName
             get { return _pythonTypeName; }
         }
 
+        /// <summary>
+        /// Formats this PythonException object into a message as would be printed
+        /// out via the Python console. See traceback.format_exception
+        /// </summary>
+        public string Format()
+        {
+            string res;
+            IntPtr gs = PythonEngine.AcquireLock();
+            try
+            {
+                if (_pyTB != IntPtr.Zero && _pyType != IntPtr.Zero && _pyValue != IntPtr.Zero)
+                {
+                    Runtime.XIncref(_pyType);
+                    Runtime.XIncref(_pyValue);
+                    Runtime.XIncref(_pyTB);
+                    using (PyObject pyType = new PyObject(_pyType))
+                    using (PyObject pyValue = new PyObject(_pyValue))
+                    using (PyObject pyTB = new PyObject(_pyTB))
+                    using (PyObject tb_mod = PythonEngine.ImportModule("traceback"))
+                    {
+                        var buffer = new StringBuilder();
+                        var values = tb_mod.InvokeMethod("format_exception", pyType, pyValue, pyTB);
+                        foreach (PyObject val in values)
+                        {
+                            buffer.Append(val.ToString());
+                        }
+                        res = buffer.ToString();
+                    }
+                }
+                else
+                {
+                    res = StackTrace;
+                }
+            }
+            finally
+            {
+                PythonEngine.ReleaseLock(gs);
+            }
+            return res;
+        }
+
         /// <summary>
         /// Dispose Method
         /// </summary>
@@ -160,12 +203,23 @@ public void Dispose()
                 if (Runtime.Py_IsInitialized() > 0 && !Runtime.IsFinalizing)
                 {
                     IntPtr gs = PythonEngine.AcquireLock();
-                    Runtime.XDecref(_pyType);
-                    Runtime.XDecref(_pyValue);
+                    if (_pyType != IntPtr.Zero)
+                    {
+                        Runtime.XDecref(_pyType);
+                        _pyType= IntPtr.Zero;
+                    }
+
+                    if (_pyValue != IntPtr.Zero)
+                    {
+                        Runtime.XDecref(_pyValue);
+                        _pyValue = IntPtr.Zero;
+                    }
+
                     // XXX Do we ever get TraceBack? //
                     if (_pyTB != IntPtr.Zero)
                     {
                         Runtime.XDecref(_pyTB);
+                        _pyTB = IntPtr.Zero;
                     }
                     PythonEngine.ReleaseLock(gs);
                 }
@@ -190,5 +244,21 @@ public static bool Matches(IntPtr ob)
         {
             return Runtime.PyErr_ExceptionMatches(ob) != 0;
         }
+
+        public static void ThrowIfIsNull(IntPtr ob)
+        {
+            if (ob == IntPtr.Zero)
+            {
+                throw new PythonException();
+            }
+        }
+
+        public static void ThrowIfIsNotZero(int value)
+        {
+            if (value != 0)
+            {
+                throw new PythonException();
+            }
+        }
     }
 }
diff --git a/src/runtime/resources/clr.py b/src/runtime/resources/clr.py
index ddb0d94e8..78fae0d3a 100644
--- a/src/runtime/resources/clr.py
+++ b/src/runtime/resources/clr.py
@@ -2,7 +2,7 @@
 Code in this module gets loaded into the main clr module.
 """
 
-__version__ = "2.4.0"
+__version__ = "2.5.0"
 
 
 class clrproperty(object):
diff --git a/src/runtime/runtime.cs b/src/runtime/runtime.cs
index 7623200e0..8cf24ba70 100644
--- a/src/runtime/runtime.cs
+++ b/src/runtime/runtime.cs
@@ -1,103 +1,15 @@
+using System.Reflection.Emit;
 using System;
+using System.Diagnostics.Contracts;
 using System.Runtime.InteropServices;
 using System.Security;
 using System.Text;
 using System.Threading;
 using System.Collections.Generic;
+using Python.Runtime.Platform;
 
 namespace Python.Runtime
 {
-    [SuppressUnmanagedCodeSecurity]
-    internal static class NativeMethods
-    {
-#if MONO_LINUX || MONO_OSX
-#if NETSTANDARD
-        private static int RTLD_NOW = 0x2;
-#if MONO_LINUX
-        private static int RTLD_GLOBAL = 0x100;
-        private static IntPtr RTLD_DEFAULT = IntPtr.Zero;
-        private const string NativeDll = "libdl.so";
-        public static IntPtr LoadLibrary(string fileName)
-        {
-            return dlopen($"lib{fileName}.so", RTLD_NOW | RTLD_GLOBAL);
-        }
-#elif MONO_OSX
-        private static int RTLD_GLOBAL = 0x8;
-        private const string NativeDll = "/usr/lib/libSystem.dylib";
-        private static IntPtr RTLD_DEFAULT = new IntPtr(-2);
-
-        public static IntPtr LoadLibrary(string fileName)
-        {
-            return dlopen($"lib{fileName}.dylib", RTLD_NOW | RTLD_GLOBAL);
-        }
-#endif
-#else
-        private static int RTLD_NOW = 0x2;
-        private static int RTLD_SHARED = 0x20;
-#if MONO_OSX
-        private static IntPtr RTLD_DEFAULT = new IntPtr(-2);
-        private const string NativeDll = "__Internal";
-#elif MONO_LINUX
-        private static IntPtr RTLD_DEFAULT = IntPtr.Zero;
-        private const string NativeDll = "libdl.so";
-#endif
-
-        public static IntPtr LoadLibrary(string fileName)
-        {
-            return dlopen(fileName, RTLD_NOW | RTLD_SHARED);
-        }
-#endif
-
-
-        public static void FreeLibrary(IntPtr handle)
-        {
-            dlclose(handle);
-        }
-
-        public static IntPtr GetProcAddress(IntPtr dllHandle, string name)
-        {
-            // look in the exe if dllHandle is NULL
-            if (dllHandle == IntPtr.Zero)
-            {
-                dllHandle = RTLD_DEFAULT;
-            }
-
-            // clear previous errors if any
-            dlerror();
-            IntPtr res = dlsym(dllHandle, name);
-            IntPtr errPtr = dlerror();
-            if (errPtr != IntPtr.Zero)
-            {
-                throw new Exception("dlsym: " + Marshal.PtrToStringAnsi(errPtr));
-            }
-            return res;
-        }
-
-        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
-        public static extern IntPtr dlopen(String fileName, int flags);
-
-        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
-        private static extern IntPtr dlsym(IntPtr handle, String symbol);
-
-        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl)]
-        private static extern int dlclose(IntPtr handle);
-
-        [DllImport(NativeDll, CallingConvention = CallingConvention.Cdecl)]
-        private static extern IntPtr dlerror();
-#else // Windows
-        private const string NativeDll = "kernel32.dll";
-
-        [DllImport(NativeDll)]
-        public static extern IntPtr LoadLibrary(string dllToLoad);
-
-        [DllImport(NativeDll)]
-        public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);
-
-        [DllImport(NativeDll)]
-        public static extern bool FreeLibrary(IntPtr hModule);
-#endif
-    }
-
     /// <summary>
     /// Encapsulates the low-level Python C API. Note that it is
     /// the responsibility of the caller to have acquired the GIL
@@ -153,8 +65,11 @@ public class Runtime
 #elif PYTHON37
         internal const string _pyversion = "3.7";
         internal const string _pyver = "37";
+#elif PYTHON38
+        internal const string _pyversion = "3.8";
+        internal const string _pyver = "38";
 #else
-#error You must define one of PYTHON34 to PYTHON37 or PYTHON27
+#error You must define one of PYTHON34 to PYTHON38 or PYTHON27
 #endif
 
 #if MONO_LINUX || MONO_OSX // Linux/macOS use dotted version string
@@ -192,22 +107,11 @@ public class Runtime
         internal static object IsFinalizingLock = new object();
         internal static bool IsFinalizing;
 
-        internal static bool Is32Bit = IntPtr.Size == 4;
+        internal static bool Is32Bit => IntPtr.Size == 4;
 
         // .NET core: System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
         internal static bool IsWindows = Environment.OSVersion.Platform == PlatformID.Win32NT;
 
-        /// <summary>
-        /// Operating system type as reported by Python.
-        /// </summary>
-        public enum OperatingSystemType
-        {
-            Windows,
-            Darwin,
-            Linux,
-            Other
-        }
-
         static readonly Dictionary<string, OperatingSystemType> OperatingSystemTypeMapping = new Dictionary<string, OperatingSystemType>()
         {
             { "Windows", OperatingSystemType.Windows },
@@ -215,22 +119,16 @@ public enum OperatingSystemType
             { "Linux", OperatingSystemType.Linux },
         };
 
-        /// <summary>
-        /// Gets the operating system as reported by python's platform.system().
-        /// </summary>
-        public static OperatingSystemType OperatingSystem { get; private set; }
+        [Obsolete]
+        public static string OperatingSystemName => OperatingSystem.ToString();
+
+        [Obsolete]
+        public static string MachineName => Machine.ToString();
 
         /// <summary>
         /// Gets the operating system as reported by python's platform.system().
         /// </summary>
-        public static string OperatingSystemName { get; private set; }
-
-        public enum MachineType
-        {
-            i386,
-            x86_64,
-            Other
-        };
+        public static OperatingSystemType OperatingSystem { get; private set; }
 
         /// <summary>
         /// Map lower-case version of the python machine name to the processor
@@ -247,6 +145,9 @@ public enum MachineType
             ["amd64"] = MachineType.x86_64,
             ["x64"] = MachineType.x86_64,
             ["em64t"] = MachineType.x86_64,
+            ["armv7l"] = MachineType.armv7l,
+            ["armv8"] = MachineType.armv8,
+            ["aarch64"] = MachineType.aarch64,
         };
 
         /// <summary>
@@ -254,11 +155,6 @@ public enum MachineType
         /// </summary>
         public static MachineType Machine { get; private set; }/* set in Initialize using python's platform.machine */
 
-        /// <summary>
-        /// Gets the machine architecture as reported by python's platform.machine().
-        /// </summary>
-        public static string MachineName { get; private set; }
-
         internal static bool IsPython2 = pyversionnumber < 30;
         internal static bool IsPython3 = pyversionnumber >= 30;
 
@@ -269,6 +165,8 @@ public enum MachineType
         /// </summary>
         internal static readonly Encoding PyEncoding = _UCS == 2 ? Encoding.Unicode : Encoding.UTF32;
 
+        private static PyReferenceCollection _pyRefs = new PyReferenceCollection();
+
         /// <summary>
         /// Initialize the runtime...
         /// </summary>
@@ -287,7 +185,6 @@ internal static void Initialize(bool initSigs = false)
 
             IsFinalizing = false;
 
-            CLRModule.Reset();
             GenericUtil.Reset();
             PyScopeManager.Reset();
             ClassManager.Reset();
@@ -295,122 +192,139 @@ internal static void Initialize(bool initSigs = false)
             TypeManager.Reset();
 
             IntPtr op;
-            IntPtr dict;
-            if (IsPython3)
-            {
-                op = PyImport_ImportModule("builtins");
-                dict = PyObject_GetAttrString(op, "__dict__");
-            }
-            else // Python2
             {
-                dict = PyImport_GetModuleDict();
-                op = PyDict_GetItemString(dict, "__builtin__");
+                var builtins = GetBuiltins();
+                SetPyMember(ref PyNotImplemented, PyObject_GetAttrString(builtins, "NotImplemented"),
+                    () => PyNotImplemented = IntPtr.Zero);
+
+                SetPyMember(ref PyBaseObjectType, PyObject_GetAttrString(builtins, "object"),
+                    () => PyBaseObjectType = IntPtr.Zero);
+
+                SetPyMember(ref PyNone, PyObject_GetAttrString(builtins, "None"),
+                    () => PyNone = IntPtr.Zero);
+                SetPyMember(ref PyTrue, PyObject_GetAttrString(builtins, "True"),
+                    () => PyTrue = IntPtr.Zero);
+                SetPyMember(ref PyFalse, PyObject_GetAttrString(builtins, "False"),
+                    () => PyFalse = IntPtr.Zero);
+
+                SetPyMember(ref PyBoolType, PyObject_Type(PyTrue),
+                    () => PyBoolType = IntPtr.Zero);
+                SetPyMember(ref PyNoneType, PyObject_Type(PyNone),
+                    () => PyNoneType = IntPtr.Zero);
+                SetPyMember(ref PyTypeType, PyObject_Type(PyNoneType),
+                    () => PyTypeType = IntPtr.Zero);
+
+                op = PyObject_GetAttrString(builtins, "len");
+                SetPyMember(ref PyMethodType, PyObject_Type(op),
+                    () => PyMethodType = IntPtr.Zero);
+                XDecref(op);
+
+                // For some arcane reason, builtins.__dict__.__setitem__ is *not*
+                // a wrapper_descriptor, even though dict.__setitem__ is.
+                //
+                // object.__init__ seems safe, though.
+                op = PyObject_GetAttrString(PyBaseObjectType, "__init__");
+                SetPyMember(ref PyWrapperDescriptorType, PyObject_Type(op),
+                    () => PyWrapperDescriptorType = IntPtr.Zero);
+                XDecref(op);
+
+                SetPyMember(ref PySuper_Type, PyObject_GetAttrString(builtins, "super"),
+                    () => PySuper_Type = IntPtr.Zero);
+
+                XDecref(builtins);
             }
-            PyNotImplemented = PyObject_GetAttrString(op, "NotImplemented");
-            PyBaseObjectType = PyObject_GetAttrString(op, "object");
-
-            PyModuleType = PyObject_Type(op);
-            PyNone = PyObject_GetAttrString(op, "None");
-            PyTrue = PyObject_GetAttrString(op, "True");
-            PyFalse = PyObject_GetAttrString(op, "False");
-
-            PyBoolType = PyObject_Type(PyTrue);
-            PyNoneType = PyObject_Type(PyNone);
-            PyTypeType = PyObject_Type(PyNoneType);
-
-            op = PyObject_GetAttrString(dict, "keys");
-            PyMethodType = PyObject_Type(op);
-            XDecref(op);
-
-            // For some arcane reason, builtins.__dict__.__setitem__ is *not*
-            // a wrapper_descriptor, even though dict.__setitem__ is.
-            //
-            // object.__init__ seems safe, though.
-            op = PyObject_GetAttrString(PyBaseObjectType, "__init__");
-            PyWrapperDescriptorType = PyObject_Type(op);
-            XDecref(op);
-
-#if PYTHON3
-            XDecref(dict);
-#endif
 
             op = PyString_FromString("string");
-            PyStringType = PyObject_Type(op);
+            SetPyMember(ref PyStringType, PyObject_Type(op),
+                () => PyStringType = IntPtr.Zero);
             XDecref(op);
 
             op = PyUnicode_FromString("unicode");
-            PyUnicodeType = PyObject_Type(op);
+            SetPyMember(ref PyUnicodeType, PyObject_Type(op),
+                () => PyUnicodeType = IntPtr.Zero);
             XDecref(op);
 
 #if PYTHON3
             op = PyBytes_FromString("bytes");
-            PyBytesType = PyObject_Type(op);
+            SetPyMember(ref PyBytesType, PyObject_Type(op),
+                () => PyBytesType = IntPtr.Zero);
             XDecref(op);
 #endif
 
             op = PyTuple_New(0);
-            PyTupleType = PyObject_Type(op);
+            SetPyMember(ref PyTupleType, PyObject_Type(op),
+                () => PyTupleType = IntPtr.Zero);
             XDecref(op);
 
             op = PyList_New(0);
-            PyListType = PyObject_Type(op);
+            SetPyMember(ref PyListType, PyObject_Type(op),
+                () => PyListType = IntPtr.Zero);
             XDecref(op);
 
             op = PyDict_New();
-            PyDictType = PyObject_Type(op);
+            SetPyMember(ref PyDictType, PyObject_Type(op),
+                () => PyDictType = IntPtr.Zero);
             XDecref(op);
 
             op = PyInt_FromInt32(0);
-            PyIntType = PyObject_Type(op);
+            SetPyMember(ref PyIntType, PyObject_Type(op),
+                () => PyIntType = IntPtr.Zero);
             XDecref(op);
 
             op = PyLong_FromLong(0);
-            PyLongType = PyObject_Type(op);
+            SetPyMember(ref PyLongType, PyObject_Type(op),
+                () => PyLongType = IntPtr.Zero);
             XDecref(op);
 
             op = PyFloat_FromDouble(0);
-            PyFloatType = PyObject_Type(op);
+            SetPyMember(ref PyFloatType, PyObject_Type(op),
+                () => PyFloatType = IntPtr.Zero);
             XDecref(op);
 
-#if PYTHON3
+#if !PYTHON2
             PyClassType = IntPtr.Zero;
             PyInstanceType = IntPtr.Zero;
-#elif PYTHON2
-            IntPtr s = PyString_FromString("_temp");
-            IntPtr d = PyDict_New();
+#else
+            {
+                IntPtr s = PyString_FromString("_temp");
+                IntPtr d = PyDict_New();
 
-            IntPtr c = PyClass_New(IntPtr.Zero, d, s);
-            PyClassType = PyObject_Type(c);
+                IntPtr c = PyClass_New(IntPtr.Zero, d, s);
+                SetPyMember(ref PyClassType, PyObject_Type(c),
+                    () => PyClassType = IntPtr.Zero);
 
-            IntPtr i = PyInstance_New(c, IntPtr.Zero, IntPtr.Zero);
-            PyInstanceType = PyObject_Type(i);
+                IntPtr i = PyInstance_New(c, IntPtr.Zero, IntPtr.Zero);
+                SetPyMember(ref PyInstanceType, PyObject_Type(i),
+                    () => PyInstanceType = IntPtr.Zero);
 
-            XDecref(s);
-            XDecref(i);
-            XDecref(c);
-            XDecref(d);
+                XDecref(s);
+                XDecref(i);
+                XDecref(c);
+                XDecref(d);
+            }
 #endif
 
             Error = new IntPtr(-1);
 
+            // Initialize data about the platform we're running on. We need
+            // this for the type manager and potentially other details. Must
+            // happen after caching the python types, above.
+            InitializePlatformData();
+
             IntPtr dllLocal = IntPtr.Zero;
+            var loader = LibraryLoader.Get(OperatingSystem);
 
             if (_PythonDll != "__Internal")
             {
-                dllLocal = NativeMethods.LoadLibrary(_PythonDll);
+                dllLocal = loader.Load(_PythonDll);
             }
-            _PyObject_NextNotImplemented = NativeMethods.GetProcAddress(dllLocal, "_PyObject_NextNotImplemented");
+            _PyObject_NextNotImplemented = loader.GetFunction(dllLocal, "_PyObject_NextNotImplemented");
+            PyModuleType = loader.GetFunction(dllLocal, "PyModule_Type");
 
-#if !(MONO_LINUX || MONO_OSX)
             if (dllLocal != IntPtr.Zero)
             {
-                NativeMethods.FreeLibrary(dllLocal);
+                loader.Free(dllLocal);
             }
-#endif
-            // Initialize data about the platform we're running on. We need
-            // this for the type manager and potentially other details. Must
-            // happen after caching the python types, above.
-            InitializePlatformData();
 
             // Initialize modules that depend on the runtime class.
             AssemblyManager.Initialize();
@@ -423,7 +337,7 @@ internal static void Initialize(bool initSigs = false)
             string rtdir = RuntimeEnvironment.GetRuntimeDirectory();
             IntPtr path = PySys_GetObject("path");
             IntPtr item = PyString_FromString(rtdir);
-            PyList_Append(path, item);
+            PyList_Append(new BorrowedReference(path), item);
             XDecref(item);
             AssemblyManager.UpdatePath();
         }
@@ -438,6 +352,7 @@ internal static void Initialize(bool initSigs = false)
         /// </summary>
         private static void InitializePlatformData()
         {
+#if !NETSTANDARD
             IntPtr op;
             IntPtr fn;
             IntPtr platformModule = PyImport_ImportModule("platform");
@@ -445,13 +360,13 @@ private static void InitializePlatformData()
 
             fn = PyObject_GetAttrString(platformModule, "system");
             op = PyObject_Call(fn, emptyTuple, IntPtr.Zero);
-            OperatingSystemName = GetManagedString(op);
+            string operatingSystemName = GetManagedString(op);
             XDecref(op);
             XDecref(fn);
 
             fn = PyObject_GetAttrString(platformModule, "machine");
             op = PyObject_Call(fn, emptyTuple, IntPtr.Zero);
-            MachineName = GetManagedString(op);
+            string machineName = GetManagedString(op);
             XDecref(op);
             XDecref(fn);
 
@@ -461,18 +376,47 @@ private static void InitializePlatformData()
             // Now convert the strings into enum values so we can do switch
             // statements rather than constant parsing.
             OperatingSystemType OSType;
-            if (!OperatingSystemTypeMapping.TryGetValue(OperatingSystemName, out OSType))
+            if (!OperatingSystemTypeMapping.TryGetValue(operatingSystemName, out OSType))
             {
                 OSType = OperatingSystemType.Other;
             }
             OperatingSystem = OSType;
 
             MachineType MType;
-            if (!MachineTypeMapping.TryGetValue(MachineName.ToLower(), out MType))
+            if (!MachineTypeMapping.TryGetValue(machineName.ToLower(), out MType))
             {
                 MType = MachineType.Other;
             }
             Machine = MType;
+#else
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
+                OperatingSystem = OperatingSystemType.Linux;
+            else if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+                OperatingSystem = OperatingSystemType.Darwin;
+            else if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                OperatingSystem = OperatingSystemType.Windows;
+            else
+                OperatingSystem = OperatingSystemType.Other;
+
+            switch (RuntimeInformation.ProcessArchitecture)
+            {
+                case Architecture.X86:
+                    Machine = MachineType.i386;
+                    break;
+                case Architecture.X64:
+                    Machine = MachineType.x86_64;
+                    break;
+                case Architecture.Arm:
+                    Machine = MachineType.armv7l;
+                    break;
+                case Architecture.Arm64:
+                    Machine = MachineType.aarch64;
+                    break;
+                default:
+                    Machine = MachineType.Other;
+                    break;
+            }
+#endif
         }
 
         internal static void Shutdown()
@@ -481,6 +425,9 @@ internal static void Shutdown()
             Exceptions.Shutdown();
             ImportHook.Shutdown();
             Finalizer.Shutdown();
+            // TOOD: PyCLRMetaType's release operation still in #958
+            PyCLRMetaType = IntPtr.Zero;
+            ResetPyMembers();
             Py_Finalize();
         }
 
@@ -494,6 +441,19 @@ internal static int AtExit()
             return 0;
         }
 
+        private static void SetPyMember(ref IntPtr obj, IntPtr value, Action onRelease)
+        {
+            // XXX: For current usages, value should not be null.
+            PythonException.ThrowIfIsNull(value);
+            obj = value;
+            _pyRefs.Add(value, onRelease);
+        }
+
+        private static void ResetPyMembers()
+        {
+            _pyRefs.Release();
+        }
+
         internal static IntPtr Py_single_input = (IntPtr)256;
         internal static IntPtr Py_file_input = (IntPtr)257;
         internal static IntPtr Py_eval_input = (IntPtr)258;
@@ -502,6 +462,7 @@ internal static int AtExit()
         internal static IntPtr PyModuleType;
         internal static IntPtr PyClassType;
         internal static IntPtr PyInstanceType;
+        internal static IntPtr PySuper_Type;
         internal static IntPtr PyCLRMetaType;
         internal static IntPtr PyMethodType;
         internal static IntPtr PyWrapperDescriptorType;
@@ -518,6 +479,8 @@ internal static int AtExit()
         internal static IntPtr PyNoneType;
         internal static IntPtr PyTypeType;
 
+        internal static IntPtr Py_NoSiteFlag;
+
 #if PYTHON3
         internal static IntPtr PyBytesType;
 #endif
@@ -536,6 +499,16 @@ internal static int AtExit()
         internal static IntPtr PyNone;
         internal static IntPtr Error;
 
+        public static PyObject None
+        {
+            get
+            {
+                var none = Runtime.PyNone;
+                Runtime.XIncref(none);
+                return new PyObject(none);
+            }
+        }
+
         /// <summary>
         /// Check if any Python Exceptions occurred.
         /// If any exist throw new PythonException.
@@ -666,6 +639,15 @@ internal static unsafe void XIncref(IntPtr op)
 #endif
         }
 
+        /// <summary>
+        /// Increase Python's ref counter for the given object, and get the object back.
+        /// </summary>
+        internal static IntPtr SelfIncRef(IntPtr op)
+        {
+            XIncref(op);
+            return op;
+        }
+
         internal static unsafe void XDecref(IntPtr op)
         {
 #if PYTHON_WITH_PYDEBUG || NETSTANDARD
@@ -703,6 +685,7 @@ internal static unsafe void XDecref(IntPtr op)
 #endif
         }
 
+        [Pure]
         internal static unsafe long Refcount(IntPtr op)
         {
             var p = (void*)op;
@@ -856,13 +839,45 @@ public static extern int Py_Main(
         internal static extern int PyRun_SimpleString(string code);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern IntPtr PyRun_String(string code, IntPtr st, IntPtr globals, IntPtr locals);
+#if PYTHON2
+        internal static extern NewReference PyRun_String(string code, IntPtr st, IntPtr globals, IntPtr locals);
+#else
+        internal static extern NewReference PyRun_String([MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(Utf8Marshaler))] string code, IntPtr st, IntPtr globals, IntPtr locals);
+#endif
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyEval_EvalCode(IntPtr co, IntPtr globals, IntPtr locals);
 
+#if PYTHON2
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern IntPtr Py_CompileString(string code, string file, IntPtr tok);
+        internal static extern IntPtr Py_CompileString(string code, string file, int start);
+#else
+        /// <summary>
+        /// Return value: New reference.
+        /// This is a simplified interface to Py_CompileStringFlags() below, leaving flags set to NULL.
+        /// </summary>
+        internal static IntPtr Py_CompileString(string str, string file, int start)
+        {
+            return Py_CompileStringFlags(str, file, start, IntPtr.Zero);
+        }
+
+        /// <summary>
+        /// Return value: New reference.
+        /// This is a simplified interface to Py_CompileStringExFlags() below, with optimize set to -1.
+        /// </summary>
+        internal static IntPtr Py_CompileStringFlags(string str, string file, int start, IntPtr flags)
+        {
+            return Py_CompileStringExFlags(str, file, start, flags, -1);
+        }
+
+        /// <summary>
+        /// Return value: New reference.
+        /// Like Py_CompileStringObject(), but filename is a byte string decoded from the filesystem encoding(os.fsdecode()).
+        /// </summary>
+        /// <returns></returns>
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
+        internal static extern IntPtr Py_CompileStringExFlags(string str, string file, int start, IntPtr flags, int optimize);
+#endif
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyImport_ExecCodeModule(string name, IntPtr code);
@@ -873,8 +888,10 @@ public static extern int Py_Main(
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyCFunction_Call(IntPtr func, IntPtr args, IntPtr kw);
 
+#if PYTHON2
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyClass_New(IntPtr bases, IntPtr dict, IntPtr name);
+#endif
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyInstance_New(IntPtr cls, IntPtr args, IntPtr kw);
@@ -1032,7 +1049,7 @@ internal static int PyObject_Compare(IntPtr value1, IntPtr value2)
 
         internal static long PyObject_Size(IntPtr pointer)
         {
-            return (long) _PyObject_Size(pointer);
+            return (long)_PyObject_Size(pointer);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PyObject_Size")]
@@ -1116,10 +1133,6 @@ internal static IntPtr PyInt_FromInt64(long value)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl,
             EntryPoint = "PyLong_FromString")]
         internal static extern IntPtr PyInt_FromString(string value, IntPtr end, int radix);
-
-        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl,
-            EntryPoint = "PyLong_GetMax")]
-        internal static extern int PyInt_GetMax();
 #elif PYTHON2
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         private static extern IntPtr PyInt_FromLong(IntPtr value);
@@ -1142,8 +1155,21 @@ internal static bool PyLong_Check(IntPtr ob)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyLong_FromLong(long value);
 
-        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern IntPtr PyLong_FromUnsignedLong(uint value);
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl,
+            EntryPoint = "PyLong_FromUnsignedLong")]
+        internal static extern IntPtr PyLong_FromUnsignedLong32(uint value);
+
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl,
+            EntryPoint = "PyLong_FromUnsignedLong")]
+        internal static extern IntPtr PyLong_FromUnsignedLong64(ulong value);
+
+        internal static IntPtr PyLong_FromUnsignedLong(object value)
+        {
+            if (Is32Bit || IsWindows)
+                return PyLong_FromUnsignedLong32(Convert.ToUInt32(value));
+            else
+                return PyLong_FromUnsignedLong64(Convert.ToUInt64(value));
+        }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyLong_FromDouble(double value);
@@ -1160,8 +1186,21 @@ internal static bool PyLong_Check(IntPtr ob)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern int PyLong_AsLong(IntPtr value);
 
-        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern uint PyLong_AsUnsignedLong(IntPtr value);
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl,
+            EntryPoint = "PyLong_AsUnsignedLong")]
+        internal static extern uint PyLong_AsUnsignedLong32(IntPtr value);
+
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl,
+            EntryPoint = "PyLong_AsUnsignedLong")]
+        internal static extern ulong PyLong_AsUnsignedLong64(IntPtr value);
+
+        internal static object PyLong_AsUnsignedLong(IntPtr value)
+        {
+            if (Is32Bit || IsWindows)
+                return PyLong_AsUnsignedLong32(value);
+            else
+                return PyLong_AsUnsignedLong64(value);
+        }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern long PyLong_AsLongLong(IntPtr value);
@@ -1316,7 +1355,7 @@ internal static int PySequence_DelSlice(IntPtr pointer, long i1, long i2)
 
         internal static long PySequence_Size(IntPtr pointer)
         {
-            return (long) _PySequence_Size(pointer);
+            return (long)_PySequence_Size(pointer);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PySequence_Size")]
@@ -1341,7 +1380,7 @@ internal static IntPtr PySequence_Repeat(IntPtr pointer, long count)
 
         internal static long PySequence_Count(IntPtr pointer, IntPtr value)
         {
-            return (long) _PySequence_Count(pointer, value);
+            return (long)_PySequence_Count(pointer, value);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PySequence_Count")]
@@ -1384,7 +1423,7 @@ internal static IntPtr PyString_FromString(string value)
 
         internal static long PyBytes_Size(IntPtr op)
         {
-            return (long) _PyBytes_Size(op);
+            return (long)_PyBytes_Size(op);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PyBytes_Size")]
@@ -1414,6 +1453,10 @@ internal static IntPtr PyUnicode_FromStringAndSize(IntPtr value, long size)
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         private static extern IntPtr PyUnicode_FromStringAndSize(IntPtr value, IntPtr size);
+
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
+        internal static extern IntPtr PyUnicode_AsUTF8(IntPtr unicode);
+
 #elif PYTHON2
         internal static IntPtr PyString_FromStringAndSize(string value, long size)
         {
@@ -1516,6 +1559,8 @@ internal static IntPtr PyUnicode_FromString(string s)
             return PyUnicode_FromUnicode(s, s.Length);
         }
 
+        internal static string GetManagedString(in BorrowedReference borrowedReference)
+            => GetManagedString(borrowedReference.DangerousGetAddress());
         /// <summary>
         /// Function to access the internal PyUnicode/PyString object and
         /// convert it to a managed string with the correct encoding.
@@ -1598,7 +1643,7 @@ internal static bool PyDict_Check(IntPtr ob)
         internal static extern IntPtr PyDict_Values(IntPtr pointer);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern IntPtr PyDict_Items(IntPtr pointer);
+        internal static extern NewReference PyDict_Items(IntPtr pointer);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyDict_Copy(IntPtr pointer);
@@ -1611,7 +1656,7 @@ internal static bool PyDict_Check(IntPtr ob)
 
         internal static long PyDict_Size(IntPtr pointer)
         {
-            return (long) _PyDict_Size(pointer);
+            return (long)_PyDict_Size(pointer);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PyDict_Size")]
@@ -1638,13 +1683,13 @@ internal static IntPtr PyList_New(long size)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyList_AsTuple(IntPtr pointer);
 
-        internal static IntPtr PyList_GetItem(IntPtr pointer, long index)
+        internal static BorrowedReference PyList_GetItem(IntPtr pointer, long index)
         {
             return PyList_GetItem(pointer, new IntPtr(index));
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        private static extern IntPtr PyList_GetItem(IntPtr pointer, IntPtr index);
+        private static extern BorrowedReference PyList_GetItem(IntPtr pointer, IntPtr index);
 
         internal static int PyList_SetItem(IntPtr pointer, long index, IntPtr value)
         {
@@ -1654,22 +1699,22 @@ internal static int PyList_SetItem(IntPtr pointer, long index, IntPtr value)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         private static extern int PyList_SetItem(IntPtr pointer, IntPtr index, IntPtr value);
 
-        internal static int PyList_Insert(IntPtr pointer, long index, IntPtr value)
+        internal static int PyList_Insert(BorrowedReference pointer, long index, IntPtr value)
         {
             return PyList_Insert(pointer, new IntPtr(index), value);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        private static extern int PyList_Insert(IntPtr pointer, IntPtr index, IntPtr value);
+        private static extern int PyList_Insert(BorrowedReference pointer, IntPtr index, IntPtr value);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern int PyList_Append(IntPtr pointer, IntPtr value);
+        internal static extern int PyList_Append(BorrowedReference pointer, IntPtr value);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern int PyList_Reverse(IntPtr pointer);
+        internal static extern int PyList_Reverse(BorrowedReference pointer);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern int PyList_Sort(IntPtr pointer);
+        internal static extern int PyList_Sort(BorrowedReference pointer);
 
         internal static IntPtr PyList_GetSlice(IntPtr pointer, long start, long end)
         {
@@ -1689,7 +1734,7 @@ internal static int PyList_SetSlice(IntPtr pointer, long start, long end, IntPtr
 
         internal static long PyList_Size(IntPtr pointer)
         {
-            return (long) _PyList_Size(pointer);
+            return (long)_PyList_Size(pointer);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PyList_Size")]
@@ -1738,7 +1783,7 @@ internal static IntPtr PyTuple_GetSlice(IntPtr pointer, long start, long end)
 
         internal static long PyTuple_Size(IntPtr pointer)
         {
-            return (long) _PyTuple_Size(pointer);
+            return (long)_PyTuple_Size(pointer);
         }
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl, EntryPoint = "PyTuple_Size")]
@@ -1789,6 +1834,9 @@ internal static bool PyIter_Check(IntPtr pointer)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyImport_Import(IntPtr name);
 
+        /// <summary>
+        /// Return value: New reference.
+        /// </summary>
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyImport_ImportModule(string name);
 
@@ -1845,6 +1893,11 @@ internal static bool PyObject_TypeCheck(IntPtr ob, IntPtr tp)
             return (t == tp) || PyType_IsSubtype(t, tp);
         }
 
+        internal static bool PyType_IsSameAsOrSubtype(IntPtr type, IntPtr ofType)
+        {
+            return (type == ofType) || PyType_IsSubtype(type, ofType);
+        }
+
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyType_GenericNew(IntPtr type, IntPtr args, IntPtr kw);
 
@@ -1937,7 +1990,7 @@ internal static IntPtr PyMem_Realloc(IntPtr ptr, long size)
         internal static extern IntPtr PyErr_Occurred();
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern void PyErr_Fetch(ref IntPtr ob, ref IntPtr val, ref IntPtr tb);
+        internal static extern void PyErr_Fetch(out IntPtr ob, out IntPtr val, out IntPtr tb);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern void PyErr_Restore(IntPtr ob, IntPtr val, IntPtr tb);
@@ -1948,6 +2001,15 @@ internal static IntPtr PyMem_Realloc(IntPtr ptr, long size)
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern void PyErr_Print();
 
+        //====================================================================
+        // Cell API
+        //====================================================================
+
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
+        internal static extern NewReference PyCell_Get(BorrowedReference cell);
+
+        [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
+        internal static extern int PyCell_Set(BorrowedReference cell, IntPtr value);
 
         //====================================================================
         // Miscellaneous
@@ -1964,5 +2026,61 @@ internal static IntPtr PyMem_Realloc(IntPtr ptr, long size)
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern int Py_MakePendingCalls();
+
+        internal static void SetNoSiteFlag()
+        {
+            var loader = LibraryLoader.Get(OperatingSystem);
+
+            IntPtr dllLocal = _PythonDll != "__Internal"
+                ? loader.Load(_PythonDll)
+                : IntPtr.Zero;
+
+            try
+            {
+                Py_NoSiteFlag = loader.GetFunction(dllLocal, "Py_NoSiteFlag");
+                Marshal.WriteInt32(Py_NoSiteFlag, 1);
+            }
+            finally
+            {
+                if (dllLocal != IntPtr.Zero)
+                {
+                    loader.Free(dllLocal);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Return value: New reference.
+        /// </summary>
+        internal static IntPtr GetBuiltins()
+        {
+            return IsPython3 ? PyImport_ImportModule("builtins")
+                    : PyImport_ImportModule("__builtin__");
+        }
+    }
+
+
+    class PyReferenceCollection
+    {
+        private List<KeyValuePair<IntPtr, Action>> _actions = new List<KeyValuePair<IntPtr, Action>>();
+
+        /// <summary>
+        /// Record obj's address to release the obj in the future,
+        /// obj must alive before calling Release.
+        /// </summary>
+        public void Add(IntPtr ob, Action onRelease)
+        {
+            _actions.Add(new KeyValuePair<IntPtr, Action>(ob, onRelease));
+        }
+
+        public void Release()
+        {
+            foreach (var item in _actions)
+            {
+                Runtime.XDecref(item.Key);
+                item.Value?.Invoke();
+            }
+            _actions.Clear();
+        }
     }
 }
diff --git a/src/runtime/slots/mp_length.cs b/src/runtime/slots/mp_length.cs
new file mode 100644
index 000000000..b0a2e8d79
--- /dev/null
+++ b/src/runtime/slots/mp_length.cs
@@ -0,0 +1,50 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+
+namespace Python.Runtime.Slots
+{
+    internal static class mp_length_slot
+    {
+        /// <summary>
+        /// Implements __len__ for classes that implement ICollection
+        /// (this includes any IList implementer or Array subclass)
+        /// </summary>
+        public static int mp_length(IntPtr ob)
+        {
+            var co = ManagedType.GetManagedObject(ob) as CLRObject;
+            if (co == null)
+            {
+                Exceptions.RaiseTypeError("invalid object");
+            }
+
+            // first look for ICollection implementation directly
+            if (co.inst is ICollection c)
+            {
+                return c.Count;
+            }
+
+            Type clrType = co.inst.GetType();
+
+            // now look for things that implement ICollection<T> directly (non-explicitly)
+            PropertyInfo p = clrType.GetProperty("Count");
+            if (p != null && clrType.GetInterfaces().Any(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ICollection<>)))
+            {
+                return (int)p.GetValue(co.inst, null);
+            }
+
+            // finally look for things that implement the interface explicitly
+            var iface = clrType.GetInterfaces().FirstOrDefault(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ICollection<>));
+            if (iface != null)
+            {
+                p = iface.GetProperty(nameof(ICollection<int>.Count));
+                return (int)p.GetValue(co.inst, null);
+            }
+
+            Exceptions.SetError(Exceptions.TypeError, $"object of type '{clrType.Name}' has no len()");
+            return -1;
+        }
+    }
+}
diff --git a/src/runtime/typemanager.cs b/src/runtime/typemanager.cs
index d19c8737f..3df873eb5 100644
--- a/src/runtime/typemanager.cs
+++ b/src/runtime/typemanager.cs
@@ -1,11 +1,15 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 using System.Reflection;
 using System.Runtime.InteropServices;
+using Python.Runtime.Platform;
+using Python.Runtime.Slots;
 
 namespace Python.Runtime
 {
+
     /// <summary>
     /// The TypeManager class is responsible for building binary-compatible
     /// Python type objects that are implemented in managed code.
@@ -83,7 +87,7 @@ internal static IntPtr CreateType(Type impl)
             // Set tp_basicsize to the size of our managed instance objects.
             Marshal.WriteIntPtr(type, TypeOffset.tp_basicsize, (IntPtr)ob_size);
 
-            var offset = (IntPtr)ObjectOffset.DictOffset(type);
+            var offset = (IntPtr)ObjectOffset.TypeDictOffset(type);
             Marshal.WriteIntPtr(type, TypeOffset.tp_dictoffset, offset);
 
             InitializeSlots(type, impl);
@@ -121,7 +125,6 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
 
             IntPtr base_ = IntPtr.Zero;
             int ob_size = ObjectOffset.Size(Runtime.PyTypeType);
-            int tp_dictoffset = ObjectOffset.DictOffset(Runtime.PyTypeType);
 
             // XXX Hack, use a different base class for System.Exception
             // Python 2.5+ allows new style class exceptions but they *must*
@@ -129,9 +132,10 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
             if (typeof(Exception).IsAssignableFrom(clrType))
             {
                 ob_size = ObjectOffset.Size(Exceptions.Exception);
-                tp_dictoffset = ObjectOffset.DictOffset(Exceptions.Exception);
             }
 
+            int tp_dictoffset = ob_size + ManagedDataOffsets.ob_dict;
+
             if (clrType == typeof(Exception))
             {
                 base_ = Exceptions.Exception;
@@ -151,6 +155,13 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
             Marshal.WriteIntPtr(type, TypeOffset.tp_itemsize, IntPtr.Zero);
             Marshal.WriteIntPtr(type, TypeOffset.tp_dictoffset, (IntPtr)tp_dictoffset);
 
+            // add a __len__ slot for inheritors of ICollection and ICollection<>
+            if (typeof(ICollection).IsAssignableFrom(clrType) || clrType.GetInterfaces().Any(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ICollection<>)))
+            {
+                InitializeSlot(type, TypeOffset.mp_length, typeof(mp_length_slot).GetMethod(nameof(mp_length_slot.mp_length)));
+            }
+
+            // we want to do this after the slot stuff above in case the class itself implements a slot method
             InitializeSlots(type, impl.GetType());
 
             if (base_ != IntPtr.Zero)
@@ -191,6 +202,12 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
             return type;
         }
 
+        static void InitializeSlot(IntPtr type, int slotOffset, MethodInfo method)
+        {
+            var thunk = Interop.GetThunk(method);
+            Marshal.WriteIntPtr(type, slotOffset, thunk.Address);
+        }
+
         internal static IntPtr CreateSubType(IntPtr py_name, IntPtr py_base_type, IntPtr py_dict)
         {
             // Utility to create a subtype of a managed type with the ability for the
@@ -263,6 +280,14 @@ internal static IntPtr CreateSubType(IntPtr py_name, IntPtr py_base_type, IntPtr
                 IntPtr cls_dict = Marshal.ReadIntPtr(py_type, TypeOffset.tp_dict);
                 Runtime.PyDict_Update(cls_dict, py_dict);
 
+                // Update the __classcell__ if it exists
+                var cell = new BorrowedReference(Runtime.PyDict_GetItemString(cls_dict, "__classcell__"));
+                if (!cell.IsNull)
+                {
+                    Runtime.PyCell_Set(cell, py_type);
+                    Runtime.PyDict_DelItemString(cls_dict, "__classcell__");
+                }
+
                 return py_type;
             }
             catch (Exception e)
@@ -307,17 +332,11 @@ internal static IntPtr CreateMetaType(Type impl)
             Marshal.WriteIntPtr(type, TypeOffset.tp_base, py_type);
             Runtime.XIncref(py_type);
 
-            // Copy gc and other type slots from the base Python metatype.
-
-            CopySlot(py_type, type, TypeOffset.tp_basicsize);
-            CopySlot(py_type, type, TypeOffset.tp_itemsize);
-
-            CopySlot(py_type, type, TypeOffset.tp_dictoffset);
-            CopySlot(py_type, type, TypeOffset.tp_weaklistoffset);
-
-            CopySlot(py_type, type, TypeOffset.tp_traverse);
-            CopySlot(py_type, type, TypeOffset.tp_clear);
-            CopySlot(py_type, type, TypeOffset.tp_is_gc);
+            // Slots will inherit from TypeType, it's not neccesary for setting them.
+            // Inheried slots:
+            // tp_basicsize, tp_itemsize,
+            // tp_dictoffset, tp_weaklistoffset,
+            // tp_traverse, tp_clear, tp_is_gc, etc.
 
             // Override type slots with those of the managed implementation.
 
@@ -333,16 +352,18 @@ internal static IntPtr CreateMetaType(Type impl)
             // 4 int-ptrs in size.
             IntPtr mdef = Runtime.PyMem_Malloc(3 * 4 * IntPtr.Size);
             IntPtr mdefStart = mdef;
+            ThunkInfo thunkInfo = Interop.GetThunk(typeof(MetaType).GetMethod("__instancecheck__"), "BinaryFunc");
             mdef = WriteMethodDef(
                 mdef,
                 "__instancecheck__",
-                Interop.GetThunk(typeof(MetaType).GetMethod("__instancecheck__"), "BinaryFunc")
+                thunkInfo.Address
             );
 
+            thunkInfo = Interop.GetThunk(typeof(MetaType).GetMethod("__subclasscheck__"), "BinaryFunc");
             mdef = WriteMethodDef(
                 mdef,
                 "__subclasscheck__",
-                Interop.GetThunk(typeof(MetaType).GetMethod("__subclasscheck__"), "BinaryFunc")
+                thunkInfo.Address
             );
 
             // FIXME: mdef is not used
@@ -413,12 +434,8 @@ internal static IntPtr AllocateTypeObject(string name)
             // the Python version of the type name - otherwise we'd have to
             // allocate the tp_name and would have no way to free it.
 #if PYTHON3
-            // For python3 we leak two objects. One for the ASCII representation
-            // required for tp_name, and another for the Unicode representation
-            // for ht_name.
-            IntPtr temp = Runtime.PyBytes_FromString(name);
-            IntPtr raw = Runtime.PyBytes_AS_STRING(temp);
-            temp = Runtime.PyUnicode_FromString(name);
+            IntPtr temp = Runtime.PyUnicode_FromString(name);
+            IntPtr raw = Runtime.PyUnicode_AsUTF8(temp);
 #elif PYTHON2
             IntPtr temp = Runtime.PyString_FromString(name);
             IntPtr raw = Runtime.PyString_AsString(temp);
@@ -504,14 +521,14 @@ public static NativeCode Active
             {
                 get
                 {
-                    switch(Runtime.Machine)
+                    switch (Runtime.Machine)
                     {
-                        case Runtime.MachineType.i386:
+                        case MachineType.i386:
                             return I386;
-                        case Runtime.MachineType.x86_64:
+                        case MachineType.x86_64:
                             return X86_64;
                         default:
-                            throw new NotImplementedException($"No support for {Runtime.MachineName}");
+                            return null;
                     }
                 }
             }
@@ -603,12 +620,14 @@ int MAP_ANONYMOUS
                 {
                     switch (Runtime.OperatingSystem)
                     {
-                        case Runtime.OperatingSystemType.Darwin:
+                        case OperatingSystemType.Darwin:
                             return 0x1000;
-                        case Runtime.OperatingSystemType.Linux:
+                        case OperatingSystemType.Linux:
                             return 0x20;
                         default:
-                            throw new NotImplementedException($"mmap is not supported on {Runtime.OperatingSystemName}");
+                            throw new NotImplementedException(
+                                $"mmap is not supported on {Runtime.OperatingSystem}"
+                            );
                     }
                 }
             }
@@ -636,13 +655,15 @@ internal static IMemoryMapper CreateMemoryMapper()
         {
             switch (Runtime.OperatingSystem)
             {
-                case Runtime.OperatingSystemType.Darwin:
-                case Runtime.OperatingSystemType.Linux:
+                case OperatingSystemType.Darwin:
+                case OperatingSystemType.Linux:
                     return new UnixMemoryMapper();
-                case Runtime.OperatingSystemType.Windows:
+                case OperatingSystemType.Windows:
                     return new WindowsMemoryMapper();
                 default:
-                    throw new NotImplementedException($"No support for {Runtime.OperatingSystemName}");
+                    throw new NotImplementedException(
+                        $"No support for {Runtime.OperatingSystem}"
+                    );
             }
         }
 
@@ -668,7 +689,7 @@ internal static void InitializeNativeCodePage()
             Marshal.Copy(NativeCode.Active.Code, 0, NativeCodePage, codeLength);
             mapper.SetReadExec(NativeCodePage, codeLength);
         }
-#endregion
+        #endregion
 
         /// <summary>
         /// Given a newly allocated Python type object and a managed Type that
@@ -703,7 +724,8 @@ internal static void InitializeSlots(IntPtr type, Type impl)
                         continue;
                     }
 
-                    InitializeSlot(type, Interop.GetThunk(method), name);
+                    var thunkInfo = Interop.GetThunk(method);
+                    InitializeSlot(type, thunkInfo.Address, name);
 
                     seen.Add(name);
                 }
@@ -711,21 +733,40 @@ internal static void InitializeSlots(IntPtr type, Type impl)
                 impl = impl.BaseType;
             }
 
-            // See the TestDomainReload test: there was a crash related to
-            // the gc-related slots. They always return 0 or 1 because we don't
-            // really support gc:
+            var native = NativeCode.Active;
+
+            // The garbage collection related slots always have to return 1 or 0
+            // since .NET objects don't take part in Python's gc:
             //   tp_traverse (returns 0)
             //   tp_clear    (returns 0)
             //   tp_is_gc    (returns 1)
-            // We can't do without: python really wants those slots to exist.
-            // We can't implement those in C# because the application domain
-            // can be shut down and the memory released.
-            InitializeNativeCodePage();
-            InitializeSlot(type, NativeCodePage + NativeCode.Active.Return0, "tp_traverse");
-            InitializeSlot(type, NativeCodePage + NativeCode.Active.Return0, "tp_clear");
-            InitializeSlot(type, NativeCodePage + NativeCode.Active.Return1, "tp_is_gc");
+            // These have to be defined, though, so by default we fill these with
+            // static C# functions from this class.
+
+            var ret0 = Interop.GetThunk(((Func<IntPtr, int>)Return0).Method).Address;
+            var ret1 = Interop.GetThunk(((Func<IntPtr, int>)Return1).Method).Address;
+
+            if (native != null)
+            {
+                // If we want to support domain reload, the C# implementation
+                // cannot be used as the assembly may get released before
+                // CPython calls these functions. Instead, for amd64 and x86 we
+                // load them into a separate code page that is leaked
+                // intentionally.
+                InitializeNativeCodePage();
+                ret1 = NativeCodePage + native.Return1;
+                ret0 = NativeCodePage + native.Return0;
+            }
+
+            InitializeSlot(type, ret0, "tp_traverse");
+            InitializeSlot(type, ret0, "tp_clear");
+            InitializeSlot(type, ret1, "tp_is_gc");
         }
 
+        static int Return1(IntPtr _) => 1;
+
+        static int Return0(IntPtr _) => 0;
+
         /// <summary>
         /// Helper for InitializeSlots.
         ///
diff --git a/src/testing/Python.Test.15.csproj b/src/testing/Python.Test.15.csproj
index da20ed2ef..0e19adf91 100644
--- a/src/testing/Python.Test.15.csproj
+++ b/src/testing/Python.Test.15.csproj
@@ -7,7 +7,7 @@
     <AssemblyName>Python.Test</AssemblyName>
     <RootNamespace>Python.Test</RootNamespace>
     <PackageId>Python.Test</PackageId>
-    <VersionPrefix>2.4.0</VersionPrefix>
+    <VersionPrefix>2.5.0</VersionPrefix>
     <OutputPath>bin\</OutputPath>
     <AppendTargetFrameworkToOutputPath Condition="'$(TargetFramework)'=='net40'">false</AppendTargetFrameworkToOutputPath>
     <DocumentationFile Condition="'$(TargetFramework)'=='net40'">$(OutputPath)\$(AssemblyName).xml</DocumentationFile>
diff --git a/src/testing/Python.Test.csproj b/src/testing/Python.Test.csproj
index 27639ed5a..63526c060 100644
--- a/src/testing/Python.Test.csproj
+++ b/src/testing/Python.Test.csproj
@@ -91,6 +91,9 @@
     <Compile Include="threadtest.cs" />
     <Compile Include="doctest.cs" />
     <Compile Include="subclasstest.cs" />
+    <Compile Include="ReprTest.cs" />
+    <Compile Include="nonexportable.cs" />
+    <Compile Include="mp_lengthtest.cs" />
   </ItemGroup>
   <ItemGroup>
     <Reference Include="Microsoft.CSharp" />
@@ -111,4 +114,4 @@
     <Copy SourceFiles="$(TargetAssembly)" DestinationFolder="$(SolutionDir)\src\tests\fixtures" />
     <!--Copy SourceFiles="$(TargetAssemblyPdb)" Condition="Exists('$(TargetAssemblyPdb)')" DestinationFolder="$(PythonBuildDir)" /-->
   </Target>
-</Project>
\ No newline at end of file
+</Project>
diff --git a/src/testing/ReprTest.cs b/src/testing/ReprTest.cs
new file mode 100644
index 000000000..48e93683a
--- /dev/null
+++ b/src/testing/ReprTest.cs
@@ -0,0 +1,108 @@
+using System;
+using System.Text;
+
+namespace Python.Test
+{
+    /// <summary>
+    /// Supports repr unit tests.
+    /// </summary>
+    public class ReprTest
+    {
+        public class Point
+        {
+            public Point(double x, double y)
+            {
+                X = x;
+                Y = y;
+            }
+
+            public double X { get; set; }
+            public double Y { get; set; }
+
+            public override string ToString()
+            {
+                return base.ToString() + ": X=" + X.ToString() + ", Y=" + Y.ToString();
+            }
+
+            public string __repr__()
+            {
+                return "Point(" + X.ToString() + "," + Y.ToString() + ")";
+            }
+        }
+
+        public class Foo
+        {
+            public string __repr__()
+            {
+                return "I implement __repr__() but not ToString()!";
+            }
+        }
+
+        public class Bar
+        {
+            public override string ToString()
+            {
+                return "I implement ToString() but not __repr__()!";
+            }
+        }
+
+        public class BazBase
+        {
+            public override string ToString()
+            {
+                return "Base class implementing ToString()!";
+            }
+        }
+
+        public class BazMiddle : BazBase
+        {
+            public override string ToString()
+            {
+                return "Middle class implementing ToString()!";
+            }
+        }
+
+        //implements ToString via BazMiddle
+        public class Baz : BazMiddle
+        {
+
+        }
+
+        public class Quux
+        {
+            public string ToString(string format)
+            {
+                return "I implement ToString() with an argument!";
+            }
+        }
+
+        public class QuuzBase
+        {
+            protected string __repr__()
+            {
+                return "I implement __repr__ but it isn't public!";
+            }
+        }
+
+        public class Quuz : QuuzBase
+        {
+
+        }
+
+        public class Corge
+        {
+            public string __repr__(int i)
+            {
+                return "__repr__ implemention with input parameter!";
+            }
+        }
+
+        public class Grault
+        {
+            public int __repr__()
+            {
+                return "__repr__ implemention with wrong return type!".Length;
+            }
+        }
+    }
+}
diff --git a/src/testing/conversiontest.cs b/src/testing/conversiontest.cs
index 06ab7cb4e..1f9d64e1b 100644
--- a/src/testing/conversiontest.cs
+++ b/src/testing/conversiontest.cs
@@ -1,7 +1,9 @@
 namespace Python.Test
 {
+    using System.Collections.Generic;
+
     /// <summary>
-    /// Supports units tests for field access.
+    /// Supports unit tests for field access.
     /// </summary>
     public class ConversionTest
     {
@@ -32,6 +34,7 @@ public ConversionTest()
 
         public byte[] ByteArrayField;
         public sbyte[] SByteArrayField;
+        public readonly List<int> ListField = new List<int>();
 
         public T? Echo<T>(T? arg) where T: struct {
             return arg;
diff --git a/src/testing/methodtest.cs b/src/testing/methodtest.cs
index cf653f9f9..91836b727 100644
--- a/src/testing/methodtest.cs
+++ b/src/testing/methodtest.cs
@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Runtime.InteropServices;
 
 namespace Python.Test
 {
@@ -651,6 +652,38 @@ public static string Casesensitive()
         {
             return "Casesensitive";
         }
+
+        public static string DefaultParams(int a=0, int b=0, int c=0, int d=0)
+        {
+            return string.Format("{0}{1}{2}{3}", a, b, c, d);
+        }
+
+        public static string OptionalParams([Optional]int a, [Optional]int b, [Optional]int c, [Optional] int d)
+        {
+            return string.Format("{0}{1}{2}{3}", a, b, c, d);
+        }
+
+        public static bool OptionalParams_TestMissing([Optional]object a)
+        {
+            return a == Type.Missing;
+        }
+
+        public static bool OptionalParams_TestReferenceType([Optional]string a)
+        {
+            return a == null;
+        }
+
+        public static string OptionalAndDefaultParams([Optional]int a, [Optional]int b, int c=0, int d=0)
+        {
+            return string.Format("{0}{1}{2}{3}", a, b, c, d);
+        }
+
+        public static string OptionalAndDefaultParams2([Optional]int a, [Optional]int b, [Optional, DefaultParameterValue(1)]int c, int d = 2)
+        {
+            return string.Format("{0}{1}{2}{3}", a, b, c, d);
+        }
+
+        
     }
 
 
diff --git a/src/testing/mp_lengthtest.cs b/src/testing/mp_lengthtest.cs
new file mode 100644
index 000000000..a4f3e8c25
--- /dev/null
+++ b/src/testing/mp_lengthtest.cs
@@ -0,0 +1,171 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Python.Test
+{
+    public class MpLengthCollectionTest : ICollection
+    {
+        private readonly List<int> items;
+
+        public MpLengthCollectionTest()
+        {
+            SyncRoot = new object();
+            items = new List<int>
+            {
+                1,
+                2,
+                3
+            };
+        }
+
+        public int Count => items.Count;
+
+        public object SyncRoot { get; private set; }
+
+        public bool IsSynchronized => false;
+
+        public void CopyTo(Array array, int index)
+        {
+            throw new NotImplementedException();
+        }
+
+        public IEnumerator GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    public class MpLengthExplicitCollectionTest : ICollection
+    {
+        private readonly List<int> items;
+        private readonly object syncRoot;
+
+        public MpLengthExplicitCollectionTest()
+        {
+            syncRoot = new object();
+            items = new List<int>
+            {
+                9,
+                10
+            };
+        }
+        int ICollection.Count => items.Count;
+
+        object ICollection.SyncRoot => syncRoot;
+
+        bool ICollection.IsSynchronized => false;
+
+        void ICollection.CopyTo(Array array, int index)
+        {
+            throw new NotImplementedException();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    public class MpLengthGenericCollectionTest<T> : ICollection<T>
+    {
+        private readonly List<T> items;
+
+        public MpLengthGenericCollectionTest() {
+            SyncRoot = new object();
+            items = new List<T>();
+        }
+
+        public int Count => items.Count;
+
+        public object SyncRoot { get; private set; }
+
+        public bool IsSynchronized => false;
+
+        public bool IsReadOnly => false;
+
+        public void Add(T item)
+        {
+            items.Add(item);
+        }
+
+        public void Clear()
+        {
+            items.Clear();
+        }
+
+        public bool Contains(T item)
+        {
+            return items.Contains(item);
+        }
+
+        public void CopyTo(T[] array, int arrayIndex)
+        {
+            items.CopyTo(array, arrayIndex);
+        }
+
+        public IEnumerator GetEnumerator()
+        {
+            return ((IEnumerable)items).GetEnumerator();
+        }
+
+        public bool Remove(T item)
+        {
+            return items.Remove(item);
+        }
+
+        IEnumerator<T> IEnumerable<T>.GetEnumerator()
+        {
+            return items.GetEnumerator();
+        }
+    }
+
+    public class MpLengthExplicitGenericCollectionTest<T> : ICollection<T>
+    {
+        private readonly List<T> items;
+
+        public MpLengthExplicitGenericCollectionTest()
+        {
+            items = new List<T>();
+        }
+
+        int ICollection<T>.Count => items.Count;
+
+        bool ICollection<T>.IsReadOnly => false;
+
+        public void Add(T item)
+        {
+            items.Add(item);
+        }
+
+        void ICollection<T>.Clear()
+        {
+            items.Clear();
+        }
+
+        bool ICollection<T>.Contains(T item)
+        {
+            return items.Contains(item);
+        }
+
+        void ICollection<T>.CopyTo(T[] array, int arrayIndex)
+        {
+            items.CopyTo(array, arrayIndex);
+        }
+
+        IEnumerator<T> IEnumerable<T>.GetEnumerator()
+        {
+            return items.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IEnumerable)items).GetEnumerator();
+        }
+
+        bool ICollection<T>.Remove(T item)
+        {
+            return items.Remove(item);
+        }
+    }
+}
diff --git a/src/testing/nonexportable.cs b/src/testing/nonexportable.cs
new file mode 100644
index 000000000..a29c78589
--- /dev/null
+++ b/src/testing/nonexportable.cs
@@ -0,0 +1,8 @@
+namespace Python.Test
+{
+    using Python.Runtime;
+
+    // this class should not be visible to Python
+    [PyExport(false)]
+    public class NonExportable { }
+}
diff --git a/src/tests/test_array.py b/src/tests/test_array.py
index 7ccadddff..b492a66d3 100644
--- a/src/tests/test_array.py
+++ b/src/tests/test_array.py
@@ -1337,3 +1337,34 @@ def test_array_abuse():
     with pytest.raises(TypeError):
         desc = Test.PublicArrayTest.__dict__['__setitem__']
         desc(0, 0, 0)
+
+
+@pytest.mark.skipif(PY2, reason="Only applies in Python 3")
+def test_iterator_to_array():
+    from System import Array, String
+
+    d = {"a": 1, "b": 2, "c": 3}
+    keys_iterator = iter(d.keys())
+    arr = Array[String](keys_iterator)
+
+    Array.Sort(arr)
+
+    assert arr[0] == "a"
+    assert arr[1] == "b"
+    assert arr[2] == "c"
+
+
+@pytest.mark.skipif(PY2, reason="Only applies in Python 3")
+def test_dict_keys_to_array():
+    from System import Array, String
+
+    d = {"a": 1, "b": 2, "c": 3}
+    d_keys = d.keys()
+    arr = Array[String](d_keys)
+
+    Array.Sort(arr)
+
+    assert arr[0] == "a"
+    assert arr[1] == "b"
+    assert arr[2] == "c"
+
diff --git a/src/tests/test_class.py b/src/tests/test_class.py
index 612ce442e..6db3c36b7 100644
--- a/src/tests/test_class.py
+++ b/src/tests/test_class.py
@@ -14,7 +14,6 @@ def test_basic_reference_type():
     """Test usage of CLR defined reference types."""
     assert System.String.Empty == ""
 
-
 def test_basic_value_type():
     """Test usage of CLR defined value types."""
     assert System.Int32.MaxValue == 2147483647
@@ -29,7 +28,6 @@ def test_class_standard_attrs():
     assert isinstance(ClassTest.__dict__, DictProxyType)
     assert len(ClassTest.__doc__) > 0
 
-
 def test_class_docstrings():
     """Test standard class docstring generation"""
     from Python.Test import ClassTest
@@ -58,6 +56,14 @@ def test_non_public_class():
     with pytest.raises(AttributeError):
         _ = Test.InternalClass
 
+def test_non_exported():
+    """Test [PyExport(false)]"""
+    with pytest.raises(ImportError):
+        from Python.Test import NonExportable
+
+    with pytest.raises(AttributeError):
+        _ = Test.NonExportable
+
 
 def test_basic_subclass():
     """Test basic subclass of a managed class."""
diff --git a/src/tests/test_conversion.py b/src/tests/test_conversion.py
index 0ba10a80e..1386a0358 100644
--- a/src/tests/test_conversion.py
+++ b/src/tests/test_conversion.py
@@ -6,6 +6,8 @@
 import System
 import pytest
 from Python.Test import ConversionTest, UnicodeString
+from Python.Runtime import PyObjectConversions
+from Python.Runtime.Codecs import RawProxyEncoder
 
 from ._compat import indexbytes, long, unichr, text_type, PY2, PY3
 
@@ -595,11 +597,10 @@ def test_object_conversion():
 
     # need to test subclass here
 
-    with pytest.raises(TypeError):
-        class Foo(object):
-            pass
-        ob = ConversionTest()
-        ob.ObjectField = Foo
+    class Foo(object):
+        pass
+    ob.ObjectField = Foo
+    assert ob.ObjectField == Foo
 
 
 def test_enum_conversion():
@@ -701,3 +702,19 @@ def test_sbyte_array_conversion():
     array = ob.SByteArrayField
     for i, _ in enumerate(value):
         assert array[i] == indexbytes(value, i)
+
+def test_codecs():
+    """Test codec registration from Python"""
+    class ListAsRawEncoder(RawProxyEncoder):
+        __namespace__ = "Python.Test"
+        def CanEncode(self, clr_type):
+            return clr_type.Name == "List`1" and clr_type.Namespace == "System.Collections.Generic"
+
+    list_raw_encoder = ListAsRawEncoder()
+    PyObjectConversions.RegisterEncoder(list_raw_encoder)
+
+    ob = ConversionTest()
+
+    l = ob.ListField
+    l.Add(42)
+    assert ob.ListField.Count == 1
diff --git a/src/tests/test_exceptions.py b/src/tests/test_exceptions.py
index a10d9a183..c2f18d443 100644
--- a/src/tests/test_exceptions.py
+++ b/src/tests/test_exceptions.py
@@ -282,11 +282,8 @@ def test_python_compat_of_managed_exceptions():
 
     assert e.args == (msg,)
     assert isinstance(e.args, tuple)
-    if PY3:
-        strexp = "OverflowException('Simple message"
-        assert repr(e)[:len(strexp)] == strexp
-    elif PY2:
-        assert repr(e) == "OverflowException(u'Simple message',)"
+    strexp = "OverflowException('Simple message"
+    assert repr(e)[:len(strexp)] == strexp
 
 
 def test_exception_is_instance_of_system_object():
diff --git a/src/tests/test_indexer.py b/src/tests/test_indexer.py
index 6f18550d9..ca4fd3b89 100644
--- a/src/tests/test_indexer.py
+++ b/src/tests/test_indexer.py
@@ -438,13 +438,13 @@ def test_object_indexer():
     ob[long(1)] = "long"
     assert ob[long(1)] == "long"
 
-    with pytest.raises(TypeError):
-        class Eggs(object):
-            pass
+    class Eggs(object):
+        pass
 
-        key = Eggs()
-        ob = Test.ObjectIndexerTest()
-        ob[key] = "wrong"
+    key = Eggs()
+    ob = Test.ObjectIndexerTest()
+    ob[key] = "eggs_key"
+    assert ob[key] == "eggs_key"
 
 
 def test_interface_indexer():
diff --git a/src/tests/test_method.py b/src/tests/test_method.py
index ad678611b..69f1b5e72 100644
--- a/src/tests/test_method.py
+++ b/src/tests/test_method.py
@@ -257,6 +257,37 @@ def test_non_params_array_in_last_place():
     result = MethodTest.TestNonParamsArrayInLastPlace(1, 2, 3)
     assert result
 
+def test_params_methods_with_no_params():
+    """Tests that passing no arguments to a params method
+    passes an empty array"""
+    result = MethodTest.TestValueParamsArg()
+    assert len(result) == 0
+
+    result = MethodTest.TestOneArgWithParams('Some String')
+    assert len(result) == 0
+
+    result = MethodTest.TestTwoArgWithParams('Some String', 'Some Other String')
+    assert len(result) == 0
+
+def test_params_methods_with_non_lists():
+    """Tests that passing single parameters to a params
+    method will convert into an array on the .NET side"""
+    result = MethodTest.TestOneArgWithParams('Some String', [1, 2, 3, 4])
+    assert len(result) == 4
+
+    result = MethodTest.TestOneArgWithParams('Some String', 1, 2, 3, 4)
+    assert len(result) == 4
+
+    result = MethodTest.TestOneArgWithParams('Some String', [5])
+    assert len(result) == 1
+
+    result = MethodTest.TestOneArgWithParams('Some String', 5)
+    assert len(result) == 1
+
+def test_params_method_with_lists():
+    """Tests passing multiple lists to a params object[] method"""
+    result = MethodTest.TestObjectParamsArg([],[])
+    assert len(result) == 2
 
 def test_string_out_params():
     """Test use of string out-parameters."""
@@ -776,6 +807,9 @@ def test_no_object_in_param():
 
     res = MethodTest.TestOverloadedNoObject(5)
     assert res == "Got int"
+    
+    res = MethodTest.TestOverloadedNoObject(i=7)
+    assert res == "Got int"
 
     with pytest.raises(TypeError):
         MethodTest.TestOverloadedNoObject("test")
@@ -787,9 +821,15 @@ def test_object_in_param():
 
     res = MethodTest.TestOverloadedObject(5)
     assert res == "Got int"
+    
+    res = MethodTest.TestOverloadedObject(i=7)
+    assert res == "Got int"
 
     res = MethodTest.TestOverloadedObject("test")
     assert res == "Got object"
+    
+    res = MethodTest.TestOverloadedObject(o="test")
+    assert res == "Got object"
 
 
 def test_object_in_multiparam():
@@ -813,6 +853,42 @@ def test_object_in_multiparam():
     res = MethodTest.TestOverloadedObjectTwo(7.24, 7.24)
     assert res == "Got object-object"
 
+    res = MethodTest.TestOverloadedObjectTwo(a=5, b=5)
+    assert res == "Got int-int"
+
+    res = MethodTest.TestOverloadedObjectTwo(5, b=5)
+    assert res == "Got int-int"
+
+    res = MethodTest.TestOverloadedObjectTwo(a=5, b="foo")
+    assert res == "Got int-string"
+
+    res = MethodTest.TestOverloadedObjectTwo(5, b="foo")
+    assert res == "Got int-string"
+
+    res = MethodTest.TestOverloadedObjectTwo(a="foo", b=7.24)
+    assert res == "Got string-object"
+
+    res = MethodTest.TestOverloadedObjectTwo("foo", b=7.24)
+    assert res == "Got string-object"
+
+    res = MethodTest.TestOverloadedObjectTwo(a="foo", b="bar")
+    assert res == "Got string-string"
+
+    res = MethodTest.TestOverloadedObjectTwo("foo", b="bar")
+    assert res == "Got string-string"
+
+    res = MethodTest.TestOverloadedObjectTwo(a="foo", b=5)
+    assert res == "Got string-int"
+
+    res = MethodTest.TestOverloadedObjectTwo("foo", b=5)
+    assert res == "Got string-int"
+
+    res = MethodTest.TestOverloadedObjectTwo(a=7.24, b=7.24)
+    assert res == "Got object-object"
+
+    res = MethodTest.TestOverloadedObjectTwo(7.24, b=7.24)
+    assert res == "Got object-object"
+
 
 def test_object_in_multiparam_exception():
     """Test method with object multiparams behaves"""
@@ -966,3 +1042,120 @@ def test_getting_overloaded_constructor_binding_does_not_leak_ref_count():
     # simple test
     refCount = sys.getrefcount(PlainOldClass.Overloads[int])
     assert refCount == 1
+
+
+def test_default_params():
+    # all positional parameters
+    res = MethodTest.DefaultParams(1,2,3,4)
+    assert res == "1234"
+
+    res = MethodTest.DefaultParams(1, 2, 3)
+    assert res == "1230"
+
+    res = MethodTest.DefaultParams(1, 2)
+    assert res == "1200"
+
+    res = MethodTest.DefaultParams(1)
+    assert res == "1000"
+
+    res = MethodTest.DefaultParams(a=2)
+    assert res == "2000"
+
+    res = MethodTest.DefaultParams(b=3)
+    assert res == "0300"
+
+    res = MethodTest.DefaultParams(c=4)
+    assert res == "0040"
+
+    res = MethodTest.DefaultParams(d=7)
+    assert res == "0007"
+
+    res = MethodTest.DefaultParams(a=2, c=5)
+    assert res == "2050"
+
+    res = MethodTest.DefaultParams(1, d=7, c=3)
+    assert res == "1037"
+
+    with pytest.raises(TypeError):
+        MethodTest.DefaultParams(1,2,3,4,5)
+
+def test_optional_params():
+    res = MethodTest.OptionalParams(1, 2, 3, 4)
+    assert res == "1234"
+
+    res = MethodTest.OptionalParams(1, 2, 3)
+    assert res == "1230"
+
+    res = MethodTest.OptionalParams(1, 2)
+    assert res == "1200"
+
+    res = MethodTest.OptionalParams(1)
+    assert res == "1000"
+
+    res = MethodTest.OptionalParams(a=2)
+    assert res == "2000"
+
+    res = MethodTest.OptionalParams(b=3)
+    assert res == "0300"
+
+    res = MethodTest.OptionalParams(c=4)
+    assert res == "0040"
+
+    res = MethodTest.OptionalParams(d=7)
+    assert res == "0007"
+
+    res = MethodTest.OptionalParams(a=2, c=5)
+    assert res == "2050"
+
+    res = MethodTest.OptionalParams(1, d=7, c=3)
+    assert res == "1037"
+
+    res = MethodTest.OptionalParams_TestMissing()
+    assert res == True
+
+    res = MethodTest.OptionalParams_TestMissing(None)
+    assert res == False
+
+    res = MethodTest.OptionalParams_TestMissing(a = None)
+    assert res == False
+
+    res = MethodTest.OptionalParams_TestMissing(a='hi')
+    assert res == False
+
+    res = MethodTest.OptionalParams_TestReferenceType()
+    assert res == True
+
+    res = MethodTest.OptionalParams_TestReferenceType(None)
+    assert res == True
+
+    res = MethodTest.OptionalParams_TestReferenceType(a=None)
+    assert res == True
+
+    res = MethodTest.OptionalParams_TestReferenceType('hi')
+    assert res == False
+
+    res = MethodTest.OptionalParams_TestReferenceType(a='hi')
+    assert res == False
+
+def test_optional_and_default_params():
+
+    res = MethodTest.OptionalAndDefaultParams()
+    assert res == "0000"
+
+    res = MethodTest.OptionalAndDefaultParams(1)
+    assert res == "1000"
+
+    res = MethodTest.OptionalAndDefaultParams(1, c=4)
+    assert res == "1040"
+
+    res = MethodTest.OptionalAndDefaultParams(b=4, c=7)
+    assert res == "0470"
+
+    res = MethodTest.OptionalAndDefaultParams2()
+    assert res == "0012"
+
+    res = MethodTest.OptionalAndDefaultParams2(a=1,b=2,c=3,d=4)
+    assert res == "1234"
+
+    res = MethodTest.OptionalAndDefaultParams2(b=2, c=3)
+    assert res == "0232"
diff --git a/src/tests/test_mp_length.py b/src/tests/test_mp_length.py
new file mode 100644
index 000000000..c96ac77d1
--- /dev/null
+++ b/src/tests/test_mp_length.py
@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+
+"""Test __len__ for .NET classes implementing ICollection/ICollection<T>."""
+
+import System
+import pytest
+from Python.Test import MpLengthCollectionTest, MpLengthExplicitCollectionTest, MpLengthGenericCollectionTest, MpLengthExplicitGenericCollectionTest
+
+def test_simple___len__():
+    """Test __len__ for simple ICollection implementers"""
+    import System
+    import System.Collections.Generic
+    l = System.Collections.Generic.List[int]()
+    assert len(l) == 0
+    l.Add(5)
+    l.Add(6)
+    assert len(l) == 2
+
+    d = System.Collections.Generic.Dictionary[int, int]()
+    assert len(d) == 0
+    d.Add(4, 5)
+    assert len(d) == 1
+
+    a = System.Array[int]([0,1,2,3])
+    assert len(a) == 4
+
+def test_custom_collection___len__():
+    """Test __len__ for custom collection class"""
+    s = MpLengthCollectionTest()
+    assert len(s) == 3
+
+def test_custom_collection_explicit___len__():
+    """Test __len__ for custom collection class that explicitly implements ICollection"""
+    s = MpLengthExplicitCollectionTest()
+    assert len(s) == 2
+
+def test_custom_generic_collection___len__():
+    """Test __len__ for custom generic collection class"""
+    s = MpLengthGenericCollectionTest[int]()
+    s.Add(1)
+    s.Add(2)
+    assert len(s) == 2
+
+def test_custom_generic_collection_explicit___len__():
+    """Test __len__ for custom generic collection that explicity implements ICollection<T>"""
+    s = MpLengthExplicitGenericCollectionTest[int]()
+    s.Add(1)
+    s.Add(10)
+    assert len(s) == 2
diff --git a/src/tests/test_repr.py b/src/tests/test_repr.py
new file mode 100644
index 000000000..5131f5d88
--- /dev/null
+++ b/src/tests/test_repr.py
@@ -0,0 +1,68 @@
+# -*- coding: utf-8 -*-
+
+"""Test __repr__ output"""
+
+import System
+import pytest
+from Python.Test import ReprTest
+
+def test_basic():
+    """Test Point class which implements both ToString and __repr__ without inheritance"""
+    ob = ReprTest.Point(1,2)
+    # point implements ToString() and __repr__()
+    assert ob.__repr__() == "Point(1,2)"
+    assert str(ob) == "Python.Test.ReprTest+Point: X=1, Y=2"
+
+def test_system_string():
+    """Test system string"""
+    ob = System.String("hello")
+    assert str(ob) == "hello"
+    assert "<System.String object at " in ob.__repr__()
+
+def test_str_only():
+    """Test class implementing ToString() but not __repr__()"""
+    ob = ReprTest.Bar()
+    assert str(ob) == "I implement ToString() but not __repr__()!"
+    assert "<Python.Test.Bar object at " in ob.__repr__()
+
+def test_hierarchy1():
+    """Test inheritance hierarchy with base & middle class implementing ToString"""
+    ob1 = ReprTest.BazBase()
+    assert str(ob1) == "Base class implementing ToString()!"
+    assert "<Python.Test.BazBase object at " in ob1.__repr__()
+
+    ob2 = ReprTest.BazMiddle()
+    assert str(ob2) == "Middle class implementing ToString()!"
+    assert "<Python.Test.BazMiddle object at " in ob2.__repr__()
+
+    ob3 = ReprTest.Baz()
+    assert str(ob3) == "Middle class implementing ToString()!"
+    assert "<Python.Test.Baz object at " in ob3.__repr__()
+
+def bad_tostring():
+    """Test ToString that can't be used by str()"""
+    ob = ReprTest.Quux()
+    assert str(ob) == "Python.Test.ReprTest+Quux"
+    assert "<Python.Test.Quux object at " in ob.__repr__()
+
+def bad_repr():
+    """Test incorrect implementation of repr"""
+    ob1 = ReprTest.QuuzBase()
+    assert str(ob1) == "Python.Test.ReprTest+QuuzBase"
+    assert "<Python.Test.QuuzBase object at " in ob.__repr__()
+    
+    ob2 = ReprTest.Quuz()
+    assert str(ob2) == "Python.Test.ReprTest+Quuz"
+    assert "<Python.Test.Quuz object at " in ob.__repr__()
+
+    ob3 = ReprTest.Corge()
+    with pytest.raises(Exception):
+        str(ob3)
+    with pytest.raises(Exception):
+        ob3.__repr__()
+
+    ob4 = ReprTest.Grault()
+    with pytest.raises(Exception):
+        str(ob4)
+    with pytest.raises(Exception):
+        ob4.__repr__()
diff --git a/src/tests/tests.pyproj b/src/tests/tests.pyproj
index cefa0d05e..074792e43 100644
--- a/src/tests/tests.pyproj
+++ b/src/tests/tests.pyproj
@@ -59,6 +59,7 @@
     <Compile Include="test_recursive_types.py" />
     <Compile Include="test_subclass.py" />
     <Compile Include="test_thread.py" />
+    <Compile Include="test_repr.py" />
     <Compile Include="utils.py" />
     <Compile Include="fixtures\argv-fixture.py" />
   </ItemGroup>
diff --git a/tools/geninterop/geninterop.py b/tools/geninterop/geninterop.py
index f8ef8e561..902296229 100644
--- a/tools/geninterop/geninterop.py
+++ b/tools/geninterop/geninterop.py
@@ -76,6 +76,8 @@ def visit(self, node):
             self.visit_struct(node)
         elif isinstance(node, c_ast.Decl):
             self.visit_decl(node)
+        elif isinstance(node, c_ast.FuncDecl):
+            self.visit_funcdecl(node)
         elif isinstance(node, c_ast.PtrDecl):
             self.visit_ptrdecl(node)
         elif isinstance(node, c_ast.IdentifierType):
@@ -110,6 +112,9 @@ def visit_struct(self, struct):
     def visit_decl(self, decl):
         self.visit(decl.type)
 
+    def visit_funcdecl(self, funcdecl):
+        self.visit(funcdecl.type)
+
     def visit_ptrdecl(self, ptrdecl):
         self.__ptr_decl_depth += 1
         self.visit(ptrdecl.type)
@@ -169,6 +174,8 @@ def preprocess_python_headers():
     include_py = sysconfig.get_config_var("INCLUDEPY")
     include_dirs.append(include_py)
 
+    include_args = [c for p in include_dirs for c in ["-I", p]]
+
     defines = [
         "-D", "__attribute__(x)=",
         "-D", "__inline__=inline",
@@ -177,6 +184,13 @@ def preprocess_python_headers():
         "-D", "_POSIX_THREADS"
     ]
 
+    if os.name == 'nt':
+        defines.extend([
+            "-D", "__inline=inline",
+            "-D", "__ptr32=",
+            "-D", "__declspec(x)=",
+        ])
+
     if hasattr(sys, "abiflags"):
         if "d" in sys.abiflags:
             defines.extend(("-D", "PYTHON_WITH_PYDEBUG"))
@@ -186,7 +200,7 @@ def preprocess_python_headers():
             defines.extend(("-D", "PYTHON_WITH_WIDE_UNICODE"))
 
     python_h = os.path.join(include_py, "Python.h")
-    cmd = ["clang", "-pthread", "-I"] + include_dirs + defines + ["-E", python_h]
+    cmd = ["clang", "-pthread"] + include_args + defines + ["-E", python_h]
 
     # normalize as the parser doesn't like windows line endings.
     lines = []
@@ -216,7 +230,7 @@ def gen_interop_code(members):
     defines_str = " && ".join(defines)
     class_definition = """
 // Auto-generated by %s.
-// DO NOT MODIFIY BY HAND.
+// DO NOT MODIFY BY HAND.
 
 
 #if %s
diff --git a/tools/vswhere/vswhere.exe b/tools/vswhere/vswhere.exe
index 3eb2df009..582e82868 100644
Binary files a/tools/vswhere/vswhere.exe and b/tools/vswhere/vswhere.exe differ
diff --git a/tox.ini b/tox.ini
deleted file mode 100644
index 3c9be5c63..000000000
--- a/tox.ini
+++ /dev/null
@@ -1,27 +0,0 @@
-[tox]
-skip_missing_interpreters=True
-envlist =
-  py27
-  py33
-  py34
-  py35
-  py36
-  py37
-  check
-
-[testenv]
-deps =
-  pytest
-commands =
-  {posargs:py.test}
-
-[testenv:check]
-ignore_errors=True
-skip_install=True
-basepython=python3.5
-deps =
-  check-manifest
-  flake8
-commands =
-  flake8 src setup.py
-  python setup.py check --strict --metadata