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koliyo opened this issue Mar 10, 2021 · 3 comments
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Crash when using nested classes with inheritance, which are instantiated from C# #1414

koliyo opened this issue Mar 10, 2021 · 3 comments
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@filmor
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bug
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3.0.0

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@koliyo
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koliyo commented Mar 10, 2021

Environment

  • Pythonnet version: 2.5.1
  • Python version: Python 3.9.2
  • Operating System: MacOS
  • .NET Runtime: 5.0.200

Details

  • Describe what you were trying to get done.

I am accessing a member of a nested C# class which inherits from the enclosing class.

When doing this, the Pythonnet runtime crashes!

Here is a minimal example:

namespace pythonnet_testing {

  public class Bar {
    public class Hej : Bar {
    }
  }

  public class Foo {
    public Bar Bar;

    public static Foo Create() {
      Foo f = new();
      f.Bar = new Bar.Hej();
      return f;
    }
  }

  class Program {
    static void Main(string[] args) {

      var pyCode = @"
import pythonnet_testing
f = pythonnet_testing.Foo.Create()
print('Trying to access member of nested subclass')
print(f'Bar: {f.Bar}')
      ";

      PythonEngine.Initialize();

      using (Py.GIL()) {
        PythonEngine.Exec(pyCode);
      }

      Console.WriteLine("EXIT");
    }
  }
}

This seem to be specifically for nested classes, where the object instance is created from the C# environment, and the class has not been explicitly referenced/used from python.

I have found a way to prevent the crash, by first explicitly using the nested class in the python code.

Eg instead of this:

import pythonnet_testing
f = pythonnet_testing.Foo.Create()
print('Trying to access member of nested subclass')
print(f'Bar: {f.Bar}')

I insert a reference to the nested Bar.Hej class

import pythonnet_testing
f = pythonnet_testing.Foo.Create()
print('Trying to access member of nested subclass')
_ = pythonnet_testing.Bar.Hej # Explicitly referencing the nested class
print(f'Bar: {f.Bar}')

If I do this, then the Pythonnet runtime no longer crashes. It is even enough to reference the enclosing baseclass Bar

_ = pythonnet_testing.Bar # Explicitly referencing the enclosing class
print(f'Bar: {f.Bar}')

When I do this, the python code executes properly:

Trying to access member of nested subclass
Bar: pythonnet_testing.Bar+Hej
@filmor
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filmor commented Mar 10, 2021

Thank you for the bug report. Two things:

  1. What does the actual stacktrace or error message look like when it's crashing?
  2. Could you check whether the problem exists on master as well?

@koliyo
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koliyo commented Mar 10, 2021

On my Mac I do not get any error message, the process dies silently. I have a report from a Windows machine with the following error:

Fatal error. System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
Repeat 2 times:
                                                                                                                                                                   --------------------------------
at Python.Runtime.Runtime.PyDict_SetItemString(IntPtr, System.String, IntPtr)
                                      --------------------------------
at Python.Runtime.ClassManager.InitClassBase(System.Type, Python.Runtime.ClassBase)
at Python.Runtime.ClassManager.GetClass(System.Type)
at Python.Runtime.TypeManager.CreateType(Python.Runtime.ManagedType, System.Type)
at Python.Runtime.TypeManager.GetTypeHandle(Python.Runtime.ManagedType, System.Type)
at Python.Runtime.ClassManager.InitClassBase(System.Type, Python.Runtime.ClassBase)
at Python.Runtime.ClassManager.GetClass(System.Type)
at Python.Runtime.Converter.ToPython(System.Object, System.Type)
at Python.Runtime.Converter.ToPython(System.Object, System.Type)
at Python.Runtime.MethodBinder.Invoke(IntPtr, IntPtr, IntPtr, System.Reflection.MethodBase, System.Reflection.MethodInfo[])
at Python.Runtime.MethodBinder.Invoke(IntPtr, IntPtr, IntPtr)
at Python.Runtime.Indexer.GetItem(IntPtr, IntPtr)
at Python.Runtime.ClassObject.mp_subscript(IntPtr, IntPtr)

I have also tested this in master, and the same crash and workaround applies.

@filmor
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filmor commented Mar 10, 2021

Thank you, I'll try to reproduce this.

@filmor filmor self-assigned this Mar 10, 2021
@filmor filmor added the bug label Mar 10, 2021
@filmor filmor added this to the 3.0.0 milestone Mar 10, 2021
@koliyo koliyo mentioned this issue Mar 18, 2021
Closed
lostmsu added a commit to losttech/pythonnet that referenced this issue Oct 1, 2021
@lostmsu
When reflecting nested types, ensure their corresponding PyType is al…
71e465f 
…located.

fixes pythonnet#1414

Without this fix attempting to use a nested .NET class that derives from its parent would cause a crash due to false mutual dependency.
@lostmsu lostmsu mentioned this issue Oct 1, 2021
Merged
2 tasks
@lostmsu lostmsu closed this as completed in f857d59 Oct 1, 2021
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