diff --git a/LibGit2Sharp/Core/ArrayMarshaler.cs b/LibGit2Sharp/Core/ArrayMarshaler.cs
index b831f6dbc..13a50c1ad 100644
--- a/LibGit2Sharp/Core/ArrayMarshaler.cs
+++ b/LibGit2Sharp/Core/ArrayMarshaler.cs
@@ -13,7 +13,7 @@ public ArrayMarshaler(T[] objs)
 
             for (var i = 0; i < objs.Length; i++)
             {
-                IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(T)));
+                IntPtr ptr = Marshal.AllocHGlobal(MarshalPortable.SizeOf<T>());
                 ptrs[i] = ptr;
                 Marshal.StructureToPtr(objs[i], ptr, false);
             }
diff --git a/LibGit2Sharp/Core/GitOdbBackend.cs b/LibGit2Sharp/Core/GitOdbBackend.cs
index ce2d1119b..a3f5777fe 100644
--- a/LibGit2Sharp/Core/GitOdbBackend.cs
+++ b/LibGit2Sharp/Core/GitOdbBackend.cs
@@ -8,7 +8,7 @@ internal struct GitOdbBackend
     {
         static GitOdbBackend()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitOdbBackend), nameof(GCHandle)).ToInt32();
+            GCHandleOffset = MarshalPortable.OffsetOf<GitOdbBackend>(nameof(GCHandle)).ToInt32();
         }
 
         public uint Version;
diff --git a/LibGit2Sharp/Core/GitOdbBackendStream.cs b/LibGit2Sharp/Core/GitOdbBackendStream.cs
index 1bd6439ba..d2771f598 100644
--- a/LibGit2Sharp/Core/GitOdbBackendStream.cs
+++ b/LibGit2Sharp/Core/GitOdbBackendStream.cs
@@ -15,7 +15,7 @@ internal class GitOdbBackendStream
     {
         static GitOdbBackendStream()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitOdbBackendStream), nameof(GCHandle)).ToInt32();
+            GCHandleOffset = MarshalPortable.OffsetOf<GitOdbBackendStream>(nameof(GCHandle)).ToInt32();
         }
 
         public IntPtr Backend;
diff --git a/LibGit2Sharp/Core/GitSmartSubtransport.cs b/LibGit2Sharp/Core/GitSmartSubtransport.cs
index 73345e511..9c545d3c0 100644
--- a/LibGit2Sharp/Core/GitSmartSubtransport.cs
+++ b/LibGit2Sharp/Core/GitSmartSubtransport.cs
@@ -8,7 +8,7 @@ internal class GitSmartSubtransport
     {
         static GitSmartSubtransport()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitSmartSubtransport), nameof(GCHandle)).ToInt32();
+            GCHandleOffset = MarshalPortable.OffsetOf<GitSmartSubtransport>(nameof(GCHandle)).ToInt32();
         }
 
         public action_callback Action;
diff --git a/LibGit2Sharp/Core/GitSmartSubtransportStream.cs b/LibGit2Sharp/Core/GitSmartSubtransportStream.cs
index 22dc6564f..90853b415 100644
--- a/LibGit2Sharp/Core/GitSmartSubtransportStream.cs
+++ b/LibGit2Sharp/Core/GitSmartSubtransportStream.cs
@@ -8,7 +8,7 @@ internal class GitSmartSubtransportStream
     {
         static GitSmartSubtransportStream()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitSmartSubtransportStream), nameof(GCHandle)).ToInt32();
+            GCHandleOffset = MarshalPortable.OffsetOf<GitSmartSubtransportStream>(nameof(GCHandle)).ToInt32();
         }
 
         public IntPtr SmartTransport;
diff --git a/LibGit2Sharp/Core/MarshalPortable.cs b/LibGit2Sharp/Core/MarshalPortable.cs
new file mode 100644
index 000000000..034675004
--- /dev/null
+++ b/LibGit2Sharp/Core/MarshalPortable.cs
@@ -0,0 +1,35 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace LibGit2Sharp.Core
+{
+    internal static class MarshalPortable
+    {
+        internal static int SizeOf<T>()
+        {
+#if NET40
+            return Marshal.SizeOf(typeof(T));
+#else
+            return Marshal.SizeOf<T>();
+#endif
+        }
+
+        internal static IntPtr OffsetOf<T>(string fieldName)
+        {
+#if NET40
+            return Marshal.OffsetOf(typeof(T), fieldName);
+#else
+            return Marshal.OffsetOf<T>(fieldName);
+#endif
+        }
+
+        internal static T PtrToStructure<T>(IntPtr ptr)
+        {
+#if NET40
+            return (T)Marshal.PtrToStructure(ptr, typeof(T));
+#else
+            return Marshal.PtrToStructure<T>(ptr);
+#endif
+        }
+    }
+}
diff --git a/LibGit2Sharp/Filter.cs b/LibGit2Sharp/Filter.cs
index 050d4a83c..723b50359 100644
--- a/LibGit2Sharp/Filter.cs
+++ b/LibGit2Sharp/Filter.cs
@@ -261,7 +261,7 @@ int StreamCreateCallback(out IntPtr git_writestream_out, GitFilter self, IntPtr
                 Marshal.StructureToPtr(state.thisStream, state.thisPtr, false);
 
                 state.nextPtr = git_writestream_next;
-                state.nextStream = (GitWriteStream)Marshal.PtrToStructure(state.nextPtr, typeof(GitWriteStream));
+                state.nextStream = MarshalPortable.PtrToStructure<GitWriteStream>(state.nextPtr);
                 
                 state.filterSource = FilterSource.FromNativePtr(filterSourcePtr);
                 state.output = new WriteStream(state.nextStream, state.nextPtr);
diff --git a/LibGit2Sharp/ObjectDatabase.cs b/LibGit2Sharp/ObjectDatabase.cs
index c662a478c..1f42d82dc 100644
--- a/LibGit2Sharp/ObjectDatabase.cs
+++ b/LibGit2Sharp/ObjectDatabase.cs
@@ -239,7 +239,7 @@ private unsafe Blob CreateBlob(Stream stream, string hintpath, long? numberOfByt
             }
 
             IntPtr writestream_ptr = Proxy.git_blob_create_fromstream(repo.Handle, hintpath);
-            GitWriteStream writestream = (GitWriteStream)Marshal.PtrToStructure(writestream_ptr, typeof(GitWriteStream));
+            GitWriteStream writestream = MarshalPortable.PtrToStructure<GitWriteStream>(writestream_ptr);
 
             try
             {