Interop COM(-isch) interface marshaling results in AccessViotlationException on simple call

Question

I am trying to write a managed interop library for a native C++ plugin standard. This native C++ library uses a COM-compatible interface design. It does NOT however, do any of t

Answer 1

We can't see what FUnknown looks like. It must be identical to IUnknown to allow the interop to work. The CLR will make calls to AddRef, Release and QueryInterface automatically. And it is very important that there are exactly three methods in FUnknown, if FUnknown has more or less then you'll end up calling the complete wrong method when your C# code calls CountClasses(). Indeed a good way to trigger an AVE.

One problem that we can see is that CountClasses is not COM compatible, methods must return an Int32 that is the HRESULT. An error code, a value that isn't zero automatically triggers an exception in the C# program. Incompatible method signatures are supported, you have to use an attribute. Like this:

[PreserveSig]
int CountClasses();

This mistake is otherwise not sufficient to explain the AVE. An interface mismatch is your likelier nemesis. In which case you need a C++/CLI wrapper.