Registration-free COM-like interop and threading

Question

I’m working on a desktop app that has WPF GUI and unmanaged C++ backend. I’ve defined API the following way (C# version, truncated):

[Guid( \"###\" ), Interface         


        

Answer 1

As hinted by Hans Passant, the key here is CoCreateFreeThreadedMarshaler API.

Add a private variable CComPtr<IUnknown> m_ftm;

Initialize that marshaller:

HRESULT FinalConstruct()
{
    IUnknown* pUnk = GetUnknown();
    return CoCreateFreeThreadedMarshaler( pUnk, &m_ftm );
}

Process IID_IMarshal in QueryInterface, by adding the following line to the interface map:

COM_INTERFACE_ENTRY_AGGREGATE( IID_IMarshal, m_ftm )

After that, C# code will happily call those objects from any thread. Of course you'll have to handle synchronization yourself, CComAutoCriticalSection / CComCritSecLock often help.

Update: MS broke my code in VS 2017, updated the answer. See edit history for VS2015 version.