COM Registration free (SxS) and proxy stub for STA->MTA

Question

I'm trying to use my C++ COM API (loaded inside a C++ application) from C# (as an addin) without any registration. I follow steps from this tutorials

So what I did is:

1. to create an activation context from my C++ application at startup. I'm passing my COM API manifest file to CreateActCtx and I enable it by ActivateActCtx
2. My C# addin is loaded by my C++ application (inside a custom AppDomain)
3. Inside my C# addin I'm trying 3 differents scenario.

• The first one is to create COM object inside the application main thread --> This is working

• The second is to create COM object and casting COM object inside a separated STA thread --> This is working

• The last case is to create COM object and casting COM object inside a separated MTA thread. And here I have a problem. I'm able to create an object but I'm not able to cast it. I received and exception like:

  Unable to cast COM object of type 'System.__ComObject' to interface type 'API.MyComObjectX'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{DE35BA2A-0566-40D3-AF1D-AD79D1133B09}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE))
  

So bellow, the code is the one I used for tested

/*
Manifest file loaded by my C++ application

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">

  <assemblyIdentity type="win32" name="ApiX.X" version="1.0.0.0"/>
  
  <file name="ApiX.dll">
    <comClass clsid="{76125B7A-E5FC-4362-93F8-3B912EB85D51}" threadingModel="Apartment"/> <!-- This define the MyComObjectX object -->
    <typelib tlbid="{4714D4C0-5AA9-47F7-82FF-458B341E7052}" version="1.0" helpdir=""/>
  </file>

  <comInterfaceExternalProxyStub name="IMyComObjectX" iid="{DE35BA2A-0566-40D3-AF1D-AD79D1133B09}" proxyStubClsid32="{00020424-0000-0000-C000-000000000046}" tlbid="{4714D4C0-5AA9-47F7-82FF-458B341E7052}" baseInterface="{00000000-0000-0000-C000-000000000046}" /> <!-- interface which is based on IDispatch -->

</assembly>

*/

public void testComFreeRegistration(object myComObjectX)
{
    try
    {
        MyComObjectX myComObjectX2 = new MyComObjectX();

        MyComObjectX myComObjectX3 = (MyComObjectX)myComObjectX; // Throw the exception

        MessageBox.Show("working");
    }
    catch (Exception e)
    {
        MessageBox.Show("fail");
        e.ToString();
    }
}


public void EntryPoint(object myComObjectX) // <-- Sent from my C++ application
{

    // The STA test which are working
    // var sta = new Thread(() => testComFreeRegistration(myComObjectX));
    // sta.SetApartmentState(ApartmentState.STA);
    // sta.Start();
    // sta.Join();

    var mta = new Thread(() => testComFreeRegistration(myComObjectX));
    mta.SetApartmentState(ApartmentState.MTA);
    mta.Start();
    mta.Join();
}

Another strange behavior is if I uncomment my STA test. Both test will work !! So I don't know what happen. It seems I have something not proerly initialized.

Do you have any idea of what happen or what I should check ? Why the STA <--> MTA mechanism seems to not load the comInterfaceExternalProxyStub configuration ?

Thanks in advance for your help ;)