c++ - CoCreateInstance returning E_NOINTERFACE even though interface is found

Question

I have a COM class CMyCOMServer implementing IMyInterface in one application, both with correct GUIDs. CMyCOMServer::QueryInterface will return S_OK (and cast itself to the right type) if IUnknown or IMyInterface is requested, otherwise it returns E_NOINTERFACE.

In another app on the same PC, I call:

HRESULT hr = ::CoCreateInstance(__uuidof(CMyCOMServer), 0, CLSCTX_SERVER,
 __uuidof(IMyInterface ),(void **)&pInterface);

It returns E_NOINTERFACE. So I assumed I was doing something wrong and added a breakpoint on CMyCOMServer::QueryInterface. I found that when CoCreateInstance is called, QueryInterface is triggered several times for different interfaces:

• First, IUnknown is requested - no problem
• Then, several interfaces like IMarshall etc are requested... these are not supported so E_NOINTERFACE is returned
• Finally, IMyInterface is requested. I verify QueryInterface returns S_OK and sets (IMyInterface *)this as the interface pointer, as expected

So my confusion is why the calling CoCreateInstance is leaving me a NULL pointer and return code of E_NOINTERFACE, when the COM server app is clearly returning the interface I ask for?

EDIT: my client app calls CoInitialize(NULL) at startup, this makes no difference.

Answer 1

This happens because COM subsystem tries to marshal your custom interface (IMyInterface) and simply has no idea how to do that. That happens either because the server is out-proc or because the server is in-proc and the thread of the consumer application that calls CoCreateInstance() has called CoInitialize()/ CoInitializeEx() incorrectly so that "multithreaded apartment" is requested as mentioned in the article user Thomas refers to in the other answer.

If you only need an in-proc server you could suppress marshalling by ensuring that the thread calling CoCreateInstance() either calls CoInitialize() or CoInitializeEx() with COINIT_APARTMENTTHREADED to enforce "single-threaded apartment".

If you need an out-proc server you can't get around marshalling. In the latter case you could do one of the following:

• implement IMarshal - least preferable
• add proxy/stubs and register them for your custom interface
• (not sure if it will work for out-proc, but it's the simplest) if your interface can be marshalled with automation marshaller simply include a typlib into the resources of your COM server and register that typelib in the registry.