CoCreateInstance returning E_NOINTERFACE even though interface is found

Mr. Boy picture Mr. Boy · Nov 23, 2009 · Viewed 15.6k times · Source

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

sharptooth picture sharptooth · Nov 23, 2009

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.