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

If your COM server is running in a different process, or a different apartment in the same process, COM needs to know how to package and transmit parameters when you make calls to your interface. This process is called "marshaling".

If you define a custom interface, you need to implement marshaling for it using one of the following approaches.

• Standard marshaling: have the MIDL compiler to generate a proxy and stub which you must register on the system. This is probably the best option since you have already defined your interface.
• OLE Automation marshaling: you define an automation compatible custom interface and use the marshaller which is already part of the COM framework
• Custom marshaling: you implement the methods of IMarshal

When you are debugging your COM server, although you see that you are returning your custom interface in the call to QueryInterface, it does not make it across the process boundary because COM cannot figure out how to marshal that interface, hence the client sees E_NOINTERFACE.

UPDATE (based on your comment)

If this is an existing COM server app then you probably already have a proxy/stub. You need to register this on both the client and server. Could it be that you were testing this on a new machine(s) and you simply forgot to register this? To register you simply do regsvr32 on the proxy/stub dll.

Answer 2

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.