VC++ thread marshalling and COM : The application called an interface that was marshalled for a different thread

Question

Hi,

My VC++ 2005 Dialog based application initializes a COM object in the dialog class and uses it in the worker thread.

I called CoInitialize(NULL) At the start of the application and the at the start of the worker thread. But when a COM method is called the error "The application called an interface that was marshalled for a different thread" follows.

If I use CoInitializeEx(0,COINIT_MULTITHREADED) then I will get the same error message

Please help me in finding the root cause.

Thanks.

Answer 1

Probably CoMarshalInterface() and CoUnMarshalInterface() are the simplest way to do this.

Look at http://support.microsoft.com/kb/206076. You can download the code example and find different implementations of your requirements in Client.cpp.

Answer 2

Did you switch to CoInitializeEx in both places main app and thread? If not then try this configuration. It will be useful if you could post some source code to better describe issue. Anyway here is a good article that could help you in understanding apartments in COM http://msdn.microsoft.com/en-us/library/ms809971.aspx

Answer 3

You created two single-threaded apartments by calling CoInitialize(NULL). An interface pointer must be marshaled from one apartment to the other before it is usable. Initializing the worker thread as MTA doesn't solve the problem. The original interface pointer was still created in a single-threaded apartment and is thus not thread-safe. In other words, you cannot call the interface methods directly from a thread. Those calls have to be marshaled to the thread that created the interface. Marshaling the interface pointer sets up the plumbing that makes that possible.

The only time you don't have to marshal is when both threads are MTA. That's almost never possible, your main thread must be STA if it creates any windows. And the COM server would actually have to be thread-safe, they very rarely are. They advertise what they need with the ThreadingModel key in the registry. COM will actually create an STA thread if necessary to find a good home for the server.

You must marshal the pointer with CoMarshalInterThreadInterfaceInStream() to avoid the error. That's a fairly unfriendly function, IGlobalInterfaceTable is easier to use. The COM server also has to support it, you typically need a proxy/stub DLL that takes care of the marshaling. You'll get E_NOINTERFACE if it doesn't.

Also beware the overhead, marshaling a call from the worker thread to the main thread is pretty expensive and subject to how responsive your main thread is. In other words, if you wrote the thread to speed up your program or to avoid blocking the user interface then this won't actually work. It is the 'there is no free lunch' principle.

Answer 4

Hi

I think one of the ways to access COM objects inside another thread would be to use Global Interface Pointers. After initialization,form the GIT pointer to the thread along with dwCookie value. Then inside the thread reinterpret-cast the pointer as a DWORD and pass it to the GI table to get our COM pointer.

Thanks