STA (Single Threaded Apartment) COM Object - Spawn worker threads?

Question

Is it a bad thing to spawn worker threads in your STA COM object (ie. COM object creates a thread to perform a task)? I think, the answer is - that depends!

For example in my case: The worker threads that I am using will not interfere/access COM or COM Services.

Reason why I am asking this is because by STA COM definition STA can only house one thread. Spawning multiple threads kind of goes against this principle unless the worker threads and the work they do NOT interfere/deal with COM/COM services. In this case I am thinking this is perfectly fine and in my opinion the worker threads should not be considered by COM as part of the logical STA.

What are your thoughts on this?

Answer 1

No, that's not a bad thing. Apartments explicitly exist to help you getting multi-threaded code working. An STA thread is a safe home for a COM server that's not thread-safe, COM's apartment threading model ensures that it is always used in a thread-safe way. All you have to do is the marshal the interface pointer you want to use in the worker thread (IGlobalInterfaceTable for example) and you can call the methods without doing anything special.

This doesn't come for free of course, there's overhead involved in marshaling the call. How much depends on how responsive the STA thread is when it pumps its message loop. If you intended to create the worker thread explicitly to use that COM server in a multi-threaded way then of course you'll not be ahead, you made it slower.