COM reference counting - thread safety

Question

If I have a COM object, is it required for the AddRef() and Release() methods to be thread-safe- i.e., that I have to use atomic operations for my ref count?

Answer 1

Yes, if you are using the free threaded aparement model, use InterlockedIncrement() and InterlockedDecrement() to handle the ref count.

Answer 2

I think the answer is no. It is not required. If you want your COM object to be thread safe then those should be thread safe. Otherwise they do not have to be.

E.g. if you look here: The Rules of the Component Object Model it is not mentioned as a requirement. Also The COM Programmer's Cookbook (Building a COM Component) you can see a sample object without thread safe reference counting.

Microsoft code snippet:

ULONG COutside::AddRef (void)
{
    return ++ m_cRef;
}

In practice most implementations would do this because otherwise the COM objects would not be thread safe. If you know the object will only be used in one thread I believe it is an allowed optimization. Not all COM objects are thread safe, I've worked with a few that weren't.

To deal with the fact that COM objects may or may not be thread safe COM offers different "apartments" in which COM objects are created. In a single threaded apartment only a single thread can access objects within that apartment whereas in multi threaded apartment objects can be shared between multiple threads. Quoting from Understanding and Using COM Threading Models:

"Although multi-threaded apartments, sometimes called free-threaded apartments, are a much simpler model, they are more difficult to develop for because the developer must implement the thread synchronization for the objects, a decidedly nontrivial task."