When implementing several COM interfaces at once how do I upcast to IUnknown?

Question

Suppose my COM object implements two or more COM interfaces:

class CMyClass : public IPersistFile, public IPersistStream {
};

when implementing

Answer 1

It doesn't matter which upcast you use, only that you use the same one always. I'd just pick a convention, such as always returning the first one declared in the inheritance list.