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

Usually in cases where you want to cast to IUnknown an object with multiple inheritence from IUnknown, you cast it to one of its interface then cast to IUnknown...