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The latest version of this topic can be found at IUnknown.

IUnknown](http://msdn.microsoft.com/library/windows/desktop/ms680509) is the base interface of every other COM interface. This interface defines three methods: QueryInterface, AddRef, and Release. QueryInterface allows an interface user to ask the object for a pointer to another of its interfaces. AddRef and Release implement reference counting on the interface.

Introduction to COM
IUnknown and Interface Inheritance