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Compiler Error C3138

Error Message

'interface' : a 'attribute' interface must inherit from IDispatch, or from an interface that inherits from IDispatch

An interface with the dual or dispinterface attributes does not have IDispatch as a direct or indirect base interface.

The following example generates C3138:

// C3138.cpp
#include <unknwn.h>

[ object, uuid("77ac9240-6e9a-11d2-97de-0000f805d73b") ]
__interface IMyCustomInterface
   HRESULT mf1(void);

[ dispinterface, uuid("3536f8a0-6e9a-11d2-97de-0000f805d73b") ]
__interface IMyDispInterface : IUnknown
   [id(1)] HRESULT mf2(void);

[ object, dual, uuid("34e90a10-6e9a-11d2-97de-0000f805d73b") ]
__interface IMyDualInterface : IMyCustomInterface  // C3138 expected
   HRESULT mf3(void);