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Compiler Error C3706

'function' : must be a COM interface to fire COM events

The event interface that you use to fire COM events must be a COM interface. In this situation, the interface should either be defined using a Visual C++ attribute, or imported using #import from a type library with #import's embedded_idl attribute.

Note that the #include lines of the ATL header files shown in the sample below are required for using COM events. To fix this error, make IEvents (the eventing interface) a COM interface by applying one of the following attributes to the interface definition: object, dual, or dispinterface.

If an interface is from a header file generated by MIDL, the compiler will not recognize it as a COM interface.

The following sample generates C3706:

// C3706.cpp
// compile with: /c
// C3706 expected
#define _ATL_ATTRIBUTES 1
#include <atlbase.h>
#include <atlcom.h>
#include <atlctl.h>

[module(dll, name="idid", uuid="12341234-1234-1234-1234-123412341234")];

// Uncomment the following line to resolve.
// [object, uuid="12341234-1234-1234-1234-123412341237"]
__interface IEvents : IUnknown {
   HRESULT event1(/*[in]*/ int i);   // uncomment [in]
};

[dual, uuid="12341234-1234-1234-1234-123412341235"]
__interface IBase {
   HRESULT fireEvents();
};

[coclass, event_source(com), uuid="12341234-1234-1234-1234-123412341236"]
class CEventSrc : public IBase {
   public:
   __event __interface IEvents;

   HRESULT fireEvents() {
      HRESULT hr = IEvents_event1(123);
      return hr;
   }
};
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