Catching events from a COM object in an ActiveX control

[Pavel Kotrc]

I have a MFC ActiveX control which displays data from a COM object (which is in a separate DLL). I want to be able to catch events fired from this object. I tried to derive the ActiveX implementation class from the IDispEventImpl class and add an event sink map, but when the event is fired I get a crash in the CComCriticalSection::Lock() method. I suppose this is due to the fact the ActiveX is not a regular COM object.

An interesting thing is that the crash appears only the first time an event is fired. The subsequent firings are all OK. I was tracing the code and found out that this occurs in CComTypeInfoHolder::GetTI(LCID lcid, ITypeInfo** ppInfo) (in atlcom.h file) because the m_pInfo member is initially NULL and the GetTI(lcid) method (in which the crash appears) is called. After that the m_pInfo member is filled and the code works.

Any thoughts how to solve this?

[Pavel Kotrc]

Problem solved!

The trick is to manually provide information about the event methods. So first in the header file for the ActiveX control declare one _ATL_FUNC_INFO object for each event in the sink map:

extern _ATL_FUNC_INFO OnMyEventInfo;

The declaration must not be inside the class definition.

Then in the implementation file fill these object with the correct information:

_ATL_FUNC_INFO OnMyEventInfo = {CC_STDCALL, VT_EMPTY, 0, {}};

(the values will be different for your methods)

Finally, use the SINK_ENTRY_INFO macros instead of SINK_ENTRY_EX ones in the sink map:

	
BEGIN_SINK_MAP(CReportCtrl)
  SINK_ENTRY_INFO(101, DIID__IMyObjectEvents, 1, OnMyEvent, &OnMyEventInfo)
END_SINK_MAP()

And that's it