C# Active events cause error in C++ client but work in VB6

See more:

C++

C#

ActiveX

interop

I am writing an ActiveX in C#, to be used in a legacy C++ application.
It all works ok, apart from event handling.
The problem is that when the event is raised in C# I get the following error:

An unhandled exception of type 'System.InvalidCastException' occurred in AwDotNetPinchOcx.dll
	Additional information: Unable to cast COM object of type 'System.__ComObject' to interface type 'AwDotNetPinchOcx.__AwPinchGrid'. 
	This operation failed because the QueryInterface call on the COM component for the interface with IID '{76F027EC-3D40-449E-B7E6-EFDF7CC63963}' 
	failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

However, the C# ActiveX works fine in a test VB6 app.
And, a native VB6 ActiveX works fine in the C++ application!

My work-around at the moment is therefore a VB6 ActiveX wrapper that contains the C# ActiveX,
and passes data and events across.

I want to get rid of this wrapper though.
Can anyone help?

=====================================
The C++ app uses the ConnectionPoint event sink method:
=====================================

DWORD EventSinkHolder::AddEventSink(REFIID iid, IUnknown* pYourEventSink)
   {
   DWORD sinkCookie = 0;
   IUnknown* pEventSource = EventSourceInterface();
   HASSERT(pEventSource);
   LPCONNECTIONPOINTCONTAINER pConnectionPointContainer = NULL;
   if (SUCCEEDED(pEventSource->QueryInterface(IID_IConnectionPointContainer, (void**)&pConnectionPointContainer)))
      {
      HASSERT(pConnectionPointContainer);
      LPCONNECTIONPOINT pConnectionPoint = NULL;
      if (SUCCEEDED(pConnectionPointContainer->FindConnectionPoint(iid, &pConnectionPoint)))
         {
         HASSERT(pConnectionPoint);
         // connect passed event sink interface to event source
         pConnectionPoint->Advise(pYourEventSink, &sinkCookie);
         pConnectionPoint->Release();
         // record connection for benefit of RemoveAllEventSinks
         mySinkEntries.Append(new EventSinkEntry(iid, sinkCookie));
         }
      pConnectionPointContainer->Release();
      }
   return sinkCookie;
   }

=====================================
=====================================
The C# ActiveX follows the standard method used in the Codeplex sample "CSActiveX"
http://1code.codeplex.com/SourceControl/changeset/view/45838#394692