PRB: FromIDispatch Returns NULL for OLE Control

    COleDispatchImpl dispatch;

    ASSERT(*(DWORD*)&dispatch != 0);    // null vtable ptr?
    if (*(DWORD*)lpDispatch != *(DWORD*)&dispatch)
        return NULL;    // not our Idispatch*
				

RESOLUTION

It is possible to access the object of the class derived from COleControl given a valid IDispatch pointer. The object can be accessed from another control or outside of a control with the aid of a helper control. A helper control would be necessary because the following formula relies on details that are internal to a COleControl class.

To access the object of the class derived from COleControl successfully, you need to use a formula similar to this one:

   COleControl * pCtrl = (COleControl*)
       ((BYTE*)lpDisp - m_xDispatch.m_nOffset);
					

In this formula, lpDisp is a valid LPDISPATCH, m_xDispatch is the XDispatch object member of COleControl that contains the control's implementation of IDispatch, and m_nOffset is the offset generated by the INIT_INTERFACE_PART macro for IDispatch.

m_xDispatch is generated by the END_INTERFACE_PART macro as follows:

   #define END_INTERFACE_PART(localClass) \ 
      } m_x##localClass; \ 
      friend class X##localClass; \ 
				

In this case, localClass is "Dispatch" so m_x##localClass becomes m_xDispatch.

m_nOffset is generated by the INIT_INTERFACE_PART macro as follows:

    #ifndef _AFX_NO_NESTED_DERIVATION
    #define INIT_INTERFACE_PART(theClass, localClass) \ 
          size_t m_nOffset; \ 
             INIT_INTERFACE_PART_DERIVE(theClass,localClass) \ 

    #define INIT_INTERFACE_PART_DERIVE(theClass, localClass) \ 
          X##localClass() \ 
        { m_nOffset = offsetof(theClass, m_x##localClass); } \ 
m_nOffset becomes offsetof(COleControl, m_xDispatch).