Article: Q138414
Product(s): Microsoft C Compiler
Version(s): 2.00 2.10 | 1.50 1.51 1.52 1.52
Operating System(s):
Keyword(s): kbprb
Last Modified: 30-JUL-2001
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The information in this article applies to:
- The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 4.1
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.51, 1.52, 1.52a
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SYMPTOMS
========
Calling CCmdTarget::FromIDispatch() inside a member function of a class derived
from COleControl or attempting to call FromIDispatch() passing an IDispatch
pointer belonging to an OLE Control, will always return NULL.
CAUSE
=====
CCmdTarget::FromIDispatch() does the following test to determine if the passed
IDispatch pointer belongs to a CCmdTarget-derived class or not:
COleDispatchImpl dispatch;
ASSERT(*(DWORD*)&dispatch != 0); // null vtable ptr?
if (*(DWORD*)lpDispatch != *(DWORD*)&dispatch)
return NULL; // not our Idispatch*
The problem here is that classes derived from COleControl don't use the
COleDispatchImpl implementation of IDispatch directly. They have their own
implementation using BEGIN_INTERFACE_PART and END_INTERFACE_PART that delegates
to the COleDispatchImpl object in the CCmdTarget base class. Because of this,
the v-tables in the previous test won't match and the function will return NULL.
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);
" (without the quotation marks) 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).
STATUS
======
This behavior is by design.
MORE INFORMATION
================
This difference between CCmdTarget and COleControl's implementation of IDispatch
disappears in Visual C++ version 4.0. COleControl uses COleDispatchImpl in
version 4.0 and therefore, FromIDispatch() will work.
Additional query words: 2.00 2.10 1.50 1.51 1.52 1.52a
======================================================================
Keywords : kbprb
Technology : kbAudDeveloper kbMFC
Version : 2.00 2.10 | 1.50 1.51 1.52 1.52
Issue type : kbprb
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