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Q131099: PRB: MFC App’s CWinApp::ProcessMessageFilter() Never Invoked

Article: Q131099
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbnokeyword kbMFC kbVC kbVC100 kbVC200 kbVC410 kbGrpDSMFCATL
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 4.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

When running a 16-bit MFC application under Windows version 3.x, the
application's CWinApp::ProcessMessageFilter() is never invoked.

When running a 32-bit DEBUG MFC application under Win32s, the application
generates an assertion in the CWnd::CreateEx() function. When the application is
built for RELEASE mode, Win32s reports an Unhandled Exception.

CAUSE
=====

Calling the SetMessageQueue() function from within an MFC application causes the
problems listed above. Depending on the version of MFC used, MFC sets up one or
more Window hooks when the application is first loaded. Calling
SetMessageQueue() under Windows version 3.x, allocates a new message queue for
your application and deletes the old one. After these hooks have been
established, calling SetMessageQueue() destroys the hooks.

This problem does not occur when running under Windows NT, because
SetMessageQueue() does nothing on Windows NT. Windows NT automatically increases
the application's message queue as needed.

RESOLUTION
==========

Newer versions of MFC, starting with Visual C++ version 1.5, automatically
attempt to set the application's message queue to a size of 96 using a loop
similar to this one:

          // attempt to make the message queue bigger
          for (int cMsg = 96; !SetMessageQueue(cMsg) && (cMsg -= 8); )
              ;

Note that an attempt is made to set the message queue to 96. If this attempt
fails, another attempt is made with a smaller value.

If a larger message queue is needed or a failure is required in the event so that
the message queue cannot be set at a desired size, you will need to provide a
custom WinMain() function that duplicates the functionality of MFC's
implementation of WinMain(). You can find the MFC implementation of WinMain() in
\MFC\SRC\WINMAIN.CPP. Use it as a guideline for writing a custom WinMain()
function for your application.

STATUS
======

This behavior is by design.

REFERENCES
==========

For more information on SetMessageQueue() and SetWindowsHookEx(), please consult
the Visual C++ Help menu.

Additional query words: 1.00 1.50 2.00 2.10 4.00 4.10

======================================================================
Keywords          : kbnokeyword kbMFC kbVC kbVC100 kbVC200 kbVC410 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbprb

=============================================================================

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