Microsoft KB Archive/131099

= PRB: MFC App's CWinApp::ProcessMessageFilter Never Invoked =

Article ID: 131099

Article Last Modified on 11/21/2006

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APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 1.0 Professional Edition

 Microsoft Visual C++ 1.5 Professional Edition

 Microsoft Visual C++ 1.51

 Microsoft Visual C++ 1.52 Professional Edition</li></ul>

 Microsoft Visual C++ 5.0 Learning Edition</li></ul>

 Microsoft Visual C++ 2.0 Professional Edition</li></ul>

 Microsoft Visual C++ 2.1</li></ul>

 Microsoft Visual C++ 4.0 Professional Edition</li></ul>

 Microsoft Visual C++ 4.1 Subscription</li></ul> </li></ul>

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This article was previously published under Q131099

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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.

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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.

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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.

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STATUS
This behavior is by design.

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