PRB: MFC App's CWinApp::ProcessMessageFilter() Never Invoked

WINDOWS             | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with:

  - Microsoft Visual C++ for Windows, 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); )
            ;

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.