PRB: Windows Flash and Disappear in Dialog-Based Applications

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 5.0

SYMPTOMS

In a default dialog-based application, windows created after returning from DoModal flash and disappear.

CAUSE

In an MFC application when the main window associated with the application is destroyed, a WM_QUIT message is posted to the application. This message is posted by calling AfxPostQuitMessage from CWnd::OnNcDestroy for the main window. AfxPostQuitMessage calls PostQuitMessage, which performs some processing and then posts the WM_QUIT message to the application.

The PostQuitMessage function indicates to Windows that a thread has made a request to terminate. Any window that is created after calling the PostQuitMessage function will be immediately destroyed. The effect is that the window flashes for a brief moment and then disappears. If DoModal is called to display another modal dialog box, control returns immediately from this function.

RESOLUTION

Change the line that sets the m_pMainWnd to point to dialog object, into a comment. Or set m_pMainWnd for the CWinApp-derived object to NULL before control gets to CWnd::OnNcDestroy for the dialog object. One way of doing this is to override OnNcDestroy for the CDialog-derived object. In the overridden function, set the m_pMainWnd to NULL before calling the base class.

STATUS

This behavior is by design.

MORE INFORMATION

The following code can be implemented in the CDialog-derived class to prevent PostQuitMessage() from being called when the dialog box in a default dialog-based application is dismissed.

Sample Code to Resolve Behavior

   void CMyDialog::OnNcDestroy()
   {
      AfxGetApp()->m_pMainWnd = NULL;
      CDialog::OnNcDestroy();
   }