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Q138681: PRB: Windows Flash and Disappear in Dialog-Based Applications

Article: Q138681
Product(s): Microsoft C Compiler
Version(s): 2.0,2.1,2.2,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbDlg kbMFC kbVC200 kbVC400 kbVC500 kbVC600 kbGrpDSMFCATL kbDialog
Last Modified: 27-MAR-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0 
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
   - Microsoft Visual C++.NET (2002) 
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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.

Steps to Resolve Behavior
-------------------------

1. Place a message map entry for the WM_NCDESTROY message in the CMyDialog class
  implementation file.

           ON_WM_NCDESTROY()

2. Add the OnNcDestory member function to the CMyDialog class and set the
  m_pMainWnd member variable of the application object to NULL.

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

Additional query words: messagebox

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Keywords          : kbcode kbDlg kbMFC kbVC200 kbVC400 kbVC500 kbVC600 kbGrpDSMFCATL kbDialog 
Technology        : kbAudDeveloper kbMFC kbVCNET
Version           : :2.0,2.1,2.2,4.0,5.0,6.0
Issue type        : kbprb

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