Q161874: FIX: Displaying CPropertySheet in COleControl Causes Assert

Article: Q161874
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1,4.2
Operating System(s): 
Keyword(s): kbole kbActiveX kbCOMt kbCtrl kbLocalSvr kbMFC KbUIDesign kbVC400bug kbVC500fix kbNoUpd
Last Modified: 31-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 4.0, 4.1 
- Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
- Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SYMPTOMS
========

Calling CPropertySheet::DoModal() from a method or message handler in a
COleControl-derived class may cause an assert in WINOCC.CPP, line 325, when the
property sheet is dismissed.

CAUSE
=====

The assert is in CWnd::EnableWindow() where it is checking if the m_hWnd member
of the CWnd object is valid.

In CPropertySheet::DoModal(), MFC gets the parent window via:

     CWnd* pParentWnd = CWnd::GetSafeOwner(m_pParentWnd,&hWndTop);

This creates a temporary CWnd* called pParentWnd. RunModalLoop() is called next
and returns after the property sheet is dismissed. When MFC tries to call
EnableWindow() through the pParentWnd pointer, the m_hWnd is no longer valid.

This behavior occurs because during RunModalLoop(), COleControl::OnPaint() will
be called. COleControl::OnPaint(), in turn, calls AfxUnlockTempMaps(). This
function deletes temporary objects that were previously created. In this case,
the "pParentWnd" pointer is being destroyed.

RESOLUTION
==========

You can prevent temporary objects from being destroyed by calling
AfxLockTempMaps() before CPropertySheet::DoModal(). You will need to call
AfxUnlockTempMaps() after CPropertySheet::DoModal() to allow temporary objects
to be cleaned up.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been corrected in Visual C++ version
5.0.

MORE INFORMATION
================

AfxLockTempMaps() and AfxUnlockTempMaps() are undocumented functions and may
change in future versions of MFC.

Sample Code
-----------

     /* Compile options needed: default
     This code snippet displays a CPropertySheet in response to a right-mouse
     click in the COleControl.
     */ 
     void CTestCtrl::OnRButtonDown(UINT nFlags, CPoint point)
     {
        CPropertySheet sheet;
        CMyPage page;
        sheet.AddPage (&page);

             // Prevent temporary objects from being destroyed.
        AfxLockTempMaps ();

        sheet.DoModal ();

             // Allow temporary objects to be cleaned up.
        AfxUnlockTempMaps ();

        COleControl::OnRButtonDown(nFlags, point);
     }

Additional query words: ocx

======================================================================
Keywords          : kbole kbActiveX kbCOMt kbCtrl kbLocalSvr kbMFC KbUIDesign kbVC400bug kbVC500fix kbNoUpdate 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC410 kbVC420 kbVC32bitSearch
Version           : winnt:4.0,4.1,4.2
Issue type        : kbbug
Solution Type     : kbfix

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