FIX: Displaying CPropertySheet in COleControl Causes Assert

• Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

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 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);
   }