FIX: Displaying CPropertySheet in COleControl Causes Assert

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 When MFC tries to call EnableWindow() through the pParentWnd pointer, the m_hWnd is no longer valid.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.

   /* 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);
   }