FIX: Drop-down Combobox Does Not Move with Property Sheet

• Microsoft Visual C++ for Windows, 16-bit edition, versions 1.52, 1.52b
• Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2

SYMPTOMS

When a combo box on the active page of a CPropertySheet object is opened and the property sheet is moved by dragging it with the mouse, the drop-down portion of the combo box stays at its current screen position; it does not move with the underlying property sheet's dialog box.

NOTE: This bug consistently occurs in 16-bit applications running under Windows 95 or Windows version 3.x. However, it does not occur when running under Windows NT, and it occurs intermittently in 32-bit applications running under Windows 95.

CAUSE

The _AfxCancelModes internal MFC function is not called from CPropertySheet::PreTranslateMessage() as it is for CFrameWnd. In 32-bit Visual C++, _AfxCancelModes is defined as AfxCancelModes.

RESOLUTION

To work around this problem, derive a class from CPropertySheet and call _AfxCancelModes from the derived class's PreTranslateMessage when the WM_LBUTTONDOWN or WM_NCLBUTTONDOWN messages are received. See the "Step- By-Step Workaround" section in this article for an example.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Visual C++, 32-bit Edition, version 4.0.

MORE INFORMATION

Step-By-Step Workaround

1. Create a new class derived from CPropertySheet.
   
   
   1. Open the ClassWizard.
   
   
   
   2. Click the Add Class button.
   
   
   
   3. Choose a Class Type of generic CWnd. This is necessary because ClassWizard does not support creating a CPropertySheet class in 16-bit Visual C++. If you are using 32-bit Visual C++, you should be able to derive a class from CPropertySheet directly.
   
   
   
   4. Enter a Class Name such as CMySheet.
   
   
   
   5. Click the Create Class button.
   
   
   



2. If you are using 32-bit Visual C++, please skip steps 2.a, 2.b, and 2.c. In the new .h file (mysheet.h), make these changes:
   
   
   1. Change this:

      
               class CMySheet : public CWnd 

      To this:

      
               class CMySheet : public CPropertySheet 

   
   
   
   2. Add the following as the first line in the class definition:

      
            DECLARE_DYNAMIC(CMySheet) 

   
   
   
   3. Delete the default constructor declaration and add the following two constructors in its place:

      
            CMySheet(UINT nIDCaption, CWnd *pParentWnd = NULL,
               UINT iSelectPage = 0 ) : CPropertySheet(nIDCaption,
               pParentWnd, iSelectPage) { }
            CMySheet(LPCTSTR pszCaption, CWnd *pParentWnd = NULL,
               UINT iSelectPage = 0 ) : CPropertySheet(pszCaption,
               pParentWnd, iSelectPage) { } 

   
   
   
   4. Add a prototype for an override of PreTranslateMessage in a public section of the class definition:

      
            public:
                virtual BOOL PreTranslateMessage(MSG* pMsg); 

   
   
   



3. If you are using 32-bit Visual C++, please skip step 3.a. In the new .cpp file (mysheet.cpp), make these changes:
   
   
   1. Delete the default constructor function body.
   
   
   
   2. Add the definition of PreTranslateMessage:

      
            BOOL CMySheet::PreTranslateMessage(MSG* pMsg)
            {
                // check for special cancel modes for ComboBoxes
                if (pMsg->message == WM_LBUTTONDOWN ||
                pMsg->message == WM_NCLBUTTONDOWN)
              _AfxCancelModes(pMsg->hwnd);    // filter clicks
      
             return CPropertySheet::PreTranslateMessage(pMsg);
            } 

   
   
   
   3. Use The following sample code to define _AfxCanelModes. Place this code should be placed above the CMySheet::PreTranslateMessage definition. The code is needed because if the project is linking to the DLL version of MFC, you need to copy the code for this function and one other function that is called and insert it into the project. The code can be found in msvc\mfc\src\winutil.cpp. If the project links to a static MFC library, all you need to do is add a prototype for the function. The code handles both situations by using the #ifdef - #else - #endif preprocessor directives.

      
      
            /* Compile options needed: Default AppWizard options
            */ 
      
            #ifdef _AFXDLL  // define the functions
            BOOL PASCAL _AfxIsComboBoxControl(HWND hWnd, UINT nStyle)
            {
               if (hWnd == NULL)
               return FALSE;
               // do cheap style compare first
               if ((UINT)(::GetWindowLong(hWnd, GWL_STYLE) & 0x0F) != nStyle)
               return FALSE;
      
               // do expensive classname compare next
      
               // If using 16-bit Visual C++, use the following code
                  static char BASED_CODE szComboBox[] = "combobox";
                  char szCompare[sizeof(szComboBox) + 1];
                  ::GetClassName(hWnd, szCompare, sizeof(szCompare));
      
                // else if using 32-bit Visual C++, use the following code
                // and comment out the code above.
                // 
                //  static const TCHAR szComboBox[] = _T("combobox");
                //  TCHAR szCompare[sizeof(szComboBox)/sizeof(szCompare[0])+1];
                //  ::GetClassName(hWnd, szCompare,
                //                 sizeof(szCompare)/sizeof(szCompare[0]) );
      
                return (lstrcmpi(szCompare, szComboBox) == 0);
                  }
      
                void PASCAL _AfxCancelModes(HWND hWndRcvr)
                {
                // if we receive a message destined for a window, cancel any
                // combobox popups that could be in toolbars or dialog bars
                HWND hWndCancel = ::GetFocus();
                if (hWndCancel == NULL)
                return;     // nothing to cancel
      
                if (hWndCancel == hWndRcvr)
                return;     // let input go to window with focus
      
                // focus is in part of a combo-box
                if (!_AfxIsComboBoxControl(hWndCancel, (UINT)CBS_DROPDOWNLIST))
                {
                // try as a dropdown
                hWndCancel = ::GetParent(hWndCancel);// parent of edit is combo
                if (hWndCancel == hWndRcvr)
                return;     // let input go to part of combo
      
                if (!_AfxIsComboBoxControl(hWndCancel, (UINT)CBS_DROPDOWN))
                   return;     // not a combo-box that is active
                }
      
                // combo-box is active, but if receiver is a popup, do nothing
                if (hWndRcvr != NULL &&
                  (::GetWindowLong(hWndRcvr, GWL_STYLE) & WS_CHILD) != 0 &&
                  ::GetParent(hWndRcvr) == ::GetDesktopWindow())
                return;
      
                // finally, you should cancel the mode !
                ::SendMessage(hWndCancel, CB_SHOWDROPDOWN, FALSE, 0L);
            }
      
            #else   // just prototype it
            void PASCAL _AfxCancelModes(HWND hWndRcvr);
      
            #endif 

   
   
   

Additional query words: 2.00 2.51 2.52 3.00 3.10 3.20

Keywords : kbcode kbMFC KbUIDesign kbVC
Version : 1.52 1.52b | 2.00 2.10 2.20
Platform : NT WINDOWS
Issue type :