FIX: CCheckListBox Fails with Style LBS_MULTICOLUMN

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1

SYMPTOMS

When using the CCheckListBox class and specifying a style of LBS_MULTICOLUMN, the user will be unable to check or uncheck items that are not displayed in the first column.

CAUSE

The hit-testing that is done in CCheckListBox::OnLButtonDown does not take into consideration which column the user is clicking.

RESOLUTION

Derive a class from CCheckListBox and override OnLButtonDown to do proper hit-testing. It will also be necessary to override OnLButtonDblClk and the constructor.

Step-by-Step Example

1. Bring up ClassWizard. Use "Add Class" to add a new class called

       CMyCheckListBox derived from CListBox. In the .h and .cpp files
       generated for the new class, change all occurrences of CListBox
       to CCheckListBox.
   

2. Add a protected member variable of type int called m_nCheckWidth

       to the new class.
   

3. Modify the default constructor for CMyCheckListBox to look like this:

       CMyCheckListBox::CMyCheckListBox()
       {
           // the following code initializes the member variable
           // m_nCheckWidth to be the width of a check box.
   
       CBitmap bitmap;
       BOOL bWin4 = (BYTE)GetVersion() >= 4;
       HINSTANCE hInst = bWin4 ? NULL : LoadLibraryA("CTL3D32.DLL");
       FARPROC pfnProc = (NULL == hInst) ? NULL : GetProcAddress(hInst,
                                                      (LPCSTR)21);
   
       if (bWin4 || pfnProc != NULL)
          VERIFY(bitmap.LoadBitmap(AFX_IDB_CHECKLISTBOX_95));
       else
          VERIFY(bitmap.LoadBitmap(AFX_IDB_CHECKLISTBOX_NT));
   
       BITMAP bm;
       bitmap.GetObject(sizeof (BITMAP), &bm);
       m_nCheckWidth = bm.bmWidth / 3;
   
       if (hInst)
          FreeLibrary(hInst);
       }
   
   

4. Use ClassWizard to add a handler for WM_LBUTTONDOWN, and implement it

       as follows:
   

       void CMyCheckListBox::OnLButtonDown(UINT nFlags, CPoint point)
       {
           CRect itemRect;
           CRect clientRect;
   
           GetClientRect(clientRect);
           for(int nIndex = GetTopIndex(); nIndex < GetCount(); nIndex++)
           {
               GetItemRect(nIndex, &itemRect);
               if (!clientRect.PtInRect(itemRect.TopLeft()))
                   break;
               if (itemRect.PtInRect(point) && IsEnabled(nIndex))
               {
                   if (m_nStyle != BS_CHECKBOX && m_nStyle != BS_3STATE)
                   {
                       if (point.x - itemRect.left < m_nCheckWidth + 2)
                       {
                           CWnd* pParent = GetParent();
                           ASSERT_VALID(pParent);
                           int nModulo = (m_nStyle == BS_AUTO3STATE) ? 3 : 2;
                           int nCheck = GetCheck(nIndex);
                           nCheck = (nCheck == nModulo) ? nCheck - 1 : nCheck;
                           SetCheck(nIndex, (nCheck + 1) % nModulo);
                           InvalidateCheck(nIndex);
                           CListBox::OnLButtonDown(nFlags, point);
                           // Inform of check
                           pParent->SendMessage(WM_COMMAND,
                               MAKEWPARAM(GetDlgCtrlID(), CLBN_CHKCHANGE),
                               (LPARAM)m_hWnd);
                           return;
   
                       }
                   }
                   else
                       return; // Swallow LButtons for disabled items
               }
           }
           // call CListBox::OnLButtonDown
           // DO NOT call CCheckListBox::OnLButtonDown
           CListBox::OnLButtonDown(nFlags, point);
       }
   
   

5. Use ClassWizard to add a handler for WM_LBUTTONDDBLCLK and just call

       CMyCheckListBox::OnLButtonDown:
   

       void CMyCheckListBox::OnLButtonDblClk(UINT nFlags, CPoint point)
       {
           OnLButtonDown(nFlags, point);
       }
   
   

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 Visual C++, 32-bit Edition, version 4.2.