Q142960: FIX: CCheckListBox Fails with Style LBS_MULTICOLUMN

Article: Q142960
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1
Operating System(s): 
Keyword(s): kbcode kbListBox kbMFC KbUIDesign kbVC400bug kbVC410bug kbVC420fix kbGrpDSMFCATL kbNoUp
Last Modified: 06-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, 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.

Additional query words: kbVC400bug 4.00 4.10 vcfixlist420

======================================================================
Keywords          : kbcode kbListBox kbMFC KbUIDesign kbVC400bug kbVC410bug kbVC420fix kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0,4.1
Issue type        : kbbug
Solution Type     : kbfix

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