FIX: Disabled DDX Radio Button Causes Infinite Loop

WINDOWS   | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
      - Microsoft Visual C++ 32-bit Edition, version 1.0
  

SYMPTOMS

When DDX_Radio() is used to perform dialog data exchange (DDX) with a group of radio buttons, it loops continuously if the first button in the group has been disabled (either programmatically or with a resource editor such as App Studio).

CAUSE

This behavior is a result of the way DDX_Radio() determines when it has traversed every control in a group. The handle of the initial control is stored. A loop runs, which gets the handle of each successive control in the group with ::GetNextDlgGroupItem, and terminates when it sees the handle of the first control again. Unfortunately the function ::GetNextDlgGroupItem skips disabled controls, which is by design. Therefore the handle to the first control is not returned from the function (because it is disabled) and therefore the loop does not terminate.

RESOLUTION

To work around this problem, do one of the following:

• Arrange the group so that the first radio button is not disabled when any of the other controls in the group are enabled.

  -or-

• Call EnableWindow() to enable the radio button before DDX_Radio() is called [for example, in DoDataExchange()].

  -or-

• Write your own DDX_Radio() replacement, as shown in the sample code in the "MORE INFORMATION" section, below.

STATUS

Microsoft has confirmed this to be a problem in Microsoft Foundation Classes, versions 2.0 and 2.5. This problem was corrected in MFC version 3.0

MORE INFORMATION

One solution is to write your own DDX_Radio() replacement, which uses ::GetWindow() to iterate through all the controls on the dialog box until it encounters one with style "WS_GROUP" or a handle of "NULL". The following sample code is a substitute for DDX_Radio():

Sample Code

   //  DDX_MyRadio(), which is a modified DDX_Radio().
   //

   void AFXAPI DDX_MyRadio(CDataExchange* pDX, int nIDC, int& value)
       // must be first in a group of auto radio buttons
   {
      HWND hWndCtrl = pDX->PrepareCtrl(nIDC);

      ASSERT(::GetWindowLong(hWndCtrl, GWL_STYLE) & WS_GROUP);
      ASSERT(::SendMessage(hWndCtrl, WM_GETDLGCODE, 0, 0L) &
   DLGC_RADIOBUTTON);

      if( pDX->m_bSaveAndValidate )
         value = -1;     // value if none found

      // walk all children in group
      int iButton = 0;
      do
      {
         if( ::SendMessage(hWndCtrl, WM_GETDLGCODE, 0, 0L) &
   DLGC_RADIOBUTTON
   )
         {
         // control in group is a radio button
            if( pDX->m_bSaveAndValidate )
            {
               if( ::SendMessage(hWndCtrl, BM_GETCHECK, 0, 0L) != 0 )
               {
                  ASSERT(value == -1);    // only set once
                  value = iButton;
               }
            }
            else
            {
            // select button
               ::SendMessage( hWndCtrl, BM_SETCHECK, (iButton == value), 0L
   );
            }
            iButton++;
         }
         else
         {
            TRACE( "Warning: skipping non-radio button in group.\n" );
         }
         hWndCtrl = ::GetWindow( hWndCtrl, GW_HWNDNEXT );
      } while(hWndCtrl!=NULL &&
   !(GetWindowLong(hWndCtrl,GWL_STYLE)&WS_GROUP));
   }

      //{{AFX_DATA_MAP(CMyDialog)
      DDX_Radio(pDX, IDC_RADIO1, m_iRadio);
      //}}AFX_DATA_MAP

   with:

      //{{AFX_DATA_MAP(CMyDialog)
      DDX_MyRadio(pDX, IDC_RADIO1, m_iRadio);
      //}}AFX_DATA_MAP