SAMPLE: Infinite Loop When Moving Through Dialog Box Control

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SUMMARY

An application enters an infinite loop when you use the TAB or arrow keys to navigate through dialog box controls. The problem occurs only when the dialog box has another dialog box as a child. When you use the TAB key, the problem occurs only when the child dialog box has the WS_MAXIMIZEBOX or WS_TABSTOPS style.

This article describes the problem in detail together with two possible solutions. A sample is also provided that demonstrates the problem and one of the workarounds.

SYMPTOMS

Problem 1

When arrow keys are used to maneuver through a dialog box's last control (the last control in the parent dialog box), the focus is passed to the next child (in this case, the child dialog box) and the application goes into an infinite loop.

Problem 2

When the TAB key is used to maneuver through the dialog box's last control, the focus is then passed to the next child (again the child dialog box) and the application goes into an infinite loop.

The infinite loop is clearly visible when spying on the child dialog box window. The child dialog box window continually receives WM_GETDLGCODE messages.

CAUSE

The dialog box manager confuses the child dialog box window with a child control.

When maneuvering through controls in a dialog box, the dialog box manager looks through its children and checks for the next child to pass focus to. In order to do that, the dialog box manager sets the focus to the child control and generates messages to complete the process.

Using the Arrow Keys to Navigate Through Controls

WS_GROUP defines the start of a group within a dialog box. All controls defined between two WS_GROUP styles are members of a group. Arrow keys can then be used to navigate between members of a group.

Normally a dialog box does not have WS_GROUP for a style; therefore, the child dialog box does not have a WS_GROUP but a previous control does have one. Based on the definition of a group, the child dialog box is considered a member of the last group by the dialog box manager. When the arrow key is used on the last control, the dialog box manager checks for the next child control in that group to pass the focus to, in this case the child dialog box window. The dialog box manager then tries to set focus to the window and generates necessary messages to complete the process. These generated messages are meaningful to a control but not to a window. One of these generated messages is WM_GETDLGCODE, which causes the infinite loop.

Using the TAB Key to Navigate Through Controls

WS_TABSTOPS defines which control has a tab stop. When you tab through controls, the dialog box manager looks for the next child that has the WS_TABSTOPS style and changes the focus to that child.

Normally a dialog box does not use WS_TABSTOPS for a style. It is quite normal, however, to use WS_MAXIMIZEBOX for a dialog box style. The child dialog box in this case has WS_MAXIMIZEBOX. The values for WS_TABSTOPS and WS_MAXIMIZEBOX are the same in WINDOWS.H. Because the dialog box manager expects a child control and not another window, the WS_MAXIMIZEBOX is interpreted as WS_TABSTOPS. So, when you tab on the last control of the parent dialog box, the dialog box manager tries to set the focus to the next child with WS_TABSTOPS, in this case the child dialog box. Then the dialog box manager generates the necessary messages to complete the process. These generated messages are meaningful to a control but not to a window. One of these messages is WM_GETDLGCODE, which causes the infinite loop.

RESOLUTION

Workarounds are described in the MORE INFORMATION section below.

MORE INFORMATION

• Do not include the WS_MAXIMIZEBOX style for the child dialog box to resolve the TAB key problem, and add WS_GROUP to the child dialog box to resolve the arrow key problem.
• Subclass the last control and handle the change of focus in the dialog box. This method is outlined below:

  1. Subclass the last control in the parent dialog box.

  2. In the subclass procedure for the control, process the WM_GETDLGCODE

        message by:
  

        a. Passing the WM_GETDLGCODE to the button window procedure.
        b. Or'ing the return value of (a.) with DLGC_WANTMESSAGE and
           returning it.
  
        The following is sample code demonstrating the WM_GETDLGCODE message:
  
           lpButtonProcOrg is a pointer to the button window
           procedure. This is a global and was set before setting
           the pointer to the subclass procedure for the button.
           To do this call:
  
                    lpButtonProgOrg = (FARPROC)
                    GetWindowLong(GetDlgItem(hCancelBtn,GWL_WNDPROC);
                    :
                    :
  
             case WM_GETDLGCODE:
               lRet = CallWindowProc(lpButtonProcOrg,hWnd,iMessage,
                                     wParam,lParam);
               if (lParam)
                 {
                 lpmsg= (LPMSG) lParam;
                 if (lpmsg->message == WM_KEYDOWN)
                  {
                  if ( (lpmsg->wParam == VK_TAB)   ||
                       (lpmsg->wParam == VK_DOWN)  ||
                       (lpmsg->wParam == VK_UP)    ||
                       (lpmsg->wParam == VK_RIGHT) ||
                       (lpmsg->wParam == VK_LEFT) )
                    lRet |= DLGC_WANTMESSAGE;
                  }
                 }
               return (lRet);
  
        For more information on WM_GETDLGCODE, please see the following
        article in the Microsoft Knowledge Base:
  
           ARTICLE-ID: Q83302
           TITLE     : Using the WM_GETDLGCODE Message
  
     3. In the subclass procedure, process the WM_KEYDOWN message by:
  
        a. Set the focus to the first control.
        b. If the first control is not a push button, then the process is
           complete; otherwise, the dark border needs to be removed from
           whichever button that has it, and placed on the first button, as
           follows:
  
           1) First send a WM_GETDLGCODE.
  
           2) If the return value is DLGC_DEFPUSHBUTTON, then send a
              BM_SETSTYLE message to the button that has the dark border,
              with BS_PUSHBUTTON for wParam. This will cause the dark border
              to be removed.
  
           3) Now the dark border needs to be placed on the button that has
              the focus (that is, the first button). To do this, send
              BM_SETSTYLE to the first button, with BS_DEFPUSHBUTTON for
              wParam.
  
        Sample Code Demonstrating the WM_KEYDOWN Message
        ------------------------------------------------
  
           case WM_KEYDOWN:
  
              // Check for keys that need this processing such as the
              // TAB and arrow keys.
              if ( (wParam == VK_TAB)   ||
                   (wParam == VK_DOWN)  || (wParam == VK_UP) ||
                   (wParam == VK_RIGHT) || (wParam == VK_LEFT) )
                 {
                 // Set the focus to the first control, Button #1
                 // in this case.
                 hwndBtn1 = GetDlgItem(hParentDlg,IDBUTTON1);
                 SetFocus(hwndBtn1);
  
                 // Loop through all the controls and remove the
                 // dark border that the previous default push
                 // button has.
                 hwndCtrls = GetWindow(hParentDlg, GW_CHILD);
                 while (hwndCtrls)
                     {
                     wRet = (WORD)(DWORD)SendMessage(hwndCtrls,
                                             WM_GETDLGCODE, 0, 0L);
  
                    if (wRet & DLGC_DEFPUSHBUTTON)
                      SendMessage(hwndCtrls, BM_SETSTYLE,
                                 (WPARAM)BS_PUSHBUTTON, TRUE);
  
                    hwndCtrls = GetWindow(hwndCtrls, GW_HWNDNEXT);
                    }
  
                // Give the hwndBtn1 button the default push button
                // border.
                 SendMessage(hwndBtn1,  BM_SETSTYLE,
                             (WPARAM)BS_DEFPUSHBUTTON, TRUE);
  
                 return(0L);
                 }
  
              break;
  
  

Download DLGTAB.EXE, a self-extracting file, from the Microsoft Software Library (MSL) on the following services:

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download DLGTAB.EXE (size: 32946 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get DLGTAB.EXE (size: 32946 bytes) 
  

There are two option buttons in the Parent dialog box: one demonstrates the problem and the other the solution. When the Demo Problem option button is selected, the sample demonstrates the problem. When the Fix Problem option button is selected, the sample demonstrates the workaround. The default button is the workaround for the problem.