PRB: Modeless Dialog Box in a DLL Does Not Process TAB Key

• Microsoft Win32 Software Development Kit (SDK), used with:
  • Microsoft Windows versions 95, 98, 98 Second Edition
  • Microsoft Windows NT, versions 4.0, 4.0 SP4, 5.0

SYMPTOMS

When a modeless dialog box is launched from a dynamic-link library (DLL), the TAB key and the arrow keys do not move the focus from control to control as you would expect.

CAUSE

For a modeless dialog box to process a TAB key, the message pump needs to call the IsDialogMessage API. However, if you are writing a DLL and do not have access to the .exe's source code, you cannot modify the message pump to do this.

RESOLUTION

To work around this problem, you can use a WH_GETMESSAGE hook to capture the keystroke messages and call the IsDialogMessage API. If IsDialogMessage returns TRUE, then do not pass the message on to the message pump. Set the hook when handling WM_INITDIALOG and unset it when handling the WM_DESTROY message.

STATUS

This behavior is by design.

MORE INFORMATION

The following code illustrates how to set and unset the hook as well as how to use IsDialogMessage() to process TAB key messages:

BOOL CALLBACK DllDlgProc( HWND hwndDlg, UINT uMsg, WPARAM wParam, LPARAM lParam )
{
   switch ( uMsg )
   {
     case WM_INITDIALOG:
         hHook = SetWindowsHookEx( WH_GETMESSAGE, GetMsgProc,
                                   NULL, GetCurrentThreadId() );
         return TRUE;

     case WM_COMMAND:
         if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL) 
         {
            DestroyWindow( hwndDlg );
            hwndDllDlg = NULL;
         }
         return TRUE;

     case WM_DESTROY:
         UnhookWindowsHookEx( hHook );
         return FALSE;
   }
   return FALSE;
} 

LRESULT FAR PASCAL GetMsgProc(int nCode, WPARAM wParam, LPARAM lParam)
{
   LPMSG lpMsg = (LPMSG) lParam;

   if ( nCode >= 0 && PM_REMOVE == wParam )
   {
      // Don't translate non-input events.
      if ( (lpMsg->message >= WM_KEYFIRST && lpMsg->message <= WM_KEYLAST) )
      {
         if ( IsDialogMessage(hwndDllDlg, lpMsg) )
         {
            // The value returned from this hookproc is ignored, 
            // and it cannot be used to tell Windows the message has been handled.
            // To avoid further processing, convert the message to WM_NULL 
            // before returning.
            lpMsg->message = WM_NULL;
            lpMsg->lParam  = 0;
            lpMsg->wParam  = 0;
         }
      }
   }

   return CallNextHookEx(hHook, nCode, wParam, lParam);
} 

Additional query words: usrwndw usrdlg usrhooks usrhkgm usrtab

Keywords          : kbDlg kbHook kbNTOS400 kbWinOS2000 kbSDKPlatform kbSDKWin32 kbGrpUser kbWinOS95 kbWinOS98 kbDSupport 
Version           : WINDOWS:95,98,98 Second Edition
Platform          : WINDOWS 
Issue type        : kbprb 

Last Reviewed: August 5, 1999