Limiting the Number of Entries in a List Box

• Microsoft Windows Software Development Kit (SDK) versions 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

Although there is no single message that restricts the number of entries (lines) allowed in a list box, the limit can be imposed through the use of subclassing.

MORE INFORMATION

The following code fragment is an excerpt from a subclassing function that can be used to restrict the number of entries in a list box to no more than the constant MAXENTRIES where the lpfnOldLBFn variable points to the original list box window procedure:

long FAR PASCAL SubClassFn(hWnd, message, wParam, lParam) HWND hWnd; unsigned message; WORD wParam; LONG lParam; {

   int iCount;

   switch (message)
      {
   case LB_ADDSTRING:
   case LB_INSERTSTRING:
      iCount = SendMessage(hWnd, LB_GETCOUNT, 0, 0L);
      if (iCount > MAXENTRIES)
         { /* Insert action here to inform user of limit violation */
         break;
         }
      /* fall through if less entries than maximum */

   default:
      return CallWindowProc(lpfnOldLBProc, hWnd, message, wParam,
            lParam);
      }