PRB: CScrollView Scroll Range Limited to 32K (166473)


The information in this article applies to:

  • The Microsoft Foundation Classes (MFC), when used with:
    • Microsoft Visual C++, 32-bit Editions 4.0
    • Microsoft Visual C++, 32-bit Editions 4.1
    • Microsoft Visual C++, 32-bit Editions 4.2
    • Microsoft Visual C++, 32-bit Editions 5.0
    • Microsoft Visual C++, 32-bit Editions 6.0
This article was previously published under Q166473

SYMPTOMS

If the scroll range exceeds 32K when you are using the Scroll Thumb to scroll, CScrollView does not handle scrolling correctly. Unusual behavior occurs when you try to scroll beyond 32K.

CAUSE

This behavior is by design. The code to handle ranges greater than 32K was not added to CScrollView because it could fail if a WM_xSCROLL message is sent to the view to programatically scroll the contents. In Visual C++ version 6.0 on Windows NT, the limit has been extended to 2 gigabytes.

RESOLUTION

You can work around the problem easily by adding support to handle scroll ranges greater than 32K to a CScrollView derived class. Override the function as follows: 
   BOOL CMyScrollView::OnScroll(UINT nScrollCode, UINT nPos, BOOL
                                bDoScroll)
   {
      SCROLLINFO info;
      info.cbSize = sizeof(SCROLLINFO);
      info.fMask = SIF_TRACKPOS;

      if (LOBYTE(nScrollCode) == SB_THUMBTRACK)
      {
         GetScrollInfo(SB_HORZ, &info);
         nPos = info.nTrackPos;
      }

      if (HIBYTE(nScrollCode) == SB_THUMBTRACK)
      {
         GetScrollInfo(SB_VERT, &info);
         nPos = info.nTrackPos;
      }

      return CScrollView::OnScroll(nScrollCode, nPos, bDoScroll);
   }
In the code sample above, note that you can not depend on the value of nPos that is sent to the handler. It will be invalid for positions greater than 32K. Instead, use GetScrollInfo() to assign the correct position to nPos.

For additional information on this problem, click the article number below to view the article in the Microsoft Knowledge Base:

152252 HOWTO: How To Get 32-bit Scroll Position During Scroll Messages

NOTE: If you are trying to programatically scroll the contents in CscrollView, use the SetScrollInfo to correctly set the scroll bar information and then send or post the WM_xSCROLL message.

NOTE: Windows 95, Windows 98, Windows Millennium Edition (Me) and Win32s only support logical and device coordinates up to 32K. Because CScrollView uses logical units in SetScrollSizes(), you should specify a scroll range < 32K on when using CSrollView derived classes on these operating systems.

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Sridhar S. Madhugiri, Microsoft Corporation
Modification Type:MajorLast Reviewed:12/10/2003
Keywords:kbDocView kbprb kbProgramming kbScrollBar KbUIDesign kbusage KB166473
©2003 Microsoft Corporation. All rights reserved.