Q147684: FIX: Sending WM_xSCROLL Message Causes Invalid ASSERT

Article: Q147684
Product(s): Microsoft C Compiler
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbMFC KbUIDesign kbVC400bug kbVC410fix kbGrpDSMFCATL kbNoUpdate
Last Modified: 06-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), included with:
   - Microsoft Visual C++, 32-bit Editions, version 4.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

Forcing a window to scroll to an absolute location by sending it a WM_HSCROLL or
WM_VSCROLL message with the SB_THUMBPOSITION or SB_THUMBTRACK scrolling codes
might cause an assertion. The assertion is in Wincore.cpp file on line 1938.


CAUSE
=====

The assertion tries to ensure that for the current message, the nTrackPos member
of the SCROLLINFO structure retrieved from a call to ::GetScrollInfo is the same
as the thumb position specified in the message. The value of this variable is
retrieved by calling ::GetScrollInfo, and it reflects the current scroll thumb
position only when the user is dragging the thumb. In the case of a
programmatically sent scroll message with the SB_THUMBPOSITION code, this value
is not valid. Because this value is indeterminate, it will not likely match the
thumb position specified in the message, and the assertion will fail.

Also, even if the assertion is ignored, the WM_xSCROLL message handler will be
called with an invalid scroll position when the WM_xSCROLL message is sent
programmatically.

RESOLUTION
==========

If the assertion is ignored, you cannot rely on the position value passed to the
WM_xSCROLL handler and must make a call to GetCurrentMessage() to get the values
specified in the message instead. If this is done properly, then the assertion
can be safely ignored.


For more information on this, please refer to the sample code included in this
article.

If the assertion occurs too often to manually select 'Ignore' in the assertion
failure message box, you can programmatically turn off assertion failures with
the following code:

  #ifdef _DEBUG
      int nPrevMode = _CrtSetReportMode(_CRT_ASSERT,0);
  #endif
     // Code that causes the assertion, for example:
     // SendMessage(WM_VSCROLL,...)
  #ifdef _DEBUG
      _CrtSetReportMode(_CRT_ASSERT,nPrevMode);
  #endif

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was corrected in Visual C++, 32-bit
Edition, version 4.1.

MORE INFORMATION
================

This assertion is part of some new code added in MFC 4.0 to accomplish 32 bit
scrolling and exists in the function CWnd::OnWndMsg.

Sample Code to Reproduce and Resolve the problem
------------------------------------------------

The following code demonstrates the problem:

  // Assume that nScrollPos is not 0

  WPARAM wParam = MAKELONG ( SB_THUMBPOSITION, nScrollPos);
  pView->SendMessage( WM_VSCROLL, wParam );

  // As mentioned above, ignore the assert.
  // Change the OnVScroll handler to the following :
  void CMyView::OnVScroll(UINT nSBCode, UINT nPos,
                          CScrollBar* pScrollBar)
  {
      if(nSBCode = SB_THUMBPOSITION )
      {
          const MSG* pMsg = GetCurrentMessage();
          nPos = (short)HIWORD(pMsg->wParam);
      }
  // Do your processing here
      CEditView::OnVScroll(nSBCode, nPos, pScrollBar);
  }

NOTE: CMyView is derived from CEditView. However the same technique works for any
CWnd derived class. Remember, the same problem also occurs for scroll messages
programmatically sent with the SB_THUMBTRACK code.

Additional query words: 4.00 Windows 95 vcrel 1948 vcfixlist410

======================================================================
Keywords          : kbMFC KbUIDesign kbVC400bug kbVC410fix kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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