FIX: Sending WM_xSCROLL Message Causes Invalid ASSERT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, 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);

   // Code that causes the assertion, for example:
   // SendMessage(WM_VSCROLL,...)

    _CrtSetReportMode(_CRT_ASSERT,nPrevMode);

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

// 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.