FIX: Sending WM_xSCROLL Message Causes Invalid ASSERT

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.

#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
				

// 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);
}