how to make the string rolling perfectly with the vc?

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VC6

C++

Now,I can make the string rolling in the dialog of vc, but the problem is that it can't show very perfectly,when it rolling from the bottom to the top,where it rolling, a long line would following, the code is in function OnTimer(),a fraction of code as follows：

m_nWidth+=5;            //defined in class
CClientDC dc(this);
TEXTMETRIC tm;
dc.GetTextMetrics(&tm);
CRect rect;
rect.left=0;
rect.top=200;
rect.right=m_nWidth;
rect.bottom=rect.top+tm.tmHeight;

dc.SetTextColor(RGB(0,255,0));
dc.SetBkColor(RGB(240,240,240));
CString str;
str.LoadString(IDS_STRING);

rect.top=150;
rect.bottom=rect.top+tm.tmHeight;
dc.DrawText(str,rect,DT_RIGHT);

CSize sz=dc.GetTextExtent(str);
if(m_nWidth==sz.cx)
{
    m_nWidth-=5;
    a--;
    if(a==-tm.tmHeight)
        a=150;
    rect.top=a;
    rect.bottom=a+tm.tmHeight;
    dc.DrawText(str,rect,DT_BOTTOM|DT_SINGLELINE);
}
CDialog::OnTimer(nIDEvent);

maybe help you understand me clearly,as follows is my question link： http://topic.csdn.net/u/20111125/12/52e25e41-24ce-487e-90ca-b6cab574af59.html[^]