在TrackingLayer中画线

<P>请大家帮忙看看我的代码有什么问题？为什么线总是画不出来呢？</P>
<P>CMainFrame* pframe=(CMainFrame*)AfxGetMainWnd();
CMapView* pview=(CMapView*)pframe->GetActiveView();
 
CMoTrackingLayer tl;</P>
<P>tl=pview->m_MapDisp.GetTrackingLayer();
tl.SetSymbolCount(1);
tl.GetSymbol(0).SetSymbolType(moLineSymbol);
tl.GetSymbol(0).SetStyle(moSolidLine);
tl.GetSymbol(0).SetColor(color);
tl.GetSymbol(0).SetSize(1);
  
CMoLine line;
CMoPoint pt;
CMoPoints pts;
CMoParts parts;
VERIFY(line.CreateDispatch(TEXT("MapObjects2.Line")));
VERIFY(pt.CreateDispatch(TEXT("MapObjects2.Point")));
VERIFY(pts.CreateDispatch(TEXT("MapObjects2.Points")));</P>
<P>pt.SetX(m_fCtlXmin);
pt.SetY(m_fCtlYmin);
pts.Add(pt);
pt.SetX(m_fCtlXmax);
pt.SetY(m_fCtlYmax);
pts.Add(pt);</P>
<P>parts=line.GetParts();
parts.Add(pts);
  
if(LPDISPATCH(line))
 tl.AddEvent(line,0);</P>
<P>pview->m_MapDisp.Refresh();</P>