在TrackingLayer中画线

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？

CMainFrame* pframe=(CMainFrame*)AfxGetMainWnd(); CMapView* pview=(CMapView*)pframe->GetActiveView();   CMoTrackingLayer tl;

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")));

pt.SetX(m_fCtlXmin); pt.SetY(m_fCtlYmin); pts.Add(pt); pt.SetX(m_fCtlXmax); pt.SetY(m_fCtlYmax); pts.Add(pt);

parts=line.GetParts(); parts.Add(pts);    if(LPDISPATCH(line))  tl.AddEvent(line,0);

pview->m_MapDisp.Refresh();