mo的 Buffer方法的使用，老出错，帮忙看一下程序，错在哪？？

<P> MapObjects2.Polygon buffline;<BR>  MapObjects2.Line line;<BR>  MapObjects2.Symbol sym1;<BR>  MapObjects2.Symbol sym2;<BR>  const double Bdist=1;<BR>  private void Form1_Load(object sender, System.EventArgs e)<BR>  {<BR>   sym1=new MapObjects2.SymbolClass();<BR>   sym1.SymbolType=MapObjects2.SymbolTypeConstants.moLineSymbol;<BR>   sym1.Style=(short)0;<BR>   sym1.Color=(uint)MapObjects2.ColorConstants.moRed;<BR>   sym2=new MapObjects2.SymbolClass();<BR>   sym2.SymbolType=MapObjects2.SymbolTypeConstants.moFillSymbol;<BR>   sym2.Style=(short)4;<BR>   sym2.Color=(uint)MapObjects2.ColorConstants.moGreen;<BR>  }</P>
<P>  private void axMap1_AfterTrackingLayerDraw(object sender, AxMapObjects2._DMapEvents_AfterTrackingLayerDrawEvent e)<BR>  {<BR>   axMap1.DrawShape(line,sym1);<BR>   if(buffline==null)<BR>   {<BR>    MessageBox.Show("fail");<BR>   }<BR>   else<BR>   {<BR>    axMap1.DrawShape(buffline,sym2);<BR>   }<BR>   <BR>  }</P>
<P>  private void axMap1_MouseDownEvent(object sender, AxMapObjects2._DMapEvents_MouseDownEvent e)<BR>  {<BR>   <BR>   line=axMap1.TrackLine();<BR>   buffline=(MapObjects2.Polygon)line.Buffer(Bdist,axMap1.FullExtent);<BR>   axMap1.TrackingLayer.Refresh(true,axMap1.Extent);<BR>   <BR>  }</P>
<P>运行之后在动态层上面画线，只显示直线，没有产生一个缓冲区域！为什么？</P>

  buffline=(MapObjects2.Polygon)line.Buffer(Bdist,axMap1.FullExtent);<BR>buffline是个polygon吗？Bdist是多少？