AO中选择要素

<P>希望实现在MapControl上拖出一个矩形框选择要素,代码如下:<BR>void CShapefile2View::OnOnMouseDownMapcontrol1(long button, long shift, long X, long Y, double mapX, double mapY) <BR>{<BR> // TOD Add your control notification handler code here<BR> switch(m_operate)<BR> {<BR> case IDM_SELECT:<BR>  {<BR>   IActiveViewPtr ipView(m_ipMap);<BR>   IEnvelopePtr ipEnvelope;<BR>   ipEnvelope=m_MapControl.TrackRectangle();</P>
<P>   IGraphicsContainerPtr ipGraphicsContainer(m_ipMap);<BR>   IGraphicsContainerSelectPtr ipGCS(m_ipMap);<BR>   VARIANT_BOOL isEmpty;<BR>   ipEnvelope->get_IsEmpty(;isEmpty);<BR>   if(isEmpty) return;<BR>   IEnumElementPtr ipEnumElement;<BR>   ipGraphicsContainer->LocateElementsByEnvelope(ipEnvelope,;ipEnumElement);<BR>   if(ipEnumElement==NULL) <BR>   {<BR>    MessageBox("Nothing to be Selected!");<BR>    return;<BR>   }<BR>   else<BR>   {<BR>    ipGCS->UnselectAllElements();<BR>    ipEnumElement->Reset();<BR>    ipGCS->SelectElements(ipEnumElement);<BR>    ipView->Refresh();<BR>   }<BR>  }<BR>  break;<BR> default:<BR>  break;<BR> }<BR>}<BR>为何选择后总是出现"Nothing to be Selected!"?请帮帮忙,谢谢.</P>