ArcEngine中绘制圆、矩形、多边形，并测量其面积

//画圆            
IMap map = axMapControl1.Map;
IActiveView pActiveView = (IActiveView)map;
IGeometry pCir = axMapControl1.TrackCircle();
ISimpleFillSymbol pSimpleFillsym = new SimpleFillSymbolClass();
pSimpleFillsym.Style = esriSimpleFillStyle.esriSFSDiagonalCross;                
                IFillShapeElement pCirEle = new ESRI.ArcGIS.Carto.CircleElementClass();
                pCirEle.Symbol = pSimpleFillsym;
                ((IElement)pCirEle).Geometry = pCir;
                IGraphicsContainer pContainer = map as IGraphicsContainer;
                pContainer.AddElement((IElement)pCirEle, 0);
                //以下为测面积功能
                Double dArea;
                IArea pArea = (IArea)pCir;
                dArea = Math.Abs(pArea.Area);
                pActiveView.PartialRefresh(esriViewDrawPhase.esriViewGraphics, null, null);
                MessageBox.Show(dArea.ToString());





//画矩形
IMap map = axMapControl1.Map;
                IActiveView pActiveView = (IActiveView)map;
                IEnvelope pRec = axMapControl1.TrackRectangle();
                ISimpleFillSymbol pSimpleFillsym = new SimpleFillSymbolClass();
                pSimpleFillsym.Style = esriSimpleFillStyle.esriSFSDiagonalCross;                
                IFillShapeElement pRecEle = new ESRI.ArcGIS.Carto.RectangleElementClass();
                pRecEle.Symbol = pSimpleFillsym;