Arcengine将圆形、矩形转换为多边形 ；ICircularArc、IEnvelope转化为IPolygon

1、将ICircularArc转化为IPolygon

ICircularArc pCircularArc = null;
IRubberBand pRubberBand = new RubberCircleClass();
pCircularArc = pRubberBand.TrackNew(axMapControl_Main.ActiveView.ScreenDisplay, null) as ICircularArc;
if (pCircularArc == null)
    return;
object missing = Type.Missing;
ISegmentCollection pSegmentColl = new RingClass();
pSegmentColl.AddSegment((ISegment)pCircularArc, ref missing, ref missing);
IRing pRing = (IRing)pSegmentColl;
pRing.Close(); //得到闭合的环
IGeometryCollection pGeometryCollection = new PolygonClass();
pGeometryCollection.AddGeometry(pRing, ref missing, ref missing); //环转面
IPolygon pPolygon = (IPolygon)pGeometryCollection;

2、将IEnvelope转化为IPolygon

IEnvelope pEnvelope = null;
IRubberBand pRubberBand = new RubberEnvelopeClass();
pEnvelope = pRubberBand.TrackNew(axMapControl_Main.ActiveView.ScreenDisplay, null) as IEnvelope;
if (pEnvelope == null)
    return;
IPoint p1 = new PointClass();
IPoint p2 = new PointClass();
IPoint p3 = new PointClass();
IPoint p4 = new PointClass();
p1 = pEnvelope.UpperLeft;
p2 = pEnvelope.LowerLeft;
p3 = pEnvelope.LowerRight;
p4 = pEnvelope.UpperRight;
IPointCollection pPointCollection = new PolygonClass();
object missing = Type.Missing;
pPointCollection.AddPoint(p1, ref missing, ref missing);
pPointCollection.AddPoint(p2, ref missing, ref missing);
pPointCollection.AddPoint(p3, ref missing, ref missing);
pPointCollection.AddPoint(p4, ref missing, ref missing);
IPolygon pPolygon = (IPolygon)pPointCollection;

注意：在给IPointCollection添加点的时候要按照一定的顺序添加（按照顺时针或者逆时针的顺序选取起点进行添加，否则会使得图形发生变化）