外环构面

            //g为已有的geometry

            IPolygon pg = g.ShapeCopy as IPolygon;
            IRing r = new RingClass();
            pg.QueryExteriorRings(ref r);
            IPolygon po = new PolygonClass();
            IGeometryCollection gc = po as IGeometryCollection;
            object o=Type.Missing ;
            gc.AddGeometry(r as IGeometry ,ref o,ref o);
            gc.GeometriesChanged();