//假設現有實體ent
                Entity ent = item as Entity;
                //如果實體為圓弧
                if (ent is Arc) 
                {
                    Arc arc = ent as Arc;
                    double R = arc.Radius;
                    Point3d startPoint = arc.StartPoint;
                    Point3d endPoint = arc.EndPoint;
                    Point2d p1, p2;
                    p1 = new Point2d(startPoint.X, startPoint.Y);
                    p2 = new Point2d(endPoint.X, endPoint.Y);
                    Double L = p1.GetDistanceTo(p2);
                    double H = R - Math.Sqrt(R * R - L * L / 4);

                    Polyline poly = new Polyline();
                    poly.AddVertexAt(0, p1, 2 * H / L, 0, 0);
                    poly.AddVertexAt(1, p2, 0, 0, 0);
                    poly.Color = reg.Color;
                    poly.LineWeight = reg.LineWeight;
                    poly.LinetypeId = reg.LinetypeId;
                    btr.AppendEntity(poly);
                    trans.AddNewlyCreatedDBObject(poly, true);
                }
                //如果實體為圓
                else if (ent is Circle) 
                {
                    Circle cir = ent as Circle;
                    double r = cir.Radius;
                    Point3d cc = cir.Center;
                    Point2d p1 = new Point2d(cc.X + r, cc.Y);
                    Point2d p2 = new Point2d(cc.X - r, cc.Y);
                    Polyline poly = new Polyline();
                    poly.AddVertexAt(0, p1, 1, 0, 0);
                    poly.AddVertexAt(1, p2, 1, 0, 0);
                    poly.AddVertexAt(2, p1, 1, 0, 0);
                    poly.Color = reg.Color;
                    poly.LineWeight = reg.LineWeight;
                    poly.LinetypeId = reg.LinetypeId;
                    btr.AppendEntity(poly);
                    trans.AddNewlyCreatedDBObject(poly, true);
                }