Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;

            //获取第一个点
            PromptPointOptions ppo = new PromptPointOptions("\n 请选择一个点 ");
            PromptPointResult ppr = ed.GetPoint(ppo);

            Point3d basePoint = ppr.Value;

            PromptCornerOptions pco = new PromptCornerOptions("\n 请选择对角点 ", basePoint);
            PromptPointResult ppr2 = ed.GetCorner(pco);

            Point3d endPoint = ppr2.Value;