asp.net 图片超过指定大小后等比例压缩图片的方法

/// <summary>
        /// 压缩图片
        /// </summary>
        /// <returns></returns>
        public string resizepic()
        {
            #region 压缩图片开始
            bool isimgfile = true;  //判断是否为图片文件
            string filepathname = "123";   //文件存储的路径（文件夹名称）
            string filename = "a.jpg";   //上传文件的原始名称
            string filesysname = datetime.now.tostring("yyyymmddhhmmssfff") + "_" + filename;  //修改后的文件名称
            string filepath = "";   //文件路径
            string strimgpath = "/fileupload/";   //上传路径
            if (isimgfile)
            {
                int maxwidth = 600;   //图片宽度最大限制
                int maxheight = 400;  //图片高度最大限制
                system.drawing.image imgphoto =
                    system.drawing.image.fromfile(server.mappath(strimgpath) + filepathname + "/" + filesysname);
                int imgwidth = imgphoto.width;
                int imgheight = imgphoto.height;
                if (imgwidth > imgheight)  //如果宽度超过高度以宽度为准来压缩
                {
                    if (imgwidth > maxwidth)  //如果图片宽度超过限制
                    {
                        float toimgwidth = maxwidth;   //图片压缩后的宽度
                        float toimgheight = imgheight / (float)(imgwidth / toimgwidth); //图片压缩后的高度

                        system.drawing.bitmap img = new system.drawing.bitmap(imgphoto,
                                                                              int.parse(toimgwidth.tostring()),
                                                                              int.parse(toimgheight.tostring()));
                        string strresizepicname = server.mappath(strimgpath) + filepathname + "/_small_" + filesysname;
                        img.save(strresizepicname);  //保存压缩后的图片
                        filepath = strimgpath + filepathname + "/_small_" + filesysname;  //返回压缩后的图片路径
                    }
                }
                else
                {
                    if (imgheight > maxheight)
                    {
                        float toimgheight1 = maxheight;
                        float toimgwidth1 = imgwidth / (float)(imgheight / toimgheight1);

                        system.drawing.bitmap img = new system.drawing.bitmap(imgphoto,
                                                                              int.parse(toimgwidth1.tostring()),
                                                                              int.parse(toimgheight1.tostring()));
                        string strresizepicname = server.mappath(strimgpath) + filepathname + "/_small_" + filesysname;
                        img.save(strresizepicname);
                        filepath = strimgpath + filepathname + "/_small_" + filesysname;
                    }
                }
            }
            return filepath;
            #endregion
        }