protected void Button1_Click(object sender, EventArgs e)
       {
           int width = 300, height = 300;//生成缩略图时定义一个最大的宽和高
           System.Drawing.Image img = System.Drawing.Image.FromFile("d:/test.jpg");
           int swidth =img.Width;//源图片的宽
           int sheight = img.Height;//源图片的高
           var info = new SImg
           {
               width = swidth,
               height = sheight
           };
           info.GetHW(width, height, info);
           //得到缩放后的宽和高
           int w = info.width;
           int h = info.height;
           //获取缩略图
           System.Drawing.Image smallimg = img.GetThumbnailImage(w, h, new System.Drawing.Image.GetThumbnailImageAbort(() => { return false; }), IntPtr.Zero);
　　
       }
 
       public class SImg
       {
           public int width { get; set; }
           public int height { get; set; }
 
           public void GetHW(int width, int height, SImg img)
           {
               //如果宽和高任意一个超过定义的宽和高 执行等比
               if (img.width > width || img.height > height)
               {
                   img.width = img.width / 2;
                   img.height = img.height / 2;
                   GetHW(width, height, img);
               }
           }
 
       }