1.添加一个html

             
     
              图片:
              
      
原图宽:0 原图高:0             原图大小:0KB
      
      
设置宽度:          高度按原图比例折算
       
      
 
     
     
     

2.添加一个处理程序:ashx

public void ProcessRequest(HttpContext context)        {            //1.获取用户上传的文件流            HttpPostedFile file = context.Request.Files[0];            //获取文件名            string fileName = file.FileName;            //获取扩展名            string Extension = Path.GetExtension(fileName).ToUpper();            //2.根据用户上传的文件流创建一个图片            using (Image originalImage = Image.FromStream(file.InputStream))            {            //获取原始图片的宽和高                int owidth = originalImage.Width;                int oheight = originalImage.Height;                //缩略图的宽                int mwidth = Convert.ToInt32(context.Request.Form["width"]);                //等比例的高,取整数                int mheight = mwidth * oheight / owidth;                //3.根据原始图片，等比例创建一个缩小后的图片                using (Image minImage = new Bitmap(mwidth, mheight))                {                   //4.把大图片内容画到小图片上                    //基于小图创建一个画布对象                    Graphics gmin = Graphics.FromImage(minImage);                    //把大图画到小图上                    gmin.DrawImage(originalImage, 0, 0, mwidth, mheight);                    //5.下载缩略图                    MemoryStream ms = new MemoryStream();                    //判断图片类型                    ImageFormat imageFormat = null;                    string ContentType = "";                    switch (Extension)                    {                        case ".JPG": imageFormat = ImageFormat.Jpeg; ContentType = "image/jpeg"; break;                        case ".PNG": imageFormat = ImageFormat.Png; ContentType = "image/png"; break;                        case ".GIF": imageFormat = ImageFormat.Gif; ContentType = "image/gif"; break;                        //................如需要其他图片格式继续添加                    }                    minImage.Save(ms,imageFormat);                    context.Response.ClearContent();                    context.Response.AddHeader("Content-Disposition", "attachment;  filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));                     context.Response.ContentType=ContentType;                    context.Response.BinaryWrite(ms.ToArray());                    context.Response.End();                }            }        }        public bool IsReusable        {            get            {                return false;            }        }