asp.net 上传图片，自动压缩裁剪图片。

     mSchool_Picture m = new mSchool_Picture();
     m.Name = tbxName.Text;
     m.PicUrl = SavePicture(fupload, "schoolpic/", 700, 0);//直接调用方法，这里制定700就是固定宽度，高度自动调整

    /// <summary>
    /// 通过FileUpload上传图片并压缩图片大小
    /// 四各形式：1)指定高 按比例缩放; 2)指定宽 按比例缩放; 3)不缩放; 4)指定宽高裁剪(不会变形).
    /// </summary>
    /// <param name="fu">FileUpload对象</param>
    /// <param name="file">文件完整路径Path</param>
    /// <param name="width">图片压缩后的宽度</param>
    /// <param name="height">图片压缩后的高度</param>
    /// <returns>返回文件存储的完整路径</returns>
    public static string SavePicture(FileUpload fu, string file, int width, int height)
    {
        Random myRd = new Random();
        string prefix = System.IO.Path.GetExtension(fu.FileName).ToLower();
        string fileName = "";
        if (prefix.Equals(".jpg") || prefix.Equals(".gif") || prefix.Equals(".jpeg") || prefix.Equals(".pjpeg") || prefix.Equals(".png") || prefix.Equals(".bmp"))
        {
            string strFileName = DateTime.Now.ToString("yyyyMMddHHmmss") + myRd.Next(1000) + prefix;
            string name = System.Web.HttpContext.Current.Server.MapPath(file + strFileName);
            fileName = HttpContext.Current.Server.MapPath(file);
            string filepath = HttpContext.Current.Server.MapPath("/");
            try
            {
                //获取上传图片的大小
                Stream PicStream = fu.PostedFile.InputStream;
                
                if (width == 0 && height != 0)//指定高 按比例缩放
                {
                    MakeThumbnail(PicStream, fileName +  strFileName, width, height, "H", prefix);
                }
                else if (width != 0 && height == 0)//指定宽 按比例缩放
                {
                    MakeThumbnail(PicStream, fileName +  strFileName, width, height, "W", prefix);
                }
                else if (width == 0 && height == 0)//不缩放
                {
                    fu.SaveAs(fileName +  strFileName);
                }
                else//指定宽高裁剪(不会变形)
                {
                    MakeThumbnail(PicStream, fileName + strFileName, width, height, "Cut", prefix);
                }
                //返回图片保存的路径
                
                return file + strFileName;
            }
            catch
            {
                return null;
            }
        }
        else
        {
            return null;
        }
    }
   
    /// <summary>
    /// 生成缩略图
    /// </summary>
    /// <param name="originalImagePath">源图路径（物理路径）</param>
    /// <param name="thumbnailPath">缩略图路径（物理路径）</param>
    /// <param name="width">缩略图宽度</param>
    /// <param name="height">缩略图高度</param>
    /// <param name="mode">生成缩略图的方式</param>
    private static void MakeThumbnail(Stream PicStream, string thumbnailPath, int width, int height, string mode, string PicExtension)
    {
        System.Drawing.Image originalImage = System.Drawing.Image.FromStream(PicStream);
        int towidth = width;
        int toheight = height;

        int x = 0;
        int y = 0;
        int ow = originalImage.Width;
        int oh = originalImage.Height;

        switch (mode) 
        {
            case "HW"://指定高宽缩放（可能变形） 
                break;
            case "W"://指定宽，高按比例 
                toheight = originalImage.Height * width / originalImage.Width;
                break;
            case "H"://指定高，宽按比例
                towidth = originalImage.Width * height / originalImage.Height;
                break;
            case "Cut"://指定高宽裁减（不变形） 

                if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                {
                    oh = originalImage.Height;
                    ow = originalImage.Height * towidth / toheight;
                    y = 0;
                    x = (originalImage.Width - ow) / 2;
                }
                else
                {
                    ow = originalImage.Width;
                    oh = originalImage.Width * height / towidth;
                    x = 0;
                    y = (originalImage.Height - oh) / 2;
                }
                break;
            default:
                break;
        }

        //新建一个bmp图片
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);

        //新建一个画板
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);

        //设置高质量插值法
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

        //设置高质量，低速度呈现平滑程度  昆明卓联科技（网站、软件开发）
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

        //清空画布并以透明背景色填充
        g.Clear(System.Drawing.Color.Transparent);

        //在指定位置并且按指定大小绘制原图片的指定部分
        g.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, towidth, toheight),
        new System.Drawing.Rectangle(x, y, ow, oh),
        System.Drawing.GraphicsUnit.Pixel);

        try
        {
            //根据原来的图片格式，保存为原图片格式
            switch (PicExtension.ToLower())
            {
                case "jpg":
                    //以jpg格式保存缩略图
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
                    break;
                case "jpeg":
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
                    break;
                case "pjpeg":
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
                    break;
                case "gif":
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Gif);
                    break;
                case "bmp":
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Bmp);
                    break;
                case "png":
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Png);
                    break;
                default:
                    bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
                    break;
            } 

        }
        catch (System.Exception e)
        {
            throw e;
        }
        finally
        {
            originalImage.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }