.net 上传图片压缩裁剪，压缩后大小一般不会变大

 [HttpPost]
        [AuthorizeIgnore]
        public ActionResult PhoneUpload()
        {
            try
            {
                var files = Request.Files;
                if (files == null || files.Count == 0)
                    throw new Exception("请选择需要上传的图片");

                var upFile = files[0];

                var allowType = new string[] { "image/jpeg", "image/jpg", "image/png", "image/gif" };
                if (!allowType.Contains(upFile.ContentType))
                    throw new Exception("请上传“.jpg/.png/.gif”格式的图片");

                if (upFile.ContentLength > 1024 * 1024 * 20)
                    throw new Exception("图片大小不能超20M");

                string ext = upFile.FileName.Substring(upFile.FileName.LastIndexOf('.'));

                var guid = Guid.NewGuid();
                string strpath = Path.Combine("UploadFiles", "imagebanklog", guid + ext);
                string strpath_th = Path.Combine("UploadFiles", "imagebanklog", guid + "_th" + ext);

                string path = Path.Combine(Server.MapPath("~/"), strpath);
                string path_th = Path.Combine(Server.MapPath("~/"), strpath_th);

                upFile.SaveAs(path);
                 
                GetThumImage(path, 90, 1024, path_th);
                try
                {
                    System.IO.File.Delete(path);
                }
                catch { }

                return Json(new { id = 0, success = true, result = strpath_th }, "text/html");

            }
            catch (Exception ex)
            {
                return Json(new { id = 0, success = false, msg = ex.Message.ToString() }, "text/html");
            }
        }
        /// <summary>
        /// 生成缩略图
        /// </summary>
        /// <param name="sourceFile">原始图片文件</param>
        /// <param name="quality">质量压缩比,0-100,100是最清晰</param>
        /// <param name="width">最大宽度</param> 
        /// <param name="outputFile">输出文件名</param>
        /// <returns>成功返回true,失败则返回false</returns>
        private   bool GetThumImage(string sourceFile, long quality, int width, string outputFile)
        {
            try
            {
                long imageQuality = quality;
                Bitmap sourceImage = new Bitmap(sourceFile);

                var oWidth = sourceImage.Width;
                var oHeight = sourceImage.Height;
                var height = oHeight;
                if (width > oWidth)
                {
                    width = oWidth;
                }
                else
                {
                    height = width * oHeight / oWidth;
                }
                 
                var myImageCodecInfo = GetEncoderInfo("image/jpeg");
                var myEncoder = System.Drawing.Imaging.Encoder.Quality;
                var myEncoderParameters = new EncoderParameters(1);
                var myEncoderParameter = new EncoderParameter(myEncoder, imageQuality);
                myEncoderParameters.Param[0] = myEncoderParameter;
                float xWidth = sourceImage.Width;
                float yWidth = sourceImage.Height;
                Bitmap newImage = new Bitmap(width, height);
                Graphics g = Graphics.FromImage(newImage);

                g.DrawImage(sourceImage, 0, 0, width, height);
                g.Dispose();
                newImage.Save(outputFile, myImageCodecInfo, myEncoderParameters);

                sourceImage.Dispose();
                sourceImage = null;
                newImage.Dispose();
                newImage = null;

                return true;
            }
            catch
            {
                return false;
            }

        }
   
        /// <summary>
        /// 获取图片编码信息
        /// </summary>
        private static ImageCodecInfo GetEncoderInfo(String mimeType)
        {
            int j;
            ImageCodecInfo[] encoders;
            encoders = ImageCodecInfo.GetImageEncoders();
            for (j = 0; j < encoders.Length; ++j)
            {
                if (encoders[j].MimeType == mimeType)
                    return encoders[j];
            }
            return null;
        }