一個圖片縮放類(按長寬比例縮放，自定義長寬縮放)

     public   class  Thumbnail
     {

        /**//// <summary>
        /// 制作缩略图(如果原圖長大於寬以長為基准縮放,反之以寬為基准縮放)
        /// </summary>
        /// <param name="fileName">原图路径</param>
        /// <param name="newFileName">新图路径</param>
        /// <param name="maxWidth">最大宽度</param>
        /// <param name="maxHeight">最大高度</param>
        public static void MakeThumbnailImage(string fileName, string newFileName, int maxWidth, int maxHeight)
        {
            Image original = Image.FromFile(fileName);

            Size _newSize = ResizeImage(original.Width, original.Height, maxWidth, maxHeight);
            //_image.Height = _newSize.Height;
            //_image.Width = _newSize.Width;
            Image displayImage = new Bitmap(original, _newSize);

            try
            {
                Image tempImage = displayImage;
                original.Dispose();
                tempImage.Save(newFileName, GetFormat(fileName));
            }
            finally
            {
                original.Dispose();
            }

        }
        /**//// <summary>
        /// 计算新尺寸
        /// </summary>
        /// <param name="width">原始宽度</param>
        /// <param name="height">原始高度</param>
        /// <param name="maxWidth">最大新宽度</param>
        /// <param name="maxHeight">最大新高度</param>
        /// <returns></returns>
        private static Size ResizeImage(int width, int height, int maxWidth, int maxHeight)
        {
            decimal MAX_WIDTH = (decimal)maxWidth;
            decimal MAX_HEIGHT = (decimal)maxHeight;
            decimal ASPECT_RATIO = MAX_WIDTH / MAX_HEIGHT;

            int newWidth, newHeight;

            decimal originalWidth = (decimal)width;
            decimal originalHeight = (decimal)height;

            if (originalWidth > MAX_WIDTH || originalHeight > MAX_HEIGHT)
            {
                decimal factor;
                // determine the largest factor 
                if (originalWidth / originalHeight > ASPECT_RATIO)
                {
                    factor = originalWidth / MAX_WIDTH;
                    newWidth = Convert.ToInt32(originalWidth / factor);
                    newHeight = Convert.ToInt32(originalHeight / factor);
                }
                else
                {
                    factor = originalHeight / MAX_HEIGHT;
                    newWidth = Convert.ToInt32(originalWidth / factor);
                    newHeight = Convert.ToInt32(originalHeight / factor);
                }
            }
            else
            {
                newWidth = width;
                newHeight = height;
            }

            return new Size(newWidth, newHeight);

        }

        /**//// <summary>
        /// 保存图片
        /// </summary>
        /// <param name="image">Image 对象</param>
        /// <param name="savePath">保存路径</param>
        /// <param name="ici">指定格式的编解码参数</param>
        private static void SaveImage(Image image, string savePath, ImageCodecInfo ici)
        {
            //设置 原图片 对象的 EncoderParameters 对象
            EncoderParameters parameters = new EncoderParameters(1);
            parameters.Param[0] = new EncoderParameter(Encoder.Quality, ((long)100));
            image.Save(savePath, ici, parameters);
            parameters.Dispose();
        }

        /**//// <summary>
        /// 获取图像编码解码器的所有相关信息
        /// </summary>
        /// <param name="mimeType">包含编码解码器的多用途网际邮件扩充协议 (MIME) 类型的字符串</param>
        /// <returns>返回图像编码解码器的所有相关信息</returns>
        private static ImageCodecInfo GetCodecInfo(string mimeType)
        {
            ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
            foreach (ImageCodecInfo ici in CodecInfo)
            {
                if (ici.MimeType == mimeType) return ici;
            }
            return null;
        }


        /**//// <summary>
        /// 得到图片格式
        /// </summary>
        /// <param name="name">文件名称</param>
        /// <returns></returns>
        public static ImageFormat GetFormat(string name)
        {
            string ext = name.Substring(name.LastIndexOf(".") + 1);
            switch (ext.ToLower())
            {
                case "jpg":
                case "jpeg":
                    return ImageFormat.Jpeg;
                case "bmp":
                    return ImageFormat.Bmp;
                case "png":
                    return ImageFormat.Png;
                case "gif":
                    return ImageFormat.Gif;
                default:
                    return ImageFormat.Jpeg;
            }
        }



        /**//// <summary>
        /// 自定義縮放圖(縮略圖時以圖頂端為起點)
        /// </summary>
        /// <param name="fileName"></param>
        /// <param name="newFileName"></param>
        /// <param name="newWidth"></param>
        /// <param name="newHeight"></param>
        public static void MakeCustomImage(string fileName, string newFileName, int newWidth,int newHeight)
        {
            Image image = Image.FromFile(fileName);

            int i = 0;
            int width = image.Width;
            int height = image.Height;
            if (width > height)
            {
                i = height;
            }
            else
            {
                i = width;
            }
            Bitmap b = new Bitmap(newWidth, newHeight);


            try
            {
                Graphics g = Graphics.FromImage(b);
                g.InterpolationMode = InterpolationMode.High;
                g.SmoothingMode = SmoothingMode.HighQuality;

                //清除整个绘图面并以透明背景色填充
                g.Clear(Color.Transparent);
                if (width < height)
                {
                    g.DrawImage(image, new Rectangle(0, 0, newWidth, newHeight), new Rectangle((width-newWidth)/2, 0, newWidth,newHeight), GraphicsUnit.Pixel);
                }
                else
                {
                    g.DrawImage(image, new Rectangle(0, 0, newWidth, newHeight), new Rectangle((width - newWidth) / 2, 0, newWidth, newHeight), GraphicsUnit.Pixel);
                }

                image.Dispose();
                SaveImage(b, newFileName, GetCodecInfo("image/" + GetFormat(fileName).ToString().ToLower()));
            }
            finally
            {
                image.Dispose();
                b.Dispose();
            }

        }


    }