/// <summary>

        /// 通过Url获取到Image格式的文件

        /// </summary>

        /// <param name="url"></param>

        /// <returns></returns>

        public static Image UrlToImage(string url)

        {

            WebClient mywebclient = new WebClient();

            byte[] Bytes = mywebclient.DownloadData(url);

            using (MemoryStream ms = new MemoryStream(Bytes))

            {

                Image outputImg = Image.FromStream(ms);

                return outputImg;

            }

        }

        /// <summary>

        /// 再将Iamge文件转为Base64字符串

        /// </summary>

        /// <param name="img"></param>

        /// <returns></returns>

        public static string BitmapToBase64(Image img)

        {

            System.Drawing.Bitmap bitMap = new System.Drawing.Bitmap(img);

            string imgBase64 = string.Empty;

            using (MemoryStream memoryStream = new MemoryStream())

            {

                bitMap.Save(memoryStream, System.Drawing.Imaging.ImageFormat.Jpeg);

                byte[] buffer = new byte[memoryStream.Length];

                memoryStream.Position = 0;

                memoryStream.Read(buffer, 0, (int)memoryStream.Length);

                memoryStream.Close();

                imgBase64 = Convert.ToBase64String(buffer);

            }

            return imgBase64;

        }

/// <summary>

        /// 将Base64String转为图片并保存

        /// </summary>

        /// <param name="imgBase64"></param>

        public static void Base64ToImage(string imgBase64)

        {

            string format;

            string imgPath = Directory.GetCurrentDirectory() + "\\" + DateTime.Now.ToString("yyyyMMddHHmmss");

            byte[] buffer = Convert.FromBase64String(imgBase64);

            using (MemoryStream memoryStream = new MemoryStream(buffer))

            {

                System.Drawing.Bitmap bitMap = new System.Drawing.Bitmap(memoryStream);

                System.Drawing.Imaging.ImageFormat imgFormat = GetImageFormat(memoryStream, out format);

                imgPath = imgPath + format;

                bitMap.Save(imgPath, imgFormat);

                bitMap.Dispose();

            }

        }

        /// <summary>

        /// 获取Image图片格式

        /// </summary>

        /// <param name="file"></param>

        /// <param name="format"></param>

        /// <returns></returns>

        private static System.Drawing.Imaging.ImageFormat GetImageFormat(MemoryStream file, out string format)

        {

            byte[] sb = new byte[2];  //这次读取的就是直接0-1的位置长度了.

            file.Read(sb, 0, sb.Length);

            //根据文件头判断

            string strFlag = sb[0].ToString() + sb[1].ToString();

            //察看格式类型

            switch (strFlag)

            {

                //JPG格式

                case "255216":

                    format = ".jpg";

                    return System.Drawing.Imaging.ImageFormat.Jpeg;

                //GIF格式

                case "7173":

                    format = ".gif";

                    return System.Drawing.Imaging.ImageFormat.Gif;

                //BMP格式

                case "6677":

                    format = ".bmp";

                    return System.Drawing.Imaging.ImageFormat.Bmp;

                //PNG格式

                case "13780":

                    format = ".png";

                    return System.Drawing.Imaging.ImageFormat.Png;

                //其他格式

                default:

                    format = ".jpg";

                    return System.Drawing.Imaging.ImageFormat.Jpeg;

            }

        }

 static void Main(string[] args)

        {

            //string imgUrl = "D:\\c29e09b0-e5f4-43fa-9ded-8f9865a2e8d4.jpg";

            //string imgUrl = "https://www.baidu.com/img/superlogo_c4d7df0a003d3db9b65e9ef0fe6da1ec.png";

            string imgUrl = "http://file02.16sucai.com/d/file/2015/0408/779334da99e40adb587d0ba715eca102.jpg";//图片可以是本地磁盘地址、可以是http和https地址

            Image img = UrlToImage(imgUrl);//转成Image格式

            string imgBase64 = BitmapToBase64(img); //注意：生成的Base64字符串加上  data:image/jpeg;base64,  可以在前端识别

            Base64ToImage( imgBase64);//生成图片并保存

            Console.ReadKey();

        }