static void Main(string[] args)
        {
            Console.WriteLine(SHA256File(@"C:\BaiduYunDownload\66666.jpg"));
        }


        /// 计算文件的 SHA256 值 
        /// </summary> 
        /// <param name="fileName">要计算 SHA256 值的文件名和路径</param> 
        /// <returns>SHA256值16进制字符串</returns> 
        public static string SHA256File(string fileName)
        {
            return HashFile(fileName, "sha256");
        }

        /// <summary> 
        /// 计算文件的哈希值 
        /// </summary> 
        /// <param name="fileName">要计算哈希值的文件名和路径</param> 
        /// <param name="algName">算法:sha1,md5</param> 
        /// <returns>哈希值16进制字符串</returns> 
        public static string HashFile(string fileName, string algName)
        {
            if (!System.IO.File.Exists(fileName))
                return string.Empty;

            FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
            byte[] hashBytes = HashData(fs, algName);
            fs.Close();
            return ByteArrayToHexString(hashBytes);
        }


        /// <summary> 
        /// 字节数组转换为16进制表示的字符串 
        /// </summary> 
        public static string ByteArrayToHexString(byte[] buf)
        {
            string returnStr = "";
            if (buf != null)
            {
                for (int i = 0; i < buf.Length; i++)
                {
                    returnStr += buf[i].ToString("X2");
                }
            }
            return returnStr;
        } 


        /// <summary> 
        /// 计算哈希值 
        /// </summary> 
        /// <param name="stream">要计算哈希值的 Stream</param> 
        /// <param name="algName">算法:sha1,md5</param> 
        /// <returns>哈希值字节数组</returns> 
        public static byte[] HashData(Stream stream, string algName)
        {
            HashAlgorithm algorithm;
            if (algName == null)
            {
                throw new ArgumentNullException("algName 不能为 null");
            }
            if (string.Compare(algName, "sha256", true) == 0)
            {
                algorithm = SHA256.Create();
            }
            else
            {
                if (string.Compare(algName, "md5", true) != 0)
                {
                    throw new Exception("algName 只能使用 sha256 或 md5");
                }
                algorithm = MD5.Create();
            }
            return algorithm.ComputeHash(stream);
        }