/// <summary>
    /// 获取文件MD5值
    /// </summary>
    /// <param name="fileName">文件绝对路径</param>
    /// <returns>MD5值</returns>
    public static string GetMD5HashFromFile(string fileName)
    {
      try
      {
        FileStream file = new FileStream(fileName, FileMode.Open);
        System.Security.Cryptography.MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
        byte[] retVal = md5.ComputeHash(file);
        file.Close();

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < retVal.Length; i++)
        {
          sb.Append(retVal[i].ToString("x2"));
        }
        return sb.ToString();
      }
      catch (Exception ex)
      {
        throw new Exception("GetMD5HashFromFile() fail,error:" + ex.Message);
      }
    }