MD5函数（公共方法）

 #region MD5函数
        /// <summary>
        /// MD5函数
        /// </summary>
        /// <param name="str">原始字符串</param>
        /// <returns>MD5结果</returns>
        public static string MD5(string str)
        {
            byte[] b = encoding.GetBytes(str);
            b = new System.Security.Cryptography.MD5CryptoServiceProvider().ComputeHash(b);
            string ret = "";
            for (int i = 0; i < b.Length; i++)
            {
                ret += b[i].ToString("x").PadLeft(2, '0');
            }
            return ret;
        }
#endregion