/// <summary>
        /// base64 字符串编码
        /// </summary>
        /// <param name="str">要编码的字符串</param>
        /// <returns></returns>
        public static string ToBase64(string str)
        {
            byte[] data = System.Text.ASCIIEncoding.ASCII.GetBytes(str);
            return Convert.ToBase64String(data); 
        }

        /// <summary>
        /// base 64 字符串解码
        /// </summary>
        /// <param name="base64str">要解码的字符串</param>
        /// <returns></returns>
        public static string UnBase64(string base64str)
        {
            byte[] data = Convert.FromBase64String(base64str);
            return System.Text.ASCIIEncoding.ASCII.GetString(data); 
        }