签名：实现参数字典排序，然后拼接为url参数形式

        /// <summary>
        /// 生成签名
        /// </summary>
        /// <param name="paramlst">参数列表</param>
        /// <param name="IsToUpper">是否转大写</param>
        /// <param name="IsDirect">是否直接加上签名Key</param>
        /// <returns></returns>
        public static string GenerateSign(List<KeyValuePair<string, string>> lst,string privateKey)
        {
            Comparison<KeyValuePair<string, string>> Comparer = new Comparison<KeyValuePair<string, string>>(CompareKeyValuepair);
            lst.Sort(Comparer);
            var paramlst =  lst.Where(p => !string.IsNullOrEmpty(p.Value)).ToList();
            string signstr = string.Join("&", paramlst.Select(p => p.Key + "=" + p.Value)) + privateKey;
            return Sign(signstr, "UTF-8");
        }

        /// <summary>
        /// 根据Key比较排序位置
        /// </summary>
        private static int CompareKeyValuepair(KeyValuePair<string, string> p1, KeyValuePair<string, string> p2)
        {
            return string.Compare(p1.Key, p2.Key);
        }        

/// <summary>
        /// 签名字符串
        /// </summary>
        /// <param name="prestr">需要签名的字符串</param>
        /// <param name="key">密钥</param>
        /// <param name="inputCharset">编码格式</param>
        /// <returns>签名结果</returns>
        public static string Sign(string prestr, string inputCharset)
        {
            StringBuilder sb = new StringBuilder();
            MD5 md5 = new MD5CryptoServiceProvider();
            byte[] t = md5.ComputeHash(Encoding.GetEncoding(inputCharset).GetBytes(prestr));
            foreach (byte t1 in t)
            {
                sb.Append(t1.ToString("x2"));
            }
            return sb.ToString();
        }