一直提示签名错误。

使用后台生成的json字符串也不行。一直提示签名错误。

          string singurl = "http://msg.umeng.com/api/send";
            sign = md5("$POST$" + singurl + "$" + Json + "$" + IosAppsecret);

            string Url = "http://msg.umeng.com/api/send?sign=" + sign;
            var obj = HttpMethods.Post(Url, Json, "");
 




public static string md5(string str)
        {
            try
            {
                MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
                byte[] bytValue, bytHash;
                bytValue = System.Text.Encoding.UTF8.GetBytes(str);
                bytHash = md5.ComputeHash(bytValue);
                md5.Clear();
                string sTemp = "";
                for (int i = 0; i < bytHash.Length; i++)
                {
                    sTemp += bytHash[i].ToString("X").PadLeft(2, '0');
                }
                str = sTemp.ToLower();
            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }

            return str;
        }




 public static string Post(string url, string content,string Token="")
        {
            string result = "";
            try
            {
                HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
                req.Method = "POST";
                req.ContentType = "application/json;charset=utf-8";

                #region 添加Post 参数
                byte[] data = Encoding.UTF8.GetBytes(content);
                req.ContentLength = data.Length;
                using (Stream reqStream = req.GetRequestStream())
                {
                    reqStream.Write(data, 0, data.Length);
                    reqStream.Close();
                }
                #endregion
                if (!string.IsNullOrEmpty(Token))
                {
                    req.Headers.Add("token", Token);
                }

                HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
                Stream stream = resp.GetResponseStream();
                //获取响应内容
                using (StreamReader reader = new StreamReader(stream, Encoding.UTF8))
                {
                    result = reader.ReadToEnd();
                }
            }
            catch (Exception ex)
            {
                result = "{\"ret\":\"FAIL\",\"data\":{\"error_code\":\"xx\",    \"error_msg\":\""+ex.Message+"\"   }";
                //throw;
            }
            return result;
        }