Winform C#关于utf8编码问题

 public string SendDataByPost(string param, string Url)
        {
            try
            {
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
                byte[] bs = Encoding.UTF8.GetBytes(param);
                request.Method = "Post";
                request.ContentType = "application/x-www-form-urlencoded;charset=utf8";
                request.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0";
                request.Accept = "application/json, text/javascript, */*; q=0.01";
                request.ContentType = "application/x-www-form-urlencoded";
                request.ContentLength = bs.Length;
                Stream newSteam = request.GetRequestStream();
                newSteam.Write(bs, 0, bs.Length);
                newSteam.Close();
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                Stream myResponseStream = response.GetResponseStream();
                StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.UTF8);
                string retString = myStreamReader.ReadToEnd();
                myStreamReader.Close();
                myResponseStream.Close();

                return retString;
            }
            catch (Exception ex)
            {
                return ex.ToString();
            }

        }

　　

        public string MD5(string str)
        {
            //微软md5方法参考return FormsAuthentication.HashPasswordForStoringInConfigFile(str, "md5");
            byte[] b = Encoding.UTF8.GetBytes(str);
            b = new MD5CryptoServiceProvider().ComputeHash(b);
            string ret = "";
            for (int i = ; i < b.Length; i++)
                ret += b[i].ToString("x").PadLeft(, '');
            return ret;
        }