c# PUT POST HTTPS 基础连接已经关闭: 发送时发生错误

public void Put()
        {
            string data = "";
            //Web访问对象，构造请求的url地址
            string serviceUrl = string.Format("https://");
            //构造http请求的对象
            ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072; //一定要有这一句

            HttpWebRequest myRequest = (HttpWebRequest)WebRequest.Create(serviceUrl);
            //转成网络流
            byte[] buf = System.Text.Encoding.GetEncoding("UTF-8").GetBytes(data);
            //设置

            ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
            ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;


            myRequest.Method = "PUT";
            myRequest.ContentLength = buf.Length;
            myRequest.ContentType = "application/json";
            myRequest.MaximumAutomaticRedirections = 1;
            myRequest.AllowAutoRedirect = true;
            // 发送请求
            Stream newStream = myRequest.GetRequestStream();
            newStream.Write(buf, 0, buf.Length);
            newStream.Close();
            // 获得接口返回值
            HttpWebResponse myResponse = (HttpWebResponse)myRequest.GetResponse();
            StreamReader reader = new StreamReader(myResponse.GetResponseStream(), Encoding.UTF8);
            string ReturnXml = reader.ReadToEnd();
            reader.Close();
            myResponse.Close();
            Key.token= ReturnXml;
        }