写入开始后无法设置此属性！在使用 GCM 的 C# WebRequest 对象上

我想将数据从c#发送到GCM，但我接受了这个错误。

代码在这里：

 WebRequest tRequest;
    tRequest = WebRequest.Create("https://android.googleapis.com/gcm/send");
    tRequest.Method = "post";
    tRequest.ContentType = " application/x-www-form-urlencoded;charset=UTF-8";
    tRequest.Headers.Add(string.Format("Authorization: key={0}", applicationID));

tRequest.Headers.Add（string.格式（"发件人： id={0}"， SENDER_ID））;

 con.Open();
            //int ekleyen_id = Convert.ToInt32(Session["kullaniciId"].ToString());
            SqlCommand cmd = new SqlCommand("select * from users", con);
            SqlDataReader rd = cmd.ExecuteReader();
            while (rd.Read()) {
                string regIdSend = rd["registered_id"].ToString();
                // string postData = "{ 'registration_id': [ '" + regId + "' ], 'data': {'message': '" + txtMsg.Text + "'}}";
                string postData = "collapse_key=score_update&time_to_live=108&delay_while_idle=1&data.message=" + value + "&data.time=" + System.DateTime.Now.ToString() + "&registration_id=" + regIdSend + "";
                Console.WriteLine(postData);
                Byte[] byteArray = Encoding.UTF8.GetBytes(postData);
                tRequest.ContentLength = byteArray.Length;
                Stream dataStream = tRequest.GetRequestStream();
                dataStream.Write(byteArray, 0, byteArray.Length);
                dataStream.Close();
                WebResponse tResponse = tRequest.GetResponse();
                dataStream = tResponse.GetResponseStream();
                StreamReader tReader = new StreamReader(dataStream);
                String sResponseFromServer = tReader.ReadToEnd();
                //lblStat.Text = sResponseFromServer;
                tReader.Close();
                dataStream.Close();
                tResponse.Close();