Ассинхронный WebRequest. Поток был недоступен для записи

        /// <summary>
        /// Послать ассинхронный post-запрос
        /// </summary>
        private void post_async(string url, string data, AsyncCallback on_async_callback)
        {
            var request = (HttpWebRequest)WebRequest.Create(server_url + url);
            request.Method = "post";
            request.ContentType = "application/x-www-form-urlencoded";
            byte[] sent_data = Encoding.GetEncoding(1251).GetBytes(data);
            request.ContentLength = sent_data.Length;
            Stream send_stream = request.GetRequestStream();
            send_stream.Write(sent_data, 0, sent_data.Length);
            send_stream.Close();
            IAsyncResult get_request_stream = request.BeginGetRequestStream(null, null);
            var writer = new StreamWriter(request.EndGetRequestStream(get_request_stream));
            writer.Write(data);
            writer.Close();
            request.BeginGetResponse(on_async_callback, request);
        }
 
        private void OnAsyncCallback(IAsyncResult async_result)
        {
            var httpWebRequest = (HttpWebRequest)async_result.AsyncState;
            WebResponse response = httpWebRequest.EndGetResponse(async_result);
            var reader = new StreamReader(response.GetResponseStream());
            string str = reader.ReadToEnd();
        }

var writer = new StreamWriter(request.EndGetRequestStream(get_request_stream));

        /// <summary>
        /// Послать ассинхронный post-запрос
        /// </summary>
        public void post_async()
        {
            var request = (HttpWebRequest)WebRequest.Create(_url_path);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.BeginGetRequestStream(GetRequestStreamCallback, request);
        }
 
        private void GetRequestStreamCallback(IAsyncResult asynchronousResult)
        {
            HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;
            Stream postStream = request.EndGetRequestStream(asynchronousResult);
            byte[] byteArray = Encoding.UTF8.GetBytes(_date);
            postStream.Write(byteArray, 0, _date.Length);
            postStream.Close();
            request.BeginGetResponse(OnAsyncCallback, request);
        }
 
        private void OnAsyncCallback(IAsyncResult async_result)
        {
            var httpWebRequest = (HttpWebRequest)async_result.AsyncState;
            WebResponse response = httpWebRequest.EndGetResponse(async_result);
            var reader = new StreamReader(response.GetResponseStream());
            string str = reader.ReadToEnd();
        }