`public class ListExampleViewModel
    {
        SecurityToken sToken = null;
        public ListExampleViewModel()
        {
            GlobalConstants.isGetToken = true;
            var listResults = REQ_RESP.postAndGetResponse((new ListService().GetList("xx","xxx")));

            listResults.Subscribe(x =>
            {
                Console.WriteLine("nn..................................2");
                Console.WriteLine("Received Response==>" + x);
            });
        }
    }`

`public class REQ_RESP
    {
        private static string receivedAction = "";
        private static string receivedPostDate = "";


        public static IObservable<string> postAndGetResponse(String postData)
        {
            if (GlobalConstants.isGetToken)
            {
    //Pre Request for every reusest
                receivedPostDate = postData;
                GlobalConstants.isGetToken = false;
                getServerTokenMethod();
                postData = receivedPostDate;
            }

            HttpWebRequest serviceRequest =
            (HttpWebRequest)WebRequest.Create(new Uri(Constants.SERVICE_URI));


            var fetchRequestStream =
                 Observable.FromAsyncPattern<Stream>(serviceRequest.BeginGetRequestStream,
                                                                 serviceRequest.EndGetRequestStream);
            var fetchResponse =
                Observable.FromAsyncPattern<WebResponse>(serviceRequest.BeginGetResponse,
                                                            serviceRequest.EndGetResponse);

            Func<Stream, IObservable<HttpWebResponse>> postDataAndFetchResponse = st =>
            {
                using (var writer = new StreamWriter(st) as StreamWriter)
                {
                    writer.Write(postData);
                    writer.Close();
                }
                return fetchResponse().Select(rp => (HttpWebResponse)rp);
            };

            Func<HttpWebResponse, IObservable<string>> fetchResult = rp =>
            {
                if (rp.StatusCode == HttpStatusCode.OK)
                {

                    using (var reader = new StreamReader(rp.GetResponseStream()))
                    {
                        string result = reader.ReadToEnd();
                        reader.Close();
                        rp.GetResponseStream().Close();
                        XDocument xdoc = XDocument.Parse(result);
                        Console.WriteLine(xdoc);
                        return Observable.Return<string>(result);
                    }
                }
                else
                {
                    var msg = "HttpStatusCode == " + rp.StatusCode.ToString();
                    var ex = new System.Net.WebException(msg,
                        WebExceptionStatus.ReceiveFailure);
                    return Observable.Throw<string>(ex);
                }

            };

            return
                from st in fetchRequestStream()
                from rp in postDataAndFetchResponse(st)
                from str in fetchResult(rp)
                select str;
        }

        public static void getServerTokenMethod()
        {
            SecurityToken token = new SecurityToken();

            var getTokenResults = REQ_RESP.postAndGetResponse((new ServerToken().GetServerToken()));

            getTokenResults.Subscribe(x =>
            {
                ServerToken serverToken = new ServerToken();
                ServiceModel sm = new ServiceModel();
    //Parsing Response
                serverToken = extract(x, sm);
                if (!(string.IsNullOrEmpty(sm.NetErrorCode)))
                {
        MessageBox.Show("Show Error Message");
                }
                else
                {
                    Console.WriteLine("nn..................................1");
                    Console.WriteLine("nnserverToken.token==>" + serverToken.token);
                    Console.WriteLine("nnserverToken.pk==>" + serverToken.pk);
                }
            },
            ex =>
            {
                MessageBox.Show("Exception = " + ex.Message);
            },
            () =>
            {
                Console.WriteLine("End of Process.. Releaseing all Resources used.");
            });
        }
    }`