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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Web;
using System.Net;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Text.RegularExpressions;
using OAuth;
namespace Social_Connect
{
    public partial class xing : Form
    {
        Manager oauth;
        public xing()
        {
            InitializeComponent();
        }

        private void xing_Load(object sender, EventArgs e)
        {
            oauth = new OAuth.Manager();
            // the URL to obtain a temporary "request token"
            var rtUrl = "https://api.xing.com/v1/request_token";
            oauth["consumer_key"] = "e9511ed63952b53206d8";
            oauth["consumer_secret"] = "146c0cc14814bd9e537fa4ba3e8c4efd80b021b7";
            oauth.AcquireRequestToken(rtUrl, "POST");
            var url = "https://api.xing.com/v1/authorize?oauth_token=" + oauth["token"];
            webBrowser1.Url = new Uri(url);
            //System.Diagnostics.Process.Start(url);
        }

        private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            string url = webBrowser1.Url.ToString();

            if (url.Contains("authorized=true"))
            {
                var divMarker = "id=\"verifier\""; // the div for twitter's oauth pin
                var index = webBrowser1.DocumentText.LastIndexOf(divMarker) + divMarker.Length;
                var pin = webBrowser1.DocumentText.Substring(index+1, 4);
                //var snip = webBrowser1.DocumentText.Substring(index);
                //var pin = Regex.Replace(snip, "(?s)[^0-9]*([0-9]+).*", "$1").Trim();
                Console.WriteLine(pin);
                //webBrowser1.Visible = false; // all done with the web UI
                oauth.AcquireAccessToken("https://api.xing.com/v1/access_token", "POST", pin);
                var appurl = "https://api.xing.com/v1/users/me";
                var authzHeader = oauth.GenerateAuthzHeader(appurl, "GET");
                var request = (HttpWebRequest)WebRequest.Create(appurl);
                request.Method = "GET";
                request.PreAuthenticate = true;
                request.AllowWriteStreamBuffering = true;
                request.Headers.Add("Authorization", authzHeader);

                using (var response = (HttpWebResponse)request.GetResponse())
                {
                    if (response.StatusCode != HttpStatusCode.OK)
                        MessageBox.Show("There's been a problem trying to tweet:" +
                                        Environment.NewLine +
                                        response.StatusDescription);
                }
            }

        }
    }
}					

1:

using (var response = (HttpWebResponse)request.GetResponse())