look at this
// https://dev.twitter.com/discussions/16281
// Using Oauth API 1.1 C#
//Oauth Keys (Replace with values that are obtained from registering the application
//https://dev.twitter.com/apps/new
var oauth_consumer_key = "xxxxxxxxxxxxx";
var oauth_consumer_secret = "xxxxxxxxxxxxxxxxxxxxxx";
//Token URL
var oauth_url = "https://api.twitter.com/oauth2/token";
var headerFormat = "Basic {0}";
var authHeader = string.Format(headerFormat,
Convert.ToBase64String(Encoding.UTF8.GetBytes(Uri.EscapeDataString(oauth_consumer_key) + ":" +
Uri.EscapeDataString((oauth_consumer_secret)))));
var postBody = "grant_type=client_credentials";
ServicePointManager.Expect100Continue = false;
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(oauth_url);
request.Headers.Add("Authorization", authHeader);
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded;charset=UTF-8";
using (Stream stream = request.GetRequestStream())
{
byte[] content = ASCIIEncoding.ASCII.GetBytes(postBody);
stream.Write(content, 0, content.Length);
}
request.Headers.Add("Accept-Encoding", "gzip");
WebResponse response = request.GetResponse();