Part of the https://gist.github.com/jasongaylord/df33fe4cd4bc7d253a10c3cdccc41104#file-default-cshtml

TwitterService.cs

public class TwitterService : ITwitterService
{
  public string ObtainBearerToken(string consumerKey, string consumerSecret)
  {
    var applicationAuthorization = Convert.ToBase64String(Encoding.UTF8.GetBytes(string.Format("{0}:{1}", Uri.EscapeDataString(consumerKey), Uri.EscapeDataString(consumerSecret))));

    HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://api.twitter.com/oauth2/token");
    request.Headers.Add("Authorization", "Basic " + applicationAuthorization);
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded;charset=UTF-8";
    request.AutomaticDecompression = DecompressionMethods.GZip;
    using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))
    {
      writer.Write("grant_type=client_credentials");
    }

    WebResponse response = request.GetResponse();
    var token = "";
    using (var reader = new StreamReader(response.GetResponseStream()))
    {
      token = reader.ReadToEnd();
    }

    return JsonConvert.DeserializeObject<Token>(token).access_token;
  }

  public async Task<List<Tweet>> RetrieveTweetsAsync(TwitterOptions options)
  {
    var tweets = new List<Tweet>();

    if (!string.IsNullOrEmpty(options.Username) && !string.IsNullOrEmpty(options.ConsumerKey) && !string.IsNullOrEmpty(options.ConsumerSecret))
    {
      var bearerToken = ObtainBearerToken(options.ConsumerKey, options.ConsumerSecret);

      var timelineUrl = 
        string.Format("https://api.twitter.com/1.1/statuses/user_timeline.json?exclude_replies={0}&screen_name={1}", 
          "false", 
          Uri.EscapeDataString(options.Username));
      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(timelineUrl);
      request.Headers.Add("Authorization", "Bearer " + bearerToken);
      request.Method = "GET";
      request.ContentType = "application/x-www-form-urlencoded;charset=UTF-8";
      request.AutomaticDecompression = DecompressionMethods.GZip;

      WebResponse response = await request.GetResponseAsync();
      string responseData = await new StreamReader(response.GetResponseStream()).ReadToEndAsync();
      var jsonResponse = JsonConvert.DeserializeObject<IEnumerable<TweetRaw>>(responseData);

      tweets = jsonResponse.Select(t => 
        new Tweet() { Message = t.text,
          Username = t.user.screen_name,
          Avatar = t.user.profile_image_url,
          Timestamp = DateTime.ParseExact(t.created_at, "ddd MMM dd HH:mm:ss %zzzz yyyy", 
            CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal).ToFriendlyDate(),
          Id = t.id_str
      }).ToList();
    }

    return tweets;
  }
}