トークンの有効性を CoreTweet.Tokens.Account.VerifyCredentials() で確認する

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using CoreTweet;

namespace testCoreTweet2
{
    /// <summary>
    /// MainWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class MainWindow : Window
    {
        internal Tokens tokens;

        public MainWindow()
        {
            InitializeComponent();

            // load twitter token
            if (!string.IsNullOrEmpty(Properties.Settings.Default.AccessToken)
                && !string.IsNullOrEmpty(Properties.Settings.Default.AccessTokenSecret))
            {
                tokens = Tokens.Create(
                    Properties.Settings.Default.ApiKey
                    , Properties.Settings.Default.ApiSecret
                    , Properties.Settings.Default.AccessToken
                    , Properties.Settings.Default.AccessTokenSecret);

                // トークン有効性確認
                try
                {
                    var userResponse = tokens.Account.VerifyCredentials();
                    updatescreennameLabel(userResponse.ScreenName);
                    Properties.Settings.Default.ScreenName = userResponse.ScreenName;
                    Properties.Settings.Default.Save();
                }
                catch (Exception ex)
                {
                    // MessageBox.Show(ex.Message);
                    tokens = null;
                }

            }
        }

        /// <summary>
        /// スクリーンネーム表示を更新する
        /// </summary>
        /// <param name="screenName">Twitter Screen Name</param>
        /// パラメータ省略時は設定ファイルを読み出す
        internal void updatescreennameLabel(string screenName = null)
        {
            string _screenName;
            if (string.IsNullOrEmpty(screenName))
            {
                _screenName = Properties.Settings.Default.ScreenName;
                // 未認証時
                if (string.IsNullOrEmpty(_screenName))
                {
                    screennameLabel.Content = "unregister";
                    return;
                }
            }
            else
            {
                _screenName = screenName;
            }
            // http://msdn.microsoft.com/ja-jp/library/system.windows.controls.label(v=vs.110).aspx
            // WPF Labelにおける文字 _ の仕様について対策する
            screennameLabel.Content = _screenName.Replace("_", "__");
        }


    }
}

CoreTweet: CoreTweet.Rest.Account Class Reference
UserResponse CoreTweet.Rest.Account.VerifyCredentials ( params Expression< Func< string, object >>[] parameters )
Returns a representation of the requesting user if authentication was successful.
Use this method to test if supplied user credentials are valid.