新手学习使用TIMER，为何没有运行。

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 System.Timers;


namespace WpfApplication1
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    ///

    public partial class MainWindow : Window
    {
        Timer t1 = new Timer();

        public MainWindow()
        {
            InitializeComponent();

            t1.Interval = 1000;
            t1.Enabled = true;

        }


        private void t1_Tick(object sender, System.EventArgs e)
        {
            t1.Enabled = false;
            MessageBox.Show("需要休息了，开机已经2小时了");
            t1.Enabled = true; //如果不先把timer1设成false,对话框会一直弹下去。

        }
    }
}