private void timer_Tick(object sender, EventArgs e)
{
    int time;
    time = ts.Seconds;
    if (time >= 10)
    {
        labTimer.Content= "剩余时间:" + time.ToString();
    }
    if (time < 10)
    {
        labTimer.Content= "剩余时间:" + "0" + time.ToString(); 
    }
    ts = ts.Subtract(new TimeSpan(0, 0, 1));// 时间以1秒 倒计时 
    if (ts.TotalSeconds < -1) 
    { 
        timer.Stop(); 
        ....//我要执行的操作 
    } 
} 

public void StartTimer() 
{ 
    timer.Tick += new EventHandler(timer_Tick); 
    timer.Interval = new TimeSpan(0, 0, 1); 
    timer.Start();//倒计时开始 
}