System.Timers.Timer t;        DateTime lastInputTime = DateTime.MinValue;               private void textBox1_PreviewKeyDown(object sender, KeyEventArgs e)        {            if (t!=null)            {                t.Stop();            }            t = new System.Timers.Timer();            t.Interval = 3000;            t.Elapsed += new System.Timers.ElapsedEventHandler(theout);//到达时间的时候执行事件；            t.AutoReset = false;//设置是执行一次（false）还是一直执行(true)；               t.Enabled = true;//是否执行System.Timers.Timer.Elapsed事件；            //if (lastInputTime == DateTime.MinValue)            //{            //    lastInputTime = DateTime.Now;            //}            //else            //{            //    var nowInputTime = DateTime.Now;            //    if ((nowInputTime - lastInputTime).Seconds > 3)            //    {            //        MessageBox.Show("大于");            //    }            //    lastInputTime = nowInputTime;            //}        }        public void theout(object source, System.Timers.ElapsedEventArgs e)        {            MessageBox.Show(e.SignalTime.ToString());        }