用c#做点击按钮后实现开始倒计时，点击按钮后再实现关闭倒计时

int lefttime;
        private void button1_Click(object sender, EventArgs e)
        {
            int minute;
            try
            {
                minute = int.Parse(this.textBox1.Text);
            }
            catch (System.Exception ex)
            {
                this.label1.Text = "输入错误";
                return;
            }
            lefttime = minute * 60;
            this.timer1.Interval = 1000;
            this.timer1.Enabled=true;
            this.timer1.Start();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            this.label1.Text = "剩余时间" + lefttime.ToString()+"秒";
            lefttime--;
        }