在asp.net C#中点击按钮后如何启动倒计时计时器？

<asp:Button ID="btnStart" runat="server" Text="Start" OnClick="btnStart_Click" />
<asp:Label ID="lblCountdown" runat="server" Text=""></asp:Label>

protected void btnStart_Click(object sender, EventArgs e)
{
    // 设置倒计时的总时长（单位：秒）
    int countdownSeconds = 60;

    // 启动倒计时计时器
    Timer countdownTimer = new Timer();
    countdownTimer.Interval = 1000; // 设置计时器的间隔为1秒
    countdownTimer.Tick += CountdownTimer_Tick;
    countdownTimer.Tag = countdownSeconds;
    countdownTimer.Start();
}

private void CountdownTimer_Tick(object sender, EventArgs e)
{
    Timer countdownTimer = (Timer)sender;
    int remainingSeconds = (int)countdownTimer.Tag;

    if (remainingSeconds > 0)
    {
        // 更新倒计时的显示
        lblCountdown.Text = remainingSeconds.ToString();
        remainingSeconds--;
        countdownTimer.Tag = remainingSeconds;
    }
    else
    {
        // 倒计时结束，停止计时器
        countdownTimer.Stop();
        lblCountdown.Text = "倒计时结束";
    }
}