软件界面：

软代码：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;namespace comport
{public partial class Form1 : Form{int count;int time;public Form1(){InitializeComponent();}private void Form1_Load(object sender, EventArgs e)//下拉框的选择{int i;for (i = 0; i < 100; i++){comboBox1.Items.Add(i.ToString() + " 秒");}comboBox1.Text = "5 秒";//下拉框选择的初始化}private void button1_Click(object sender, EventArgs e)//按键按下后的操作逻辑{string str = comboBox1.Text;//获取下拉框中选择的字符串内容time = Convert.ToInt16(str.Substring(0,2));//将下拉菜单中的字符串内容转换成整形progressBar1.Maximum = time;//进度条的最大值timer1.Start();//开始定时器}private void timer1_Tick(object sender, EventArgs e)//定时器工作处理流程{count++;//每到一定时间进入这个私有函数label3.Text = (time - count).ToString() + "秒";progressBar1.Value = count;if (count == time){timer1.Stop();System.Media.SystemSounds.Asterisk.Play();//提示音MessageBox.Show("时间到", "提示");//提示对话框}}}
}

参考：B站杜洋工作室视频