方法一

        private void button1_Click(object sender, EventArgs e)
        {
            DateTime start = DateTime.Now;
            List<Task> taskList = new List<Task>();
            taskList.Add(Task.Run(() =>
            {

                Thread.Sleep(3000);
                Console.WriteLine("三秒输出");
            }));
            taskList.Add(Task.Run(() =>
            {

                Thread.Sleep(7000);
                Console.WriteLine("七秒输出");
            }));
            taskList.Add(Task.Run(() =>
            {

                Thread.Sleep(5000);
                Console.WriteLine("五秒输出");
            }));
            //等待结束
            Task.WaitAll(taskList.ToArray());
            DateTime end = DateTime.Now;
            int num = GetTimeSpanSeconds(start, end);
            Console.WriteLine($"三个线程已执行完毕,用时:{num}s");

        }
        /// <summary>
        /// 两个时间间隔秒数
        /// </summary>
        /// <param name="startTime"></param>
        /// <param name="endTime"></param>
        /// <returns></returns>
        public static int GetTimeSpanSeconds(DateTime startTime, DateTime endTime)
        {
            TimeSpan ts = endTime.Subtract(startTime);
            int sec = (int)ts.TotalSeconds;
            return sec;
        }

 方法二

        private void button1_Click(object sender, EventArgs e)
        {
            DateTime start = DateTime.Now;
            AutoResetEvent[] watchers = new AutoResetEvent[3];
            watchers[0] = new AutoResetEvent(false);
            //启动10个线程输出10个数字
            new Thread(() =>
            {
                Thread.Sleep(3000);
                Console.WriteLine("三秒输出");
                watchers[0].Set();     //线程执行完的时候通知
            }).Start();


            watchers[1] = new AutoResetEvent(false);
            //启动10个线程输出10个数字
            new Thread(() =>
            {
                Thread.Sleep(7000);
                Console.WriteLine("七秒输出");
                watchers[1].Set();     //线程执行完的时候通知
            }).Start();


            watchers[2] = new AutoResetEvent(false);
            //启动10个线程输出10个数字
            new Thread(() =>
            {
                Thread.Sleep(5000);
                Console.WriteLine("五秒输出");
                watchers[2].Set();     //线程执行完的时候通知
            }).Start();

            WaitHandle.WaitAll(watchers);  //确保所有线程都执行完毕
            DateTime end = DateTime.Now;
            int num = GetTimeSpanSeconds(start, end);
            Console.WriteLine($"三个线程已执行完毕,用时:{num}s");            
        }
        /// <summary>
        /// 两个时间间隔秒数
        /// </summary>
        /// <param name="startTime"></param>
        /// <param name="endTime"></param>
        /// <returns></returns>
        public static int GetTimeSpanSeconds(DateTime startTime, DateTime endTime)
        {
            TimeSpan ts = endTime.Subtract(startTime);
            int sec = (int)ts.TotalSeconds;
            return sec;
        }