C#线程同步CountdownEvent

static CountdownEvent _countdown = new CountdownEvent(2);
        static void PerformOperation(string message,int seconds)
        {
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            Console.WriteLine(message);
            _countdown.Signal();
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Starting two operations");
            var t1 = new Thread(() => PerformOperation("Operation 1 is completed", 4));
            var t2 = new Thread(() => PerformOperation("Operation 2 is completed", 8));
            t1.Start();
            t2.Start();
            _countdown.Wait();
            Console.WriteLine("get _countdown");
            _countdown.Dispose();
        }