class Program
    {
        static CountdownEvent _count = new CountdownEvent(3);
        static void Main(string[] args)
        {
          
            Task.Factory.StartNew(() =>
            {
                Thread.Sleep(2000);
                Console.WriteLine("thread 1 complete");
                _count.Signal();
            });
            Task.Factory.StartNew(() =>
            {
                Thread.Sleep(5000);
                Console.WriteLine("thread 2 complete");
                _count.Signal();
            });
            Task.Factory.StartNew(() =>
            {
                Thread.Sleep(3000);
                Console.WriteLine("thread 3 complete");
                _count.Signal();
            });

            Console.WriteLine("waiting tasks....");
            _count.Wait();
            Console.WriteLine("all task completed");


            Console.ReadKey();
        }
      
    }

使用TASK的waitAll可以达到同样的效果：

 var t1 = Task.Factory.StartNew(() =>
            {
                Thread.Sleep(2000);
                Console.WriteLine("thread 1 complete");
            
            });
           var t2 = Task.Factory.StartNew(() =>
            {
                Thread.Sleep(5000);
                Console.WriteLine("thread 2 complete");
            
            });
           var t3 = Task.Factory.StartNew(() =>
            {
                Thread.Sleep(3000);
                Console.WriteLine("thread 3 complete");
             
            });
            Console.WriteLine("waiting tasks....");
            Task.WaitAll(t1, t2, t3);
            Console.WriteLine("all task completed");


            Console.ReadKey();