用tcpclient的异步连接方式，设置连接超时

static void Main(string[] args)
        {
            for (int i = 20; i <= 25; i++)
            {
                TcpClient tcp = new TcpClient();
                IAsyncResult async = tcp.BeginConnect(IPAddress.Parse("120.27.234.XXX"), i, null, null);
                async.AsyncWaitHandle.WaitOne(1000);
                if (async.IsCompleted)
                {
                    Console.WriteLine($"{i} is open.");
                }
                else
                {
                    Console.WriteLine($"{i} is closed."); ;
                }
                tcp.Close();
            }
            Console.WriteLine("ok");
            Console.ReadKey();
        }