await, anync

public Form1()
        {
            InitializeComponent();
        }

        // The following method runs asynchronously. The UI thread is not
        // blocked during the delay. You can move or resize the Form1 window
        // while Task.Delay is running.
        public async Task<string> WaitAsynchronouslyAsync()
        {
            await Task.Delay();
            MessageBox.Show("fi");
            return "Finished";
        }

        // The following method runs synchronously, despite the use of async.
        // You cannot move or resize the Form1 window while Thread.Sleep
        // is running because the UI thread is blocked.
        public async Task<string> WaitSynchronously()
        {
            // Add a using directive for System.Threading.
            Thread.Sleep();
            return "Finished";
        }

        private async void button1_Click_1(object sender, EventArgs e)
        {
            string result = string.Empty;
            WaitAsynchronouslyAsync();

            // Call the method that runs asynchronously.
            //result = await WaitAsynchronouslyAsync();
            // Call the method that runs synchronously.
            // result = await WaitSynchronously ();

            // Display the result.
            textBox1.Text += result;
            MessageBox.Show("hi");
        }
上述代码得出至少两点：
1. await WaitAsynchronouslyAsync() 执行时要等待这个调用完成，才会执下后面的代码，但在这个调用没有完成前，其thread是可以被其它代码占用的，表现就是UI仍可以接收其它消息
2. WaitAsynchronouslyAsync（）函数执行的线程与UI主线程是同一个，这也是为什么在这个函数中的messagebox可以显示，Thread.sleep可以阻止UI线程接收消息