1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Threading.Tasks;
 6 using System.Windows;
 7 using System.Windows.Controls;
 8 using System.Windows.Data;
 9 using System.Windows.Documents;
10 using System.Windows.Input;
11 using System.Windows.Media;
12 using System.Windows.Media.Imaging;
13 using System.Windows.Navigation;
14 using System.Windows.Shapes;
15
16 // Add a using directive and a reference for System.Net.Http;
17 using System.Net.Http;
18
19 namespace AsyncTracer
20 {
21     public partial class MainWindow : Window
22     {
23         public MainWindow()
24         {
25             InitializeComponent();
26         }
27
28         private async void startButton_Click(object sender, RoutedEventArgs e)
29         {
30             // The display lines in the example lead you through the control shifts.
31             resultsTextBox.Text += "ONE:   Entering startButton_Click.\r\n" +
32                 "           Calling AccessTheWebAsync.\r\n";
33
34             Task<int> getLengthTask = AccessTheWebAsync();
35
36             resultsTextBox.Text += "\r\nFOUR:  Back in startButton_Click.\r\n" +
37                 "           Task getLengthTask is started.\r\n" +
38                 "           About to await getLengthTask -- no caller to return to.\r\n";
39
40             int contentLength = await getLengthTask;
41
42             resultsTextBox.Text += "\r\nSIX:   Back in startButton_Click.\r\n" +
43                 "           Task getLengthTask is finished.\r\n" +
44                 "           Result from AccessTheWebAsync is stored in contentLength.\r\n" +
45                 "           About to display contentLength and exit.\r\n";
46
47             resultsTextBox.Text +=
48                 String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
49         }
50
51
52         async Task<int> AccessTheWebAsync()
53         {
54             resultsTextBox.Text += "\r\nTWO:   Entering AccessTheWebAsync.";
55
56             // Declare an HttpClient object.
57             HttpClient client = new HttpClient();
58
59             resultsTextBox.Text += "\r\n           Calling HttpClient.GetStringAsync.\r\n";
60
61             // GetStringAsync returns a Task<string>.
62             Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
63             //如果不需要显示这些文字，上面就可以直接await；await会阻塞本方法；
64             resultsTextBox.Text += "\r\nTHREE: Back in AccessTheWebAsync.\r\n" +
65                 "           Task getStringTask is started.";
66
67             // AccessTheWebAsync can continue to work until getStringTask is awaited.No await No asynchronous;
68
69             resultsTextBox.Text +=
70                 "\r\n           About to await getStringTask and return a Task<int> to startButton_Click.\r\n";
71
72             // Retrieve the website contents when task is complete.
73             string urlContents = await getStringTask;
74
75             resultsTextBox.Text += "\r\nFIVE:  Back in AccessTheWebAsync." +
76                 "\r\n           Task getStringTask is complete." +
77                 "\r\n           Processing the return statement." +
78                 "\r\n           Exiting from AccessTheWebAsync.\r\n";
79
80             return urlContents.Length;
81         }
82     }
83 }

ONE:   Entering startButton_Click.Calling AccessTheWebAsync.TWO:   Entering AccessTheWebAsync.Calling HttpClient.GetStringAsync.THREE: Back in AccessTheWebAsync.Task getStringTask is started.About to await getStringTask & return a Task<int> to startButton_Click.FOUR:  Back in startButton_Click.Task getLengthTask is started.About to await getLengthTask -- no caller to return to.FIVE:  Back in AccessTheWebAsync.Task getStringTask is complete.Processing the return statement.Exiting from AccessTheWebAsync.SIX:   Back in startButton_Click.Task getLengthTask is finished.Result from AccessTheWebAsync is stored in contentLength.About to display contentLength and exit.Length of the downloaded string: 33946.

Task<int> getLengthTask = AccessTheWebAsync();

　　正如在 AccessTheWebAsync 中，startButton_Click 可继续那些不依赖于异步任务结果的工作 (getLengthTask) 直到任务处于等待状态。 下面的输出行表示该工作。

FOUR:  Back in startButton_Click.Task getLengthTask is started.About to await getLengthTask -- no caller to return to.

当 getLengthTask 等待时，startButton_Click 中的进程将被挂起。 下面的赋值语句会挂起 startButton_Click 直到 AccessTheWebAsync 完成。

int contentLength = await getLengthTask;

FIVE:  Back in AccessTheWebAsync.Task getStringTask is complete.Processing the return statement.Exiting from AccessTheWebAsync.

如果 AccessTheWebAsync 发出了已完成信号，处理过程就可以继续到 startButton_Async中的 await 语句后。 实际上，程序没有其他要执行的操作。

输出的以下行表示 startButton_Async 中处理的恢复：

SIX:   Back in startButton_Click.Task getLengthTask is finished.Result from AccessTheWebAsync is stored in contentLength.About to display contentLength and exit.

int contentLength = await getLengthTask;