使用 Async 和 Await 的异步编程（C# 和 Visual Basic）[msdn.microsoft.com]

 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 AsyncFirstExample
20 {
21     public partial class MainWindow : Window
22     {
23         // Mark the event handler with async so you can use await in it.
24         private async void StartButton_Click(object sender, RoutedEventArgs e)
25         {
26             // Call and await separately.
27             //Task<int> getLengthTask = AccessTheWebAsync();
28             //// You can do independent work here.
29             //int contentLength = await getLengthTask;
30 
31             int contentLength = await AccessTheWebAsync();
32 
33             resultsTextBox.Text +=
34                 String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
35         }
36 
37 
38         // Three things to note in the signature:
39         //  - The method has an async modifier. 
40         //  - The return type is Task or Task<T>. (See "Return Types" section.)
41         //    Here, it is Task<int> because the return statement returns an integer.
42         //  - The method name ends in "Async."
43         async Task<int> AccessTheWebAsync()
44         { 
45             // You need to add a reference to System.Net.Http to declare client.
46             HttpClient client = new HttpClient();
47 
48             // GetStringAsync returns a Task<string>. That means that when you await the
49             // task you‘ll get a string (urlContents).
50             Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
51 
52             // You can do work here that doesn‘t rely on the string from GetStringAsync.
53             DoIndependentWork();
54 
55             // The await operator suspends AccessTheWebAsync.
56             //  - AccessTheWebAsync can‘t continue until getStringTask is complete.
57             //  - Meanwhile, control returns to the caller of AccessTheWebAsync.
58             //  - Control resumes here when getStringTask is complete. 
59             //  - The await operator then retrieves the string result from getStringTask.
60             string urlContents = await getStringTask;
61 
62             // The return statement specifies an integer result.
63             // Any methods that are awaiting AccessTheWebAsync retrieve the length value.
64             return urlContents.Length;
65         }
66 
67 
68         void DoIndependentWork()
69         {
70             resultsTextBox.Text += "Working . . . . . . .\r\n";
71         }
72     }
73 }
74 
75 // Sample Output:
76 
77 // Working . . . . . . .
78 
79 // Length of the downloaded string: 41564.