using System;

 using System.Collections.Generic;

 using System.Linq;

 using System.Text;

 using System.Threading.Tasks;

 using System.Windows;

 using System.Windows.Controls;

 using System.Windows.Data;

 using System.Windows.Documents;

 using System.Windows.Input;

 using System.Windows.Media;

 using System.Windows.Media.Imaging;

 using System.Windows.Navigation;

 using System.Windows.Shapes;

 // Add a using directive and a reference for System.Net.Http;

 using System.Net.Http;

 namespace AsyncFirstExample

 {

     public partial class MainWindow : Window

     {

         // Mark the event handler with async so you can use await in it.

         private async void StartButton_Click(object sender, RoutedEventArgs e)

         {

             // Call and await separately.

             //Task<int> getLengthTask = AccessTheWebAsync();

             //// You can do independent work here.

             //int contentLength = await getLengthTask;

             int contentLength = await AccessTheWebAsync();

             resultsTextBox.Text +=

                 String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);

         }

         // Three things to note in the signature:

         //  - The method has an async modifier.

         //  - The return type is Task or Task<T>. (See "Return Types" section.)

         //    Here, it is Task<int> because the return statement returns an integer.

         //  - The method name ends in "Async."

         async Task<int> AccessTheWebAsync()

         {

             // You need to add a reference to System.Net.Http to declare client.

             HttpClient client = new HttpClient();

             // GetStringAsync returns a Task<string>. That means that when you await the

             // task you'll get a string (urlContents).

             Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");

             // You can do work here that doesn't rely on the string from GetStringAsync.

             DoIndependentWork();

             // The await operator suspends AccessTheWebAsync.

             //  - AccessTheWebAsync can't continue until getStringTask is complete.

             //  - Meanwhile, control returns to the caller of AccessTheWebAsync.

             //  - Control resumes here when getStringTask is complete.

             //  - The await operator then retrieves the string result from getStringTask.

             string urlContents = await getStringTask;

             // The return statement specifies an integer result.

             // Any methods that are awaiting AccessTheWebAsync retrieve the length value.

             return urlContents.Length;

         }

         void DoIndependentWork()

         {

             resultsTextBox.Text += "Working . . . . . . .\r\n";

         }

     }

 }

 // Sample Output:

 // Working . . . . . . .

 // Length of the downloaded string: 41564.

async（C# 参考）
await（C# 参考）
Await 运算符 (Visual Basic)
Async (Visual Basic)
示例：.NET 异步编程（Windows 应用商店应用）
示例：生成 await WebClient
示例 (C#)：将集合保存到应用程序存储
示例 (Visual Basic)：将集合保存到应用程序存储