如何:使用 Async 和 Await 并行发起多个 Web 请求(C# 和 Visual Basic)

在异步方法中,任务创建时即启动。 等待 (Visual Basic) 或等待 (c#) 运算符应用于在任务完成前进程无法继续的方法中该点的任务。 如下例所示,通常任务一经创建即进入等待。

Dim result = Await someWebAccessMethodAsync(url)
var result = await someWebAccessMethodAsync(url);

但是,如果您的程序有其他工作要完成,这些工作不依赖于任务完成,则可以将创建任务与等待任务分开。

' The following line creates and starts the task.
Dim myTask = someWebAccessMethodAsync(url)

' While the task is running, you can do other work that does not depend
' on the results of the task.
' . . . . .

' The application of Await suspends the rest of this method until the task is 
' complete.
Dim result = Await myTask
// The following line creates and starts the task.
var myTask = someWebAccessMethodAsync(url);

// While the task is running, you can do other work that doesn't depend
// on the results of the task.
// . . . . .

// The application of await suspends the rest of this method until the task is complete.
var result = await myTask;

在启动任务和等待任务之间,可以开始其他任务。 其他任务并行隐式运行,但不会创建其他线程。

下面的过程启动三个异步 web 下载,然后按调用它们的顺序等待它们。 请注意,当您运行程序时,任务并不会总是以创建和等待的顺序完成。 它们在创建后立即开始运行,因此一个或多个任务在方法到达 await 表达式之前可能已经完成。

备注

要完成此项目,你必须在计算机上安装 Visual Studio 2012、Visual Studio 2013、Visual Studio Express 2012 for Windows Desktop、Visual Studio Express 2013 for Windows 或 .NET Framework 4.5 或 4.5.1。

有关同时启动多个任务的其他示例,请参见 如何:使用 Task.WhenAll 扩展异步演练(C# 和 Visual Basic)

可从 Developer Code Samples(开发人员代码示例)下载此示例的代码。

设置项目

  • 要设置 WPF 应用程序,请完成以下步骤。 您可以在演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)中找到这些步骤的详细说明。

    • 创建包含文本框和按钮的 WPF 应用程序。 命名按钮 startButton,并命名文本框 resultsTextBox。

    • 添加对 System.Net.Http 的引用。

    • 在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中,针对 System.Net.Http 添加 Imports 语句或 using 指令。

添加代码

  1. 在 MainWindow.xaml 的设计窗口中,双击该按钮以在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中创建 startButton_Click 事件处理程序。 或者,选择此按钮,选择“属性”窗口中的“所选元素的事件处理程序”图标,然后在“单击”文本框中输入 startButton_Click。

  2. 复制下列代码,然后将其粘贴到 MainWindow.xaml.vb 或 MainWindow.xaml.cs.中 startButton_Click 的主体。

    resultsTextBox.Clear()
    Await CreateMultipleTasksAsync()
    resultsTextBox.Text &= vbCrLf & "Control returned to button1_Click."
    
    resultsTextBox.Clear();
    await CreateMultipleTasksAsync();
    resultsTextBox.Text += "\r\n\r\nControl returned to startButton_Click.\r\n";
    

    代码调用驱动应用程序的异步方法 CreateMultipleTasksAsync。

  3. 将以下支持方法添加到项目中:

    • ProcessURLAsync 使用 HttpClient 方法以字节数组的形式从网站下载内容。 支持方法,ProcessURLAsync 然后显示并返回数组的长度。

    • DisplayResults 显示每个 URL 的字节数组中的字节数。 此显示演示每个任务何时完成下载。

    复制以下方法,然后将其粘贴在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中的 startButton_Click 事件处理程序后。

    Private Async Function ProcessURLAsync(url As String, client As HttpClient) As Task(Of Integer)
    
        Dim byteArray = Await client.GetByteArrayAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function 
    
    
    Private Sub DisplayResults(url As String, content As Byte())
    
        ' Display the length of each website. The string format  
        ' is designed to be used with a monospaced font, such as 
        ' Lucida Console or Global Monospace. 
        Dim bytes = content.Length
        ' Strip off the "http://". 
        Dim displayURL = url.Replace("http://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    End Sub
    
    async Task<int> ProcessURLAsync(string url, HttpClient client)
    {
        var byteArray = await client.GetByteArrayAsync(url);
        DisplayResults(url, byteArray);
        return byteArray.Length;
    }
    
    
    private void DisplayResults(string url, byte[] content)
    {
        // Display the length of each website. The string format  
        // is designed to be used with a monospaced font, such as 
        // Lucida Console or Global Monospace. 
        var bytes = content.Length;
        // Strip off the "http://".
        var displayURL = url.Replace("http://", "");
        resultsTextBox.Text += string.Format("\n{0,-58} {1,8}", displayURL, bytes);
    }
    
  4. 最后,定义执行以下步骤的方法 CreateMultipleTasksAsync。

    • 该方法声明 HttpClient 对象,你需要在 ProcessURLAsync 中访问方法 GetByteArrayAsync

    • 该方法创建并启动类型 Task 的三个任务,其中 TResult 是整数。 每个任务完成时,DisplayResults 显示任务的 URL 和下载内容的长度。 由于任务以异步方式运行,因此结果显示的顺序可能与其声明的顺序不同。

    • 该方法等待每项任务的完成。 每个 Await 或 await 运算符挂起 CreateMultipleTasksAsync 的执行,直到等待的任务完成。 运算符还从每个完成的任务中对 ProcessURLAsync 的调用中检索返回值。

    • 任务已完成且整数值已检索出后,该方法会计算网站的总长并显示结果。

    复制以下方法并将其粘贴到解决方案中。

    Private Async Function CreateMultipleTasksAsync() As Task
    
        ' Declare an HttpClient object, and increase the buffer size. The 
        ' default buffer size is 65,536. 
        Dim client As HttpClient =
            New HttpClient() With {.MaxResponseContentBufferSize = 1000000}
    
        ' Create and start the tasks. As each task finishes, DisplayResults  
        ' displays its length. 
        Dim download1 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com", client)
        Dim download2 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client)
        Dim download3 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client)
    
        ' Await each task. 
        Dim length1 As Integer = Await download1
        Dim length2 As Integer = Await download2
        Dim length3 As Integer = Await download3
    
        Dim total As Integer = length1 + length2 + length3
    
        ' Display the total count for all of the websites.
        resultsTextBox.Text &= String.Format(vbCrLf & vbCrLf &
                                             "Total bytes returned:  {0}" & vbCrLf, total)
    End Function
    
    private async Task CreateMultipleTasksAsync()
    {
        // Declare an HttpClient object, and increase the buffer size. The 
        // default buffer size is 65,536.
        HttpClient client =
            new HttpClient() { MaxResponseContentBufferSize = 1000000 };
    
        // Create and start the tasks. As each task finishes, DisplayResults  
        // displays its length.
        Task<int> download1 = 
            ProcessURLAsync("https://msdn.microsoft.com", client);
        Task<int> download2 = 
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client);
        Task<int> download3 = 
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client);
    
        // Await each task. 
        int length1 = await download1;
        int length2 = await download2;
        int length3 = await download3;
    
        int total = length1 + length2 + length3;
    
        // Display the total count for the downloaded websites.
        resultsTextBox.Text +=
            string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);
    }
    
  5. 选择 F5 键运行程序,然后选择“开始”按钮。

    多次运行程序以验证三个任务并非总是以相同顺序完成,且完成顺序并非即其创建和等待的顺序。

示例

下面的代码包括完整的示例。

' Add the following Imports statements, and add a reference for System.Net.Http. 
Imports System.Net.Http


Class MainWindow

    Async Sub startButton_Click(sender As Object, e As RoutedEventArgs) Handles startButton.Click
        resultsTextBox.Clear()
        Await CreateMultipleTasksAsync()
        resultsTextBox.Text &= vbCrLf & "Control returned to button1_Click." 
    End Sub 


    Private Async Function CreateMultipleTasksAsync() As Task

        ' Declare an HttpClient object, and increase the buffer size. The 
        ' default buffer size is 65,536. 
        Dim client As HttpClient =
            New HttpClient() With {.MaxResponseContentBufferSize = 1000000}

        ' Create and start the tasks. As each task finishes, DisplayResults  
        ' displays its length. 
        Dim download1 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com", client)
        Dim download2 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client)
        Dim download3 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client)

        ' Await each task. 
        Dim length1 As Integer = Await download1
        Dim length2 As Integer = Await download2
        Dim length3 As Integer = Await download3

        Dim total As Integer = length1 + length2 + length3

        ' Display the total count for all of the websites.
        resultsTextBox.Text &= String.Format(vbCrLf & vbCrLf &
                                             "Total bytes returned:  {0}" & vbCrLf, total)
    End Function 


    Private Async Function ProcessURLAsync(url As String, client As HttpClient) As Task(Of Integer)

        Dim byteArray = Await client.GetByteArrayAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function 


    Private Sub DisplayResults(url As String, content As Byte())

        ' Display the length of each website. The string format  
        ' is designed to be used with a monospaced font, such as 
        ' Lucida Console or Global Monospace. 
        Dim bytes = content.Length
        ' Strip off the "http://". 
        Dim displayURL = url.Replace("http://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    End Sub 
End Class
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 the following using directive, and add a reference for System.Net.Http. 
using System.Net.Http;


namespace AsyncExample_MultipleTasks
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();
            await CreateMultipleTasksAsync();
            resultsTextBox.Text += "\r\n\r\nControl returned to startButton_Click.\r\n";
        }


        private async Task CreateMultipleTasksAsync()
        {
            // Declare an HttpClient object, and increase the buffer size. The 
            // default buffer size is 65,536.
            HttpClient client =
                new HttpClient() { MaxResponseContentBufferSize = 1000000 };

            // Create and start the tasks. As each task finishes, DisplayResults  
            // displays its length.
            Task<int> download1 = 
                ProcessURLAsync("https://msdn.microsoft.com", client);
            Task<int> download2 = 
                ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client);
            Task<int> download3 = 
                ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client);

            // Await each task. 
            int length1 = await download1;
            int length2 = await download2;
            int length3 = await download3;

            int total = length1 + length2 + length3;

            // Display the total count for the downloaded websites.
            resultsTextBox.Text +=
                string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);
        }


        async Task<int> ProcessURLAsync(string url, HttpClient client)
        {
            var byteArray = await client.GetByteArrayAsync(url);
            DisplayResults(url, byteArray);
            return byteArray.Length;
        }


        private void DisplayResults(string url, byte[] content)
        {
            // Display the length of each website. The string format  
            // is designed to be used with a monospaced font, such as 
            // Lucida Console or Global Monospace. 
            var bytes = content.Length;
            // Strip off the "http://".
            var displayURL = url.Replace("http://", "");
            resultsTextBox.Text += string.Format("\n{0,-58} {1,8}", displayURL, bytes);
        }
    }
}

请参见

任务

演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)

如何:使用 Task.WhenAll 扩展异步演练(C# 和 Visual Basic)

概念

使用 Async 和 Await 的异步编程(C# 和 Visual Basic)