如何：并行发起多个 Web 请求（C# 和 Visual Basic）

项目
05/01/2013

在异步方法，那么，当其创建时，任务启动。等待 (Visual Basic)或等待 (C#)运算符应用于该点的任务在无法继续处理调用方法，直到任务完成。通常任务等待，在创建了，则，如下例所示。

Dim result = Await someWebAccessMethodAsync(url)

var result = await someWebAccessMethodAsync(url);

但是，您可以从等待任务分离创建任务，如果您的程序具有不依赖于完成完成的其他工作。

' 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;

在启动任务和等待它之间，可以开始其他任务。其他任务并行隐式运行的，但是，其他线程不会创建。

下面的过程中启动三个异步web按调用的顺序下载然后等待它们。请注意，那么，当您运行程序，任务不按顺序总是完成其创建并等待。它们开始运行，当其创建时，因此，一个或多个任务可能完成，方法到达等待表达式之前。

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

若要完成此项，您必须在计算机上安装 Visual Studio 2012 才能。有关更多信息，请参见 Microsoft 网站。

可以下载此示例的中的代码。开发人员代码示例

设置项目

若要设置WPF应用程序，请完成以下步骤。可以找到这些步骤的详细说明。演练：使用 Async 和 Await 访问 Web（C# 和 Visual Basic）。
- 创建包含文本框和一个按钮上的WPF应用程序。将按钮 startButton，并将文本框 resultsTextBox。
- 添加 System.Net.Http的引用。
- 在MainWindow.xaml.vb或MainWindow.xaml.cs中，添加一个 Imports 语句或 using 指令 System.Net.Http的。

添加代码

在"设计"窗口，MainWindow.xaml，在MainWindow.xaml.vb或MainWindow.xaml.cs双击按钮以创建 startButton_Click 事件处理程序。或者，选择按钮，选择。属性窗口的 *** 所选元素的事件处理程序 *** 图标，然后输入 startButton_Click 在 *** 单击 *** 文本框。

复制下面的代码，并将其粘贴到 startButton_Click 主体在MainWindow.xaml.vb或MainWindow.xaml.cs的。

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";

代码调用异步方法，CreateMultipleTasksAsync，驱动应用程序。

添加以下方法支持到该项目中:

ProcessURLAsync 使用一个 HttpClient 方法下载网站内容作为字节数组。支持方法，ProcessURLAsync 然后显示并返回数组的长度。
DisplayResults 显示中的字节数字节数组的每个URL的。此显示每个任务完成下载。

复制以下方法，并在MainWindow.xaml.vb或MainWindow.xaml.cs的 startButton_Click 事件处理程序后处理将它们粘贴。

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);
}

最后，定义方法 CreateMultipleTasksAsync，执行下列步骤。

方法声明 HttpClient 对象，需要在 ProcessURLAsync的访问方法 GetByteArrayAsync。
方法创建并启动类型 Task<TResult>三个任务，TResult 是整数。在每个任务完成，DisplayResults 显示下载内容的任务的URL和长度。由于任务以异步方式运行，结果出现的顺序可能与声明它们的顺序不同。
方法等待每项任务的完成。每 Await 或 await 运算符挂起 CreateMultipleTasksAsync 的执行，直到等待任务完成。运算符从调用来检索返回值到每个已完成的任务的 ProcessURLAsync。
当任务完成后，并整数值检索出，该方法总网站的长度并显示结果。

复制以下方法，并将其粘贴到您的解决方案。

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);
}

选择F5键运行程序，然后选择 *** 启动 *** 按钮。

运行程序多次验证三个任务不以相同顺序总是完成，并且它们完成的顺序不一定是其创建并等待的顺序。

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 扩展演练（C# 和 Visual Basic）

概念

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

如何：并行发起多个 Web 请求（C# 和 Visual Basic）

设置项目

添加代码

示例

请参见

任务

概念

其他资源