启动多个任务并在其完成时进行处理（C# 和 Visual Basic）

项目
02/21/2013

使用 Task.WhenAny，可以同时启动多个任务和依次对它们进行处理，并按开始的顺序完成而不是处理它们。

下面的示例使用查询创建任务的集合。每个任务下载一个指定网站的内容。在 while 循环的每次迭代，等待调用 WhenAny 返回在完成它首先下载任务的集合的任务。任务从集合中移除并处理。直到集合的循环不包含没有其他任务。

说明
若要运行此示例，需要 Visual Studio 2012 中，Visual Studio express 2012 中的 windows 桌面或在您的计算机上安装 .NET framework 4.5。

下载示例

可以下载完整的 windows 演示基础 (WPF) 项从 Async 示例：优化应用程序然后按照这些步骤。

对要下载的文件，然后启动 Visual Studio 2012。
在菜单栏上，依次选择**“文件”、“打开”和“项目/解决方案”**。
在 打开项目 对话框中，打开包含示例代码。解压缩，然后打开 AsyncFineTuningCS 或 AsyncFineTuningVB 的解决方案的文件夹 (.sln) 文件。
在 解决方案资源管理器，打开 ProcessTasksAsTheyFinish 项目的快捷菜单，然后选择 设为启动项目。
选择 F5 键运行项目。

选择密钥 Ctrl+F5 运行项目，但不对其进行调试。
运行项目多次验证下载的长度不以相同顺序总是出现。

如果不希望下载该项目，可以查看 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件本主题末尾的。

生成示例

此示例添加到在完成一个任务后取消剩余任务（C# 和 Visual Basic）中开发的代码并使用相同的 UI。

若要生成此示例，分步，后面部分的“下载示例的”命令，但是，选择 CancelAfterOneTask 作为 启动项目。添加本主题中的更改到该项目的 AccessTheWebAsync 方法。更改标记为星号。

CancelAfterOneTask 项目还包括，那么，当执行，创建任务的集合的查询。每个调用在下面的代码 ProcessURLAsync 返回 TResult 是整数的 Task<TResult>。

Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
    From url In urlList Select ProcessURLAsync(url, client, ct)

IEnumerable<Task<int>> downloadTasksQuery =
    from url in urlList select ProcessURL(url, client, ct);

在项目的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件中，对 AccessTheWebAsync 方法进行以下更改。

通过应用 Enumerable.ToList<TSource> 执行查询而不是 ToArray<TSource>。

Dim downloadTasks As List(Of Task(Of Integer)) = downloadTasksQuery.ToList()

List<Task<int>> downloadTasks = downloadTasksQuery.ToList();

添加到集合执行每个任务的以下步骤的 while 循环。

等待调用 WhenAny 标识集合中的第一个任务完成其下载。

Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

从集合中移除该任务。

downloadTasks.Remove(firstFinishedTask)

downloadTasks.Remove(firstFinishedTask);

等待 firstFinishedTask，供 ProcessURLAsync的调用返回。firstFinishedTask 变量是 TReturn 是整数的 Task<TResult>。任务已完成，但是，您等待它检索该下载网站的长度，如下例所示。

Dim length = Await firstFinishedTask
resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)

int length = await firstFinishedTask;
resultsTextBox.Text += String.Format("\r\nLength of the download:  {0}", length);

您应运行项目多次验证下载的长度不以相同顺序总是出现。

警告
在循环中使用 WhenAny，如该示例所述，解决涉及少量的任务的问题。但是，因此，如果需要处理，则大量任务其他方法更为有效的。有关更多信息和示例，请参见处理任务作为完。

完整的示例

下面的代码是 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件的全文该示例的。星号指出在本示例中添加了元素。

通知您必须添加 System.Net.Http的引用。

可以下载项目 Async 示例：优化应用程序。

' Add an Imports directive and a reference for System.Net.Http.
Imports System.Net.Http

' Add the following Imports directive for System.Threading.
Imports System.Threading

Class MainWindow

    ' Declare a System.Threading.CancellationTokenSource.
    Dim cts As CancellationTokenSource


    Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)

        ' Instantiate the CancellationTokenSource.
        cts = New CancellationTokenSource()

        resultsTextBox.Clear()

        Try
            Await AccessTheWebAsync(cts.Token)
            resultsTextBox.Text &= vbCrLf & "Downloads complete."

        Catch ex As OperationCanceledException
            resultsTextBox.Text &= vbCrLf & "Downloads canceled." & vbCrLf

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Downloads failed." & vbCrLf
        End Try

        ' Set the CancellationTokenSource to Nothing when the download is complete.
        cts = Nothing
    End Sub


    ' You can still include a Cancel button if you want to.
    Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)

        If cts IsNot Nothing Then
            cts.Cancel()
        End If
    End Sub


    ' Provide a parameter for the CancellationToken.
    ' Change the return type to Task because the method has no return statement.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task

        Dim client As HttpClient = New HttpClient()

        ' Call SetUpURLList to make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        ' ***Create a query that, when executed, returns a collection of tasks.
        Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
            From url In urlList Select ProcessURLAsync(url, client, ct)

        ' ***Use ToList to execute the query and start the download tasks. 
        Dim downloadTasks As List(Of Task(Of Integer)) = downloadTasksQuery.ToList()

        ' ***Add a loop to process the tasks one at a time until none remain.
        While downloadTasks.Count > 0
            ' ***Identify the first task that completes.
            Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

            ' ***Remove the selected task from the list so that you don't
            ' process it more than once.
            downloadTasks.Remove(firstFinishedTask)

            ' ***Await the first completed task and display the results.
            Dim length = Await firstFinishedTask
            resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)
        End While

    End Function


    ' Bundle the processing steps for a website into one async method.
    Async Function ProcessURLAsync(url As String, client As HttpClient, ct As CancellationToken) As Task(Of Integer)

        ' GetAsync returns a Task(Of HttpResponseMessage). 
        Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

        ' Retrieve the website contents from the HttpResponseMessage.
        Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

        Return urlContents.Length
    End Function


    ' Add a method that creates a list of web addresses.
    Private Function SetUpURLList() As List(Of String)

        Dim urls = New List(Of String) From
            {
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            }
        Return urls
    End Function

End Class


' Sample output:

' Length of the download:  226093
' Length of the download:  412588
' Length of the download:  175490
' Length of the download:  204890
' Length of the download:  158855
' Length of the download:  145790
' Length of the download:  44908
' Downloads complete.

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;

// Add the following using directive.
using System.Threading;


namespace ProcessTasksAsTheyFinish
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainWindow()
        {
            InitializeComponent();
        }

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();

            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            try
            {
                await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text += "\r\nDownloads complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownloads canceled.\r\n";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownloads failed.\r\n";
            }

            cts = null;
        }


        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        async Task AccessTheWebAsync(CancellationToken ct)
        {
            HttpClient client = new HttpClient();

            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();

            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<int>> downloadTasksQuery =
                from url in urlList select ProcessURL(url, client, ct);

            // ***Use ToList to execute the query and start the tasks. 
            List<Task<int>> downloadTasks = downloadTasksQuery.ToList();

            // ***Add a loop to process the tasks one at a time until none remain.
            while (downloadTasks.Count > 0)
            {
                    // Identify the first task that completes.
                    Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

                    // ***Remove the selected task from the list so that you don't
                    // process it more than once.
                    downloadTasks.Remove(firstFinishedTask);

                    // Await the completed task.
                    int length = await firstFinishedTask;
                    resultsTextBox.Text += String.Format("\r\nLength of the download:  {0}", length);
            }
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/library/windows/apps/br211380.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }


        async Task<int> ProcessURL(string url, HttpClient client, CancellationToken ct)
        {
            // GetAsync returns a Task<HttpResponseMessage>. 
            HttpResponseMessage response = await client.GetAsync(url, ct);

            // Retrieve the website contents from the HttpResponseMessage.
            byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

            return urlContents.Length;
        }
    }
}

// Sample Output:

// Length of the download:  226093
// Length of the download:  412588
// Length of the download:  175490
// Length of the download:  204890
// Length of the download:  158855
// Length of the download:  145790
// Length of the download:  44908
// Downloads complete.

请参见

通过

启动多个任务并在其完成时进行处理（C# 和 Visual Basic）

下载示例

生成示例

完整的示例

请参见

参考

概念

其他资源

其他资源