通过

在完成一个任务后取消剩余任务（C# 和 Visual Basic）

  • 项目

使用具有 CancellationToken一起 Task.WhenAny 方法，当一项任务完成时，在中，您可以取消任何其余的任务。WhenAny 方法采用任务的集合的参数。方法启动所有任务并返回单个任务。在集合中的所有任务完成后，单个任务完成。

此示例演示如何处理 WhenAny 结合使用取消标记保持第一个任务从任务的完成集合和移除余下的任务。每个任务下载网站的内容。此示例显示内容的长度第一下载完毕，并取消其他下载。

说明说明

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。

  1. 对要下载的文件，然后启动 Visual Studio 2012。

  2. 在菜单栏上，依次选择**“文件”、“打开”和“项目/解决方案”**。

  3. 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。

  4. 在 解决方案资源管理器，打开 CancelAfterOneTask 项目的快捷菜单，然后选择 设为启动项目。

  5. 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。

  6. 运行程序不同首先下载完成的多个验证。

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

本主题中的示例添加到 取消一个任务或一组任务（C# 和 Visual Basic） 中开发取消任务列表中选择项。该示例使用相同的 UI，不过，没有显式使用 取消 按钮。

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 CancelAListOfTasks 作为 启动项目。添加本主题中的更改到该项。

在 CancelAListOfTasks 项目的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件，请通过将每个网站的处理步骤开始从转换在 AccessTheWebAsync 的循环使用以下异步方法。

' ***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

// ***Bundle the processing steps for a website into one async method.
async Task<int> ProcessURLAsync(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;
}

在 AccessTheWebAsync，此示例使用查询、ToArray<TSource> 方法和 WhenAny 方法创建并启动任务。WhenAny 的应用程序设置为数组的返回，那么，当等待，因此计算结果为第一个任务到达在中完成的任务单个任务。

在 AccessTheWebAsync进行以下更改。星号指出在代码文件中的更改。

  1. 掉或删除循环。

  2. 创建，那么，当执行，导致常规任务的集合的查询。每个调用 ProcessURLAsync 返回 TResult 是整数的 Task<TResult>。

    ' ***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)

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

  3. 调用 ToArray 执行查询并启动任务。WhenAny 方法的应用程序在下一步中将执行查询并启动任务，而无需使用 ToArray，但是，其他方法不能。最安全的做法是显式强制查询的执行。

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

    // ***Use ToArray to execute the query and start the download tasks. 
Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

  4. 对任务的集合的 WhenAny。WhenAny 返回 Task(Of Task(Of Integer)) 或 Task<Task<int>>。即 WhenAny 返回计算为单个 Task(Of Integer) 或 Task<int> 的任务，当等待时。该单一任务是集合中的第一个任务完成。首先完成的任务指派给 firstFinishedTask。firstFinishedTask 的类型是 TResult 是整数的 Task<TResult>，因为它是 ProcessURLAsync的返回类型。

    ' ***Call WhenAny and then await the result. The task that finishes 
' first is assigned to firstFinishedTask.
Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

    // ***Call WhenAny and then await the result. The task that finishes 
// first is assigned to firstFinishedTask.
Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

  5. 在此示例中，您先完成的任务只关注。因此，请使用 CancellationTokenSource.Cancel 取消余下的任务。

    ' ***Cancel the rest of the downloads. You just want the first one.
cts.Cancel()

    // ***Cancel the rest of the downloads. You just want the first one.
cts.Cancel();

  6. 最后，将等待 firstFinishedTask 检索该下载内容的长度。

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

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

运行程序不同首先下载完成的多个验证。

完整的示例

下面的代码是该示例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。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 & "Download complete."

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

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Download 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()

        '' Comment out or delete the loop.
        ''For Each url In urlList
        ''    ' GetAsync returns a Task(Of HttpResponseMessage). 
        ''    ' Argument ct carries the message if the Cancel button is chosen. 
        ''    ' Note that the Cancel button can cancel all remaining downloads.
        ''    Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

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

        ''    resultsTextBox.Text &=
        ''        String.Format(vbCrLf & "Length of the downloaded string: {0}." & vbCrLf, urlContents.Length)
        ''Next

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

        ' ***Call WhenAny and then await the result. The task that finishes 
        ' first is assigned to firstFinishedTask.
        Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

        ' ***Cancel the rest of the downloads. You just want the first one.
        cts.Cancel()

        ' ***Await the first completed task and display the results
        ' Run the program several times to demonstrate that different
        ' websites can finish first.
        Dim length = Await firstFinishedTask
        resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)
    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 downloaded website:  158856

' Download 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 CancelAfterOneTask
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainWindow()
        {
            InitializeComponent();
        }


        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            resultsTextBox.Clear();

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

            // Set the CancellationTokenSource to null when the download is complete.
            cts = null;
        }


        // You can still include a Cancel button if you want to.
        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        // Provide a parameter for the CancellationToken.
        async Task AccessTheWebAsync(CancellationToken ct)
        {
            HttpClient client = new HttpClient();

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

            // ***Comment out or delete the loop.
            //foreach (var url in urlList)
            //{
            //    // GetAsync returns a Task<HttpResponseMessage>. 
            //    // Argument ct carries the message if the Cancel button is chosen. 
            //    // ***Note that the Cancel button can cancel all remaining downloads.
            //    HttpResponseMessage response = await client.GetAsync(url, ct);

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

            //    resultsTextBox.Text +=
            //        String.Format("\r\nLength of the downloaded string: {0}.\r\n", urlContents.Length);
            //}

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

            // ***Use ToArray to execute the query and start the download tasks. 
            Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

            // ***Call WhenAny and then await the result. The task that finishes 
            // first is assigned to firstFinishedTask.
            Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

            // ***Cancel the rest of the downloads. You just want the first one.
            cts.Cancel();

            // ***Await the first completed task and display the results. 
            // Run the program several times to demonstrate that different
            // websites can finish first.
            var length = await firstFinishedTask;
            resultsTextBox.Text += String.Format("\r\nLength of the downloaded website:  {0}\r\n", length);
        }


        // ***Bundle the processing steps for a website into one async method.
        async Task<int> ProcessURLAsync(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;
        }


        // Add a method that creates a list of web addresses.
        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "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;
        }
    }
    // Sample output:

    // Length of the downloaded website:  158856

    // Download complete.
}

请参见

参考

WhenAny

概念

微调异步应用程序（C# 和 Visual Basic）

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

其他资源

Async 示例：优化应用程序