在一段时间后取消任务（C# 和 Visual Basic）

项目
03/12/2013

如果您不希望等待该操作完成，可以取消异步操作一段时间后使用 CancellationTokenSource.CancelAfter 方法。此方法计划在时间不是完全中由 CancelAfter 表达式指定的所有关联的任务取消。

此示例添加到取消一个任务或一组任务（C# 和 Visual Basic）中开发下载网站列表和显示每个的内容长度的代码。

备注

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 CancelAfterTime 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。
运行程序多次验证输出可能显示所有网站、没有网站，或者有些网站的输出。

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。

若要指定最长时间，在任务被标记为已取消之前，请添加对的调用 CancelAfter 到 startButton_Click，如下面的示例所示。添加标记为星号。

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

    ' Instantiate the CancellationTokenSource.
    cts = New CancellationTokenSource()

    resultsTextBox.Clear()

    Try
        ' ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You 
        ' can adjust the time.)
        cts.CancelAfter(2500)

        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

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

    resultsTextBox.Clear();

    try
    {
        // ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You
        // can adjust the time.)
        cts.CancelAfter(2500);

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

    cts = null; 
}

运行程序多次验证输出可能显示所有网站、没有网站，或者有些网站的输出。下面的输出是示例。

Length of the downloaded string: 35990.

Length of the downloaded string: 407399.

Length of the downloaded string: 226091.

Downloads canceled.

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
            ' ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You 
            ' can adjust the time.)
            cts.CancelAfter(2500)

            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()

        ' Process each element in the list of web addresses.
        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
    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 string: 35990.

' Length of the downloaded string: 407399.

' Length of the downloaded string: 226091.

' Downloads canceled.

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 CancelAfterTime
{
    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
            {
                // ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You
                // can adjust the time.)
                cts.CancelAfter(2500);

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

            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();
            }
        }


        async Task AccessTheWebAsync(CancellationToken ct)
        {
            // Declare an HttpClient object.
            HttpClient client = new HttpClient();

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

            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 cancels 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);
            }
        }


        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/ee256749.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 string: 35990.

    // Length of the downloaded string: 407399.

    // Length of the downloaded string: 226091.

    // Downloads canceled.
}

请参见

在一段时间后取消任务（C# 和 Visual Basic）

下载示例

生成示例

完整的示例

请参见

任务

概念

其他资源

其他资源