取消一个异步任务或一组任务(C# 和 Visual Basic)

如果您不希望等待应用程序结束,则可以设置可用于取消异步应用程序的按钮。 通过按照此主题中的示例,可将取消按钮添加到下载一个网站或网站列表内容的应用程序。

描述使用 微调异步应用程序(C# 和 Visual Basic) 描述的 UI。

备注

若要运行此示例,需在计算机上安装 Visual Studio 2012、Visual Studio 2013、Visual Studio Express 2012 for Windows Desktop(Visual Studio Express 2012 for Windows Desktop)、Visual Studio Express 2013 for Windows 或 .NET Framework 4.5 或 4.5.1。

取消任务

第一个示例使“取消”按钮与单个下载任务关联。 如果在应用程序下载内容时选择按钮,下载将取消。

下载示例

您可以从 Async Sample: Fine Tuning Your Application(异步示例:微调应用程序)下载完整的 Windows Presentation Foundation (WPF) 项目,然后按照这些步骤操作。

  1. 解压下载的文件,然后启动 Visual Studio。

  2. 在菜单栏上,依次选择**“文件”“打开”“项目/解决方案”**。

  3. 在“打开项目”对话框中,打开包含您解压缩的示例代码的文件夹,然后打开 AsyncFineTuningCS 或 AsyncFineTuningVB 的解决方案 (.sln) 文件。

  4. 在“解决方案资源管理器”中,打开“CancelATask”项目的快捷菜单,然后选择“设置为启动项目”。

  5. 选择 F5 键运行项目。

    选择 Ctrl+F5 键运行项目,但不对其进行调试。

如果您不希望下载该项目,可以在本主题结尾查看 MainWindow.xaml.vb 和 MainWindow.xaml.cs 文件。

生成示例

下面的更改会将“取消”按钮添加到下载网站的应用程序。 如果不希望下载或生成该示例,则可以在此主题结束时在“完成示例”部分检查最终产品。 星号标记出代码中的更改。

要自己生成示例,请按照“下载示例”部分的说明一步一步操作,但请选择“StarterCode”作为“启动项目”而不是“CancelATask”。

然后向该项目的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件添加以下更改。

  1. 声明 CancellationTokenSource 变量,cts,这在所有可对其进行访问的方法范围内。

    Class MainWindow
    
        ' ***Declare a System.Threading.CancellationTokenSource. 
        Dim cts As CancellationTokenSource
    
    public partial class MainWindow : Window
    {
        // ***Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;
    
  2. 将下列事件处理程序添加为“取消”按钮。 当用户请求取消时,事件处理程序使用 CancellationTokenSource.Cancel 方法通知 cts。

    ' ***Add an event handler for the Cancel button. 
    Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)
    
        If cts IsNot Nothing Then
            cts.Cancel()
        End If 
    End Sub
    
    // ***Add an event handler for the Cancel button. 
    private void cancelButton_Click(object sender, RoutedEventArgs e)
    {
        if (cts != null)
        {
            cts.Cancel();
        }
    }
    
  3. 对“开始”按钮的事件处理程序作出以下改动,startButton_Click。

    • 实例化 CancellationTokenSource、cts。

      ' ***Instantiate the CancellationTokenSource.
      cts = New CancellationTokenSource()
      
      // ***Instantiate the CancellationTokenSource.
      cts = new CancellationTokenSource();
      
    • 在调用 AccessTheWebAsync(下载指定网站的内容)的过程中,将 cts 的 CancellationTokenSource.Token 属性当做一个参数进行发送。 如果请求取消,则 Token 属性会传播消息。 如果用户选择取消下载操作,则添加显示消息的 catch 块。 下面的代码显示变更。

      Try 
          ' ***Send a token to carry the message if cancellation is requested. 
          Dim contentLength As Integer = Await AccessTheWebAsync(cts.Token)
      
          resultsTextBox.Text &=
              String.Format(vbCrLf & "Length of the downloaded string: {0}." & vbCrLf, contentLength)
      
          ' *** If cancellation is requested, an OperationCanceledException results. 
      Catch ex As OperationCanceledException
          resultsTextBox.Text &= vbCrLf & "Download canceled." & vbCrLf
      
      Catch ex As Exception
          resultsTextBox.Text &= vbCrLf & "Download failed." & vbCrLf
      End Try
      
      try
      {
          // ***Send a token to carry the message if cancellation is requested. 
          int contentLength = await AccessTheWebAsync(cts.Token);
          resultsTextBox.Text +=
              String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
      }
      // *** If cancellation is requested, an OperationCanceledException results. 
      catch (OperationCanceledException)
      {
          resultsTextBox.Text += "\r\nDownload canceled.\r\n";
      }
      catch (Exception)
      {
          resultsTextBox.Text += "\r\nDownload failed.\r\n";
      }
      
  4. 在 AccessTheWebAsync 中,请使用 HttpClient 类型的 GetAsync 方法的 HttpClient.GetAsync(String, CancellationToken) 重载来下载网页的内容。 传递 ct、AccessTheWebAsync 的 CancellationToken 参数作为第二参数。 如果该用户选择“取消”按钮,则该标记将传播消息。

    下面的代码显示 AccessTheWebAsync 的变更。

    ' ***Provide a parameter for the CancellationToken.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer)
    
        Dim client As HttpClient = New HttpClient()
    
        resultsTextBox.Text &=
            String.Format(vbCrLf & "Ready to download." & vbCrLf)
    
        ' You might need to slow things down to have a chance to cancel.
        Await Task.Delay(250)
    
        ' GetAsync returns a Task(Of HttpResponseMessage).  
        ' ***The ct argument carries the message if the Cancel button is chosen. 
        Dim response As HttpResponseMessage = Await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct)
    
        ' Retrieve the website contents from the HttpResponseMessage. 
        Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()
    
        ' The result of the method is the length of the downloaded website. 
        Return urlContents.Length
    End Function
    
    // ***Provide a parameter for the CancellationToken.
    async Task<int> AccessTheWebAsync(CancellationToken ct)
    {
        HttpClient client = new HttpClient();
    
        resultsTextBox.Text +=
            String.Format("\r\nReady to download.\r\n");
    
        // You might need to slow things down to have a chance to cancel.
        await Task.Delay(250);
    
        // GetAsync returns a Task<HttpResponseMessage>.  
        // ***The ct argument carries the message if the Cancel button is chosen.
        HttpResponseMessage response = await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct);
    
        // Retrieve the website contents from the HttpResponseMessage. 
        byte[] urlContents = await response.Content.ReadAsByteArrayAsync();
    
        // The result of the method is the length of the downloaded website. 
        return urlContents.Length;
    }
    
  5. 如果不取消程序,它将生成以下输出。

    Ready to download.
    Length of the downloaded string: 158125.
    

    如果在程序完成下载内容之前选择“取消”按钮,则程序会生成以下输出。

    Ready to download.
    Download canceled.
    

取消任务列表

您可以通过将同一 CancellationTokenSource 实例与每个任务关联来扩展前面的示例以便取消许多任务。 如果选择“取消”按钮,则会取消所有未完成的任务。

下载示例

您可以从 Async Sample: Fine Tuning Your Application(异步示例:微调应用程序)下载完整的 Windows Presentation Foundation (WPF) 项目,然后按照这些步骤操作。

  1. 解压下载的文件,然后启动 Visual Studio 2012。

  2. 在菜单栏上,依次选择**“文件”“打开”“项目/解决方案”**。

  3. 在“打开项目”对话框中,打开包含您解压缩的示例代码的文件夹,然后打开 AsyncFineTuningCS 或 AsyncFineTuningVB 的解决方案 (.sln) 文件。

  4. 在“解决方案资源管理器”中,打开“CancelAListOfTasks”项目的快捷菜单,然后选择“设置为启动项目”。

  5. 选择 F5 键运行项目。

    选择 Ctrl+F5 键运行项目,但不对其进行调试。

如果您不希望下载该项目,可以在本主题结尾查看 MainWindow.xaml.vb 和 MainWindow.xaml.cs 文件。

生成示例

要自己扩展示例,请按照“下载示例”部分的说明一步一步操作,但请选择“CancelATask”作为“启动项目”。 添加以下更改到该项目。 星号标记出程序中的更改。

  1. 添加方法以创建 Web 地址列表。

    ' ***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
    
    // ***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;
    }
    
  2. 调用 AccessTheWebAsync 中的方法。

    ' ***Call SetUpURLList to make a list of web addresses. 
    Dim urlList As List(Of String) = SetUpURLList()
    
    // ***Call SetUpURLList to make a list of web addresses.
    List<string> urlList = SetUpURLList();
    
  3. 添加 AccessTheWebAsync 中的以下循环以处理该列表中的每个 Web 地址。

    ' ***Add a loop to process 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
    
    // ***Add a loop to process the list of web addresses. 
    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);
    }
    
  4. 由于 AccessTheWebAsync 显示长度,所以此方法不需要返回任何内容。 移除返回语句,并将方法的返回类型更改为 Task 而不是 Task

    Async Function AccessTheWebAsync(ct As CancellationToken) As Task
    
    async Task AccessTheWebAsync(CancellationToken ct)
    

    使用语句(而不是表达式)从 startButton_Click 调用方法

    Await AccessTheWebAsync(cts.Token)
    
    await AccessTheWebAsync(cts.Token);
    
  5. 如果不取消程序,它将生成以下输出。

    Length of the downloaded string: 35939.
    
    Length of the downloaded string: 237682.
    
    Length of the downloaded string: 128607.
    
    Length of the downloaded string: 158124.
    
    Length of the downloaded string: 204890.
    
    Length of the downloaded string: 175488.
    
    Length of the downloaded string: 145790.
    
    Downloads complete.
    

    如果在下载完成前选择“取消”按钮,则输出包括在取消前完成的下载的长度。

    Length of the downloaded string: 35939.
    
    Length of the downloaded string: 237682.
    
    Length of the downloaded string: 128607.
    
    Downloads canceled.
    

完成示例

以下各节包含之前每个示例的代码。 请注意,您必须为 System.Net.Http 添加引用。

您可以从 Async 示例:优化应用程序中下载项目。

取消任务示例

下面的代码是取消单个任务的示例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。

' 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 
            ' ***Send a token to carry the message if cancellation is requested. 
            Dim contentLength As Integer = Await AccessTheWebAsync(cts.Token)

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

            ' *** If cancellation is requested, an OperationCanceledException results. 
        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 

    ' ***Add an event handler for the Cancel button. 
    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.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer)

        Dim client As HttpClient = New HttpClient()

        resultsTextBox.Text &=
            String.Format(vbCrLf & "Ready to download." & vbCrLf)

        ' You might need to slow things down to have a chance to cancel.
        Await Task.Delay(250)

        ' GetAsync returns a Task(Of HttpResponseMessage).  
        ' ***The ct argument carries the message if the Cancel button is chosen. 
        Dim response As HttpResponseMessage = Await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct)

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

        ' The result of the method is the length of the downloaded website. 
        Return urlContents.Length
    End Function 
End Class 


' Output for a successful download: 

' Ready to download. 

' Length of the downloaded string: 158125. 


' Or, if you cancel: 

' Ready to download. 

' Download 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 for System.Threading. 

using System.Threading;
namespace CancelATask
{
    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
            {
                // ***Send a token to carry the message if cancellation is requested. 
                int contentLength = await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text +=
                    String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
            }
            // *** If cancellation is requested, an OperationCanceledException results. 
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownload canceled.\r\n";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownload failed.\r\n";
            }

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


        // ***Add an event handler for the Cancel button. 
        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


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

            resultsTextBox.Text +=
                String.Format("\r\nReady to download.\r\n");

            // You might need to slow things down to have a chance to cancel.
            await Task.Delay(250);

            // GetAsync returns a Task<HttpResponseMessage>.  
            // ***The ct argument carries the message if the Cancel button is chosen.
            HttpResponseMessage response = await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct);

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

            // The result of the method is the length of the downloaded website. 
            return urlContents.Length;
        }
    }

    // Output for a successful download: 

    // Ready to download. 

    // Length of the downloaded string: 158125. 


    // Or, if you cancel: 

    // Ready to download. 

    // Download canceled.
}

取消任务列表示例

下面的代码是取消任务列表的示例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。

' 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 
            ' ***AccessTheWebAsync returns a Task, not a Task(Of Integer).
            Await AccessTheWebAsync(cts.Token)
            '  ***Small change in the display lines.
            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 


    ' Add an event handler for the Cancel button. 
    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()

        ' ***Add a loop to process 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 


' Output if you do not choose to cancel: 

' Length of the downloaded string: 35939. 

' Length of the downloaded string: 237682. 

' Length of the downloaded string: 128607. 

' Length of the downloaded string: 158124. 

' Length of the downloaded string: 204890. 

' Length of the downloaded string: 175488. 

' Length of the downloaded string: 145790. 

' Downloads complete. 


'  Sample output if you choose to cancel: 

' Length of the downloaded string: 35939. 

' Length of the downloaded string: 237682. 

' Length of the downloaded string: 128607. 

' 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 for System.Threading. 
using System.Threading;

namespace CancelAListOfTasks
{
    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);
                // ***Small change in the display lines.
                resultsTextBox.Text += "\r\nDownloads complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownloads canceled.";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownloads failed.";
            }

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


        // Add an event handler for the Cancel button. 
        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        // Provide a parameter for the CancellationToken. 
        // ***Change the return type to Task because the method has no return statement.
        async Task AccessTheWebAsync(CancellationToken ct)
        {
            // Declare an HttpClient object.
            HttpClient client = new HttpClient();

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

            // ***Add a loop to process the list of web addresses. 
            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);
            }
        }


        // ***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;
        }
    }

    // Output if you do not choose to cancel: 

    //Length of the downloaded string: 35939. 

    //Length of the downloaded string: 237682. 

    //Length of the downloaded string: 128607. 

    //Length of the downloaded string: 158124. 

    //Length of the downloaded string: 204890. 

    //Length of the downloaded string: 175488. 

    //Length of the downloaded string: 145790. 

    //Downloads complete. 


    // Sample output if you choose to cancel: 

    //Length of the downloaded string: 35939. 

    //Length of the downloaded string: 237682. 

    //Length of the downloaded string: 128607. 

    //Downloads canceled.
}

请参见

参考

CancellationTokenSource

CancellationToken

概念

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

微调异步应用程序(C# 和 Visual Basic)

其他资源

Async Sample: Fine Tuning Your Application