取消异步任务或任务列表（Visual Basic）

2025-06-22

如果不想等待异步应用程序完成，可以设置可用于取消异步应用程序的按钮。通过遵循本主题中的示例，可以将取消按钮添加到下载一个网站或网站列表的应用程序。

这些示例使用了Fine-Tuning Your Async Application (Visual Basic) 中描述的 UI。

注释

若要运行这些示例，必须在计算机上安装 Visual Studio 2012 或更高版本以及 .NET Framework 4.5 或更高版本。

取消任务

第一个示例将 “取消” 按钮与单个下载任务相关联。如果在应用程序下载内容时选择该按钮，则会取消下载。

下载示例

可以从异步示例下载完整的 Windows Presentation Foundation （WPF）项目：微调应用程序，然后执行以下步骤。

解压缩下载的文件，然后启动 Visual Studio。
在菜单栏上，选择 “文件”、“ 打开”、“ 项目/解决方案”。
在 “打开项目 ”对话框中，打开保存解压缩的示例代码的文件夹，然后打开 AsyncFineTuningVB 的解决方案（.sln）文件。
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”。
选择要运行项目的 F5 键。

选择 Ctrl+F5 键以运行项目而不对其进行调试。

如果不想下载项目，可以在本主题末尾查看MainWindow.xaml.vb文件。

生成示例

以下更改将“ 取消” 按钮添加到下载网站的应用程序。如果不想下载或生成示例，可以在本主题末尾的“完整示例”部分查看最终产品。星号标记代码中的更改。

若要自行生成示例，请逐步按照“下载示例”部分中的说明进行作，但选择 StarterCode 作为 StartUp 项目 而不是 CancelATask。

然后将以下更改添加到该项目的MainWindow.xaml.vb文件中。

声明一个 CancellationTokenSource 变量， cts该变量位于访问该变量的所有方法的范围内。

Class MainWindow

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

为 “取消” 按钮添加以下事件处理程序。事件处理程序使用 CancellationTokenSource.Cancel 此方法在用户请求取消时通知 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

在 “开始” 按钮 startButton_Click的事件处理程序中进行以下更改。

实例化CancellationTokenSource和cts。

' ***Instantiate the CancellationTokenSource.
cts = New CancellationTokenSource()

在调用AccessTheWebAsync时，该调用用于下载指定网站的内容，请将CancellationTokenSource.Token的cts属性作为参数发送。如果请求取消，该 Token 属性将传播消息。添加一个 catch 块，当用户选择取消下载作时显示一条消息。以下代码显示了这些更改。

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

    resultsTextBox.Text &=
        vbCrLf & $"Length of the downloaded string: {contentLength}." & vbCrLf

    ' *** 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

在 AccessTheWebAsync 中，使用 HttpClient.GetAsync(String, CancellationToken) 类型中 GetAsync 方法的 HttpClient 重载来下载网站内容。将 ct（CancellationToken 的 AccessTheWebAsync 参数）作为第二个参数传递。如果用户选择 “取消” 按钮，令牌将传递消息。

以下代码显示了AccessTheWebAsync中的更改。

' ***Provide a parameter for the CancellationToken.
Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer)

    Dim client As HttpClient = New HttpClient()

    resultsTextBox.Text &= 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/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

如果未取消程序，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，程序将生成以下输出：
```
Ready to download.
Download canceled.
```

取消任务列表

可以通过将同一 CancellationTokenSource 实例与每个任务相关联来扩展上一个示例来取消许多任务。如果选择 “取消 ”按钮，则取消尚未完成的所有任务。

下载示例

可以从异步示例下载完整的 Windows Presentation Foundation （WPF）项目：微调应用程序，然后执行以下步骤。

解压缩下载的文件，然后启动 Visual Studio。
在菜单栏上，选择 “文件”、“ 打开”、“ 项目/解决方案”。
在 “打开项目 ”对话框中，打开保存解压缩的示例代码的文件夹，然后打开 AsyncFineTuningVB 的解决方案（.sln）文件。
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”。
选择要运行项目的 F5 键。

选择 Ctrl+F5 键以运行项目而不对其进行调试。

如果不想下载项目，可以在本主题末尾查看MainWindow.xaml.vb文件。

生成示例

若要自行扩展示例，请逐步遵循“下载示例”部分中的说明进行操作，但选择 CancelATask 作为 启动项目。将以下更改添加到该项目。星号标记程序中的更改。

添加用于创建 Web 地址列表的方法。

' ***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/library/hh290138.aspx",
            "https://msdn.microsoft.com/library/hh290140.aspx",
            "https://msdn.microsoft.com/library/dd470362.aspx",
            "https://msdn.microsoft.com/library/aa578028.aspx",
            "https://msdn.microsoft.com/library/ms404677.aspx",
            "https://msdn.microsoft.com/library/ff730837.aspx"
        }
    Return urls
End Function

在AccessTheWebAsync中调用该方法。

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

添加以下循环 AccessTheWebAsync 以处理列表中的每个 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 &=
        vbCrLf & $"Length of the downloaded string: {urlContents.Length}." & vbCrLf
Next

由于 AccessTheWebAsync 显示长度，因此该方法不需要返回任何内容。删除 return 语句，并将方法的返回类型更改为 Task 而不是 Task<TResult>。
```
Async Function AccessTheWebAsync(ct As CancellationToken) As Task
```
使用语句而不是表达式从 startButton_Click 中调用方法。
```
Await AccessTheWebAsync(cts.Token)
```

如果未取消程序，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.

如果在下载完成之前选择 “取消” 按钮，输出将包含取消之前完成的下载长度。

Length of the downloaded string: 35939.

Length of the downloaded string: 237682.

Length of the downloaded string: 128607.

Downloads canceled.

完整示例

以下部分包含前面每个示例的代码。请注意，必须为 System.Net.Http 添加引用。

可以从异步示例：微调应用程序中下载项目。

取消任务示例

以下代码是取消单个任务的示例的完整MainWindow.xaml.vb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 &=
                vbCrLf & $"Length of the downloaded string: {contentLength}." & vbCrLf

            ' *** 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 &=
            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/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.

取消任务列表的示例

以下代码是取消任务列表的示例的完整MainWindow.xaml.vb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 &=
                vbCrLf & $"Length of the downloaded string: {urlContents.Length}." & vbCrLf
        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/library/hh290138.aspx",
                "https://msdn.microsoft.com/library/hh290140.aspx",
                "https://msdn.microsoft.com/library/dd470362.aspx",
                "https://msdn.microsoft.com/library/aa578028.aspx",
                "https://msdn.microsoft.com/library/ms404677.aspx",
                "https://msdn.microsoft.com/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.

通过

取消异步任务或任务列表 （Visual Basic）

取消任务

下载示例

生成示例

取消任务列表

下载示例

生成示例

完整示例

取消任务示例

取消任务列表的示例

另请参阅

其他资源

取消异步任务或任务列表（Visual Basic）