取消異步工作或工作清單（Visual Basic）

如果您不想等待異步應用程式完成，您可以設定按鈕，以用來取消異步應用程式。依照本主題中的範例，您可以將取消按鈕新增至下載一個網站或網站清單的應用程式。

這些範例會使用 Fine-Tuning "您的異步應用程式" （Visual Basic）中所描述的 UI。

備註

若要執行範例，您必須在計算機上安裝Visual Studio 2012或更新版本和 .NET Framework 4.5 或更新版本。

取消工作

第一個範例會將 [取消] 按鈕與單一下載工作產生關聯。如果您在應用程式下載內容時選擇按鈕，則會取消下載。

下載範例

您可以從異步範例：微調您的應用程式，然後遵循下列步驟，下載完整的 Windows Presentation Foundation （WPF）專案。

解壓縮您下載的檔案，然後啟動 Visual Studio。
在功能表欄上，選擇 [ 檔案]、[ 開啟]、[ 專案/方案]。
在 [ 開啟專案 ] 對話框中，開啟保存解壓縮範例程式代碼的資料夾，然後開啟 AsyncFineTuningVB 的解決方案（.sln）檔案。
在 [方案總管] 中，開啟 CancelATask 專案的快捷方式功能表，然後選擇 [ 設定為啟始專案]。
選擇 F5 鍵以執行專案。

選擇 Ctrl+F5 鍵來執行專案，而不進行偵錯。

如果您不想下載專案，您可以在本主題結尾檢閱MainWindow.xaml.vb檔案。

建構範例

下列變更會將 [ 取消] 按鈕新增至下載網站的應用程式。如果您不想下載或建置範例程式碼，您可以在本主題結尾的「完整範例」部分查看最終產品。星號會標示程式代碼中的變更。

若要自行建置範例，請逐步遵循「下載範例」一節中的指示，但選擇 StarterCode 作為 StartUp 專案 ，而不是 CancelATask。

然後將下列變更新增至該專案MainWindow.xaml.vb檔案。

宣告一個CancellationTokenSource變數cts，使其在所有存取該變數的方法中都能夠使用。

Class MainWindow

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

為 [ 取消 ] 按鈕新增下列事件處理程式。事件處理程式使用 CancellationTokenSource.Cancel 方法，在使用者要求取消時通知 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 [ 開始 ] 按鈕 startButton_Click的事件處理程式中，進行下列變更。

實例化 CancellationTokenSource、 cts。

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

在呼叫 AccessTheWebAsync 時，下載指定網站內容，將 CancellationTokenSource.Token 的屬性 cts 作為參數傳送。如果要求取消，屬性 Token 就會傳播訊息。新增 catch 區塊，如果使用者選擇取消下載作業，則會顯示訊息。下列程式代碼顯示變更。

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 記錄中，使用 HttpClient.GetAsync(String, CancellationToken) 型別中的 GetAsync 方法的 HttpClient 多載來下載網站的內容。將 ct 作為 CancellationToken 的 AccessTheWebAsync 參數傳遞，並作為第二個參數。如果用戶選擇 [ 取消 ] 按鈕，令牌會攜帶訊息。

以下程式碼顯示AccessTheWebAsync中的變更。

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

    Dim client As HttpClient = New HttpClient()

    resultsTextBox.Text &= 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

如果您未取消程式，它會產生下列輸出：
```
Ready to download.
Length of the downloaded string: 158125.
```
如果您在程式完成下載內容之前選擇 [ 取消 ] 按鈕，程式會產生下列輸出：
```
Ready to download.
Download canceled.
```

取消工作清單

您可以藉由將相同 CancellationTokenSource 實例與每個工作產生關聯，來擴充上一個範例來取消許多工作。如果您選擇 [ 取消] 按鈕，則會取消尚未完成的所有工作。

下載範例

您可以從異步範例：微調您的應用程式，然後遵循下列步驟，下載完整的 Windows Presentation Foundation （WPF）專案。

解壓縮您下載的檔案，然後啟動 Visual Studio。
在功能表欄上，選擇 [ 檔案]、[ 開啟]、[ 專案/方案]。
在 [ 開啟專案 ] 對話框中，開啟保存解壓縮範例程式代碼的資料夾，然後開啟 AsyncFineTuningVB 的解決方案（.sln）檔案。
在 [方案總管] 中，開啟 CancelAListOfTasks 專案的快捷方式功能表，然後選擇 [ 設定為啟始專案]。
選擇 F5 鍵以執行專案。

選擇 Ctrl+F5 鍵來執行專案，而不進行偵錯。

如果您不想下載專案，您可以在本主題結尾檢閱MainWindow.xaml.vb檔案。

建構範例

若要自行擴充範例，請依照「下載範例」一節中的指示逐步操作，但選擇 CancelATask 設為 啟始專案。將下列變更新增至該專案。星號會標示程式中的變更。

新增方法來建立網址清單。

' ***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 中呼叫方法。

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

在中 AccessTheWebAsync 新增下列迴圈，以處理清單中的每個網址。

' ***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

因為 AccessTheWebAsync 顯示長度，因此方法不需要傳回任何東西。移除 return 語句，並將方法的傳回型別變更為 Task ， Task<TResult>而不是。
```
Async Function AccessTheWebAsync(ct As CancellationToken) As Task
```
使用語句而非表達式，從 startButton_Click 呼叫方法。
```
Await AccessTheWebAsync(cts.Token)
```

如果您未取消程式，它會產生下列輸出：

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.

如果您在下載完成之前選擇 [ 取消 ] 按鈕，輸出會包含取消之前完成的下載長度。

Length of the downloaded string: 35939.

Length of the downloaded string: 237682.

Length of the downloaded string: 128607.

Downloads canceled.

完整範例

下列各節包含上述每個範例的程序代碼。請注意，您必須新增System.Net.Http的引用。

您可以從異步範例：微調應用程式下載專案。

取消任務範例

下列程式代碼是取消單一工作的範例的完整MainWindow.xaml.vb檔案。

' 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.

取消工作清單範例

下列程式代碼是取消工作清單之範例的完整MainWindow.xaml.vb檔案。

' 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.

另請參閱

意見反應

此頁面對您有幫助嗎？

Last updated on 2025-06-17

共用方式為

取消異步工作或工作清單 （Visual Basic）

取消工作

下載範例

建構範例

取消工作清單

下載範例

建構範例

完整範例

取消任務範例

取消工作清單範例

另請參閱

意見反應

其他資源

取消異步工作或工作清單（Visual Basic）