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Cancel Remaining Async Tasks after One Is Complete (Visual Basic)

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By using the Task.WhenAny method together with a CancellationToken, you can cancel all remaining tasks when one task is complete. The WhenAny method takes an argument that’s a collection of tasks. The method starts all the tasks and returns a single task. The single task is complete when any task in the collection is complete.

This example demonstrates how to use a cancellation token in conjunction with WhenAny to hold onto the first task to finish from the collection of tasks and to cancel the remaining tasks. Each task downloads the contents of a website. The example displays the length of the contents of the first download to complete and cancels the other downloads.

Note

To run the examples, you must have Visual Studio 2012 or newer and the .NET Framework 4.5 or newer installed on your computer.

Downloading the Example

You can download the complete Windows Presentation Foundation (WPF) project from Async Sample: Fine Tuning Your Application and then follow these steps.

  1. Decompress the file that you downloaded, and then start Visual Studio.

  2. On the menu bar, choose File, Open, Project/Solution.

  3. In the Open Project dialog box, open the folder that holds the sample code that you decompressed, and then open the solution (.sln) file for AsyncFineTuningVB.

  4. In Solution Explorer, open the shortcut menu for the CancelAfterOneTask project, and then choose Set as StartUp Project.

  5. Choose the F5 key to run the project.

    Choose the Ctrl+F5 keys to run the project without debugging it.

  6. Run the program several times to verify that different downloads finish first.

If you don't want to download the project, you can review the MainWindow.xaml.vb file at the end of this topic.

Building the Example

The example in this topic adds to the project that's developed in Cancel an Async Task or a List of Tasks to cancel a list of tasks. The example uses the same UI, although the Cancel button isn’t used explicitly.

To build the example yourself, step by step, follow the instructions in the "Downloading the Example" section, but choose CancelAListOfTasks as the StartUp Project. Add the changes in this topic to that project.

In the MainWindow.xaml.vb file of the CancelAListOfTasks project, start the transition by moving the processing steps for each website from the loop in AccessTheWebAsync to the following async method.

' ***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

In AccessTheWebAsync, this example uses a query, the ToArray method, and the WhenAny method to create and start an array of tasks. The application of WhenAny to the array returns a single task that, when awaited, evaluates to the first task to reach completion in the array of tasks.

Make the following changes in AccessTheWebAsync. Asterisks mark the changes in the code file.

  1. Comment out or delete the loop.

  2. Create a query that, when executed, produces a collection of generic tasks. Each call to ProcessURLAsync returns a Task<TResult> where TResult is an integer.

    ' ***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)

  3. Call ToArray to execute the query and start the tasks. The application of the WhenAny method in the next step would execute the query and start the tasks without using ToArray, but other methods might not. The safest practice is to force execution of the query explicitly.

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

  4. Call WhenAny on the collection of tasks. WhenAny returns a Task(Of Task(Of Integer)) or Task<Task<int>>. That is, WhenAny returns a task that evaluates to a single Task(Of Integer) or Task<int> when it’s awaited. That single task is the first task in the collection to finish. The task that finished first is assigned to finishedTask. The type of finishedTask is Task<TResult> where TResult is an integer because that's the return type of ProcessURLAsync.

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

  5. In this example, you’re interested only in the task that finishes first. Therefore, use CancellationTokenSource.Cancel to cancel the remaining tasks.

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

  6. Finally, await finishedTask to retrieve the length of the downloaded content.

    Dim length = Await finishedTask
resultsTextBox.Text &= vbCrLf & $"Length of the downloaded website:  {length}" & vbCrLf

Run the program several times to verify that different downloads finish first.

Complete Example

The following code is the complete MainWindow.xaml.vb or MainWindow.xaml.cs file for the example. Asterisks mark the elements that were added for this example.

Notice that you must add a reference for System.Net.Http.

You can download the project from Async Sample: Fine Tuning Your Application.

' 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 &=
        ''        vbCrLf & $"Length of the downloaded string: {urlContents.Length}." & vbCrLf
        ''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 finishedTask.
        Dim finishedTask 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 finishedTask
        resultsTextBox.Text &= vbCrLf & $"Length of the downloaded website:  {length}" & vbCrLf
    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/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

' Sample output:

' Length of the downloaded website:  158856

' Download complete.

See also