Cancel Async Tasks after a Period of Time (C# and Visual Basic)
You can cancel an asynchronous operation after a period of time by using the CancellationTokenSource.CancelAfter method if you don't want to wait for the operation to finish. This method schedules the cancellation of any associated tasks that aren’t complete within the period of time that’s designated by the CancelAfter expression.
This example adds to the code that’s developed in Cancel an Async Task or a List of Tasks (C# and Visual Basic) to download a list of websites and to display the length of the contents of each one.
Note
To run the examples, you must have Visual Studio 2012, Visual Studio 2013, Visual Studio Express 2012 for Windows Desktop, Visual Studio Express 2013 for Windows, or the .NET Framework 4.5 or 4.5.1 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.
Decompress the file that you downloaded, and then start Visual Studio.
On the menu bar, choose File, Open, Project/Solution.
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 AsyncFineTuningCS or AsyncFineTuningVB.
In Solution Explorer, open the shortcut menu for the CancelAfterTime project, and then choose Set as StartUp Project.
Choose the F5 key to run the project.
Choose the Ctrl+F5 keys to run the project without debugging it.
Run the program several times to verify that the output might show output for all websites, no websites, or some web sites.
If you don't want to download the project, you can review the MainWindow.xaml.vb and MainWindow.xaml.cs files 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 (C# and Visual Basic) 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.
To specify a maximum time before the tasks are marked as canceled, add a call to CancelAfter to startButton_Click, as the following example shows. The addition is marked with asterisks.
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;
}
Run the program several times to verify that the output might show output for all websites, no websites, or some web sites. The following output is a sample.
Length of the downloaded string: 35990.
Length of the downloaded string: 407399.
Length of the downloaded string: 226091.
Downloads canceled.
Complete Example
The following code is the complete text of the 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
' ***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.
}
See Also
Tasks
Walkthrough: Accessing the Web by Using Async and Await (C# and Visual Basic)
Concepts
Asynchronous Programming with Async and Await (C# and Visual Basic)
Cancel an Async Task or a List of Tasks (C# and Visual Basic)
Fine-Tuning Your Async Application (C# and Visual Basic)