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Task Cancellation
03/30/2017
.net
dotnet-standard
article
tasks, cancellation
asynchronous task cancellation
3ecf1ea9-e399-4a6a-a0d6-8475f48dcb28
18
rpetrusha
ronpet
wpickett

Task Cancellation

The xref:System.Threading.Tasks.Task?displayProperty=nameWithType and xref:System.Threading.Tasks.Task%601?displayProperty=nameWithType classes support cancellation through the use of cancellation tokens in the .NET Framework. For more information, see Cancellation in Managed Threads. In the Task classes, cancellation involves cooperation between the user delegate, which represents a cancelable operation and the code that requested the cancellation. A successful cancellation involves the requesting code calling the xref:System.Threading.CancellationTokenSource.Cancel%2A?displayProperty=nameWithType method, and the user delegate terminating the operation in a timely manner. You can terminate the operation by using one of these options:

  • By simply returning from the delegate. In many scenarios this is sufficient; however, a task instance that is canceled in this way transitions to the xref:System.Threading.Tasks.TaskStatus.RanToCompletion?displayProperty=nameWithType state, not to the xref:System.Threading.Tasks.TaskStatus.Canceled?displayProperty=nameWithType state.

  • By throwing a xref:System.OperationCanceledException and passing it the token on which cancellation was requested. The preferred way to do this is to use the xref:System.Threading.CancellationToken.ThrowIfCancellationRequested%2A method. A task that is canceled in this way transitions to the Canceled state, which the calling code can use to verify that the task responded to its cancellation request.

The following example shows the basic pattern for task cancellation that throws the exception. Note that the token is passed to the user delegate and to the task instance itself.

[!code-csharpTPL_Cancellation#02] [!code-vbTPL_Cancellation#02]

For a more complete example, see How to: Cancel a Task and Its Children.

When a task instance observes an xref:System.OperationCanceledException thrown by user code, it compares the exception's token to its associated token (the one that was passed to the API that created the Task). If they are the same and the token's xref:System.Threading.CancellationToken.IsCancellationRequested%2A property returns true, the task interprets this as acknowledging cancellation and transitions to the Canceled state. If you do not use a xref:System.Threading.Tasks.Task.Wait%2A or xref:System.Threading.Tasks.Task.WaitAll%2A method to wait for the task, then the task just sets its status to xref:System.Threading.Tasks.TaskStatus.Canceled.

If you are waiting on a Task that transitions to the Canceled state, a xref:System.Threading.Tasks.TaskCanceledException?displayProperty=nameWithType exception (wrapped in an xref:System.AggregateException exception) is thrown. Note that this exception indicates successful cancellation instead of a faulty situation. Therefore, the task's xref:System.Threading.Tasks.Task.Exception%2A property returns null.

If the token's xref:System.Threading.CancellationToken.IsCancellationRequested%2A property returns false or if the exception's token does not match the Task's token, the xref:System.OperationCanceledException is treated like a normal exception, causing the Task to transition to the Faulted state. Also note that the presence of other exceptions will also cause the Task to transition to the Faulted state. You can get the status of the completed task in the xref:System.Threading.Tasks.Task.Status%2A property.

It is possible that a task may continue to process some items after cancellation is requested.

See Also

Cancellation in Managed Threads
How to: Cancel a Task and Its Children