Sunday, 20 December 2015

How to cancel a Task


It is a good practice to offer users a chance to cancel some long-running operation. Such operation usually has to run asynchronously and in a TPL-based design is modeled as a method returning a Task.

Cancellation model requires some flag which is accessible by the caller and from inside the operation. Caller sets the flag when wants operation to be cancelled. Operation regularly checks the flag or is subscribed to the event "flag has been set" and if flag has been set, operation will stop doing the work. This model is in .NET abstracted with two types: CancellationTokenSource class and CancellationToken structure. Caller uses CancellationTokenSource to initiate cancellation and operation uses CancellationToken to check whether it has been cancelled.

When designing an API, if provision for operation cancellation is required, we have to add a CancellationToken to a list of method's arguments. API caller invokes CancellationTokenSource.Cancel method which sets CancellationToken.IsCancellationRequested to true. Target method checks this property and can either silently return or throw OperationCanceledException. The latter approach is better as only in this case returning task will come to Canceled state.

The following code depicts the whole process of task cancellation:

CancellationToken.ThrowIfCancellationRequested() method simply checks CancellationToken.IsCancellationRequested and if it's true, it throws OperationCanceledException. The output of the code above is:


.........System.OperationCanceledException: The operation was canceled.
   at System.Threading.CancellationToken.ThrowIfCancellationRequested()
   at MyService.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Program.d__1.MoveNext()
Task.IsCanceled: True
Task.IsFaulted: False
Task.Exception: null


The following code shows that the await-ed task does not get aware that the task it holds got canceled if method silently returns on cancellation:

Output:

..........Task.IsCanceled: False
Task.IsFaulted: False
Task.Exception: null


Further reading:
Andrew Arnott - Recommended patterns for CancellationToken
MSDN: Task Cancellation

1 comment:

micheal pan said...

BE SMART AND BECOME RICH IN LESS THAN 3DAYS....It all depends on how fast 
you can be to get the new PROGRAMMED blank ATM card that is capable of
hacking into any ATM machine,anywhere in the world. I got to know about 
this BLANK ATM CARD when I was searching for job online about a month 
ago..It has really changed my life for good and now I can say I'm rich and 
I can never be poor again. The least money I get in a day with it is about 
$50,000.(fifty thousand USD) Every now and then I keeping pumping money 
into my account. Though is illegal,there is no risk of being caught 
,because it has been programmed in such a way that it is not traceable,it 
also has a technique that makes it impossible for the CCTVs to detect 
you..For details on how to get yours today, email the hackers on : (
atmmachinehackers1@gmail.com ). Tell your 
loved once too, and start to live large. That's the simple testimony of how 
my life changed for good...Love you all ...the email address again is ;
atmmachinehackers1@gmail.com