Canceling A Running Task

Shakti Saxena

C#.NET FrameworkmultithreadedCanceling Running Task

11 Comments

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Sanjay Shekhawat7y ago
Thanks, working Properly :)
Ammar Shaukat8y ago
Token.Cancel() is not enough to stop an ongoing task ?
Subash10y ago
Nice
Harshad Pansuriya11y ago
Nice one
Santhakumar Munuswamy11y ago
Good one
Former member11y ago
:)
Shakti Saxena11y ago
thanks for the feedback. I will think over it and incorporate the changes wherever required and will try to answer all your questions. :)
Former member11y ago
Its very good article. But, some questions clicked in my mind after reading your mentioned line "In this case, the operation will just end when cancellation is requested" in this article. This line is confusing and not correct. So, Could you please update your article with the explanation of below question for the better quality of content?1) Does calling thread forcibly ends the Task?2) What happens if Taks is already running and cancellation is requested? 3) What happens if cancellation is requested before the Task runs?
Sibeesh Venu11y ago
Nice
Banketeshvar Narayan11y ago
Nice
Raja T11y ago
Nice ,Thanks for sharing..

Canceling a running Task:

A long running Task can be cancelled in a multithreaded environment. The .NET Framework offers a special class that can help you cancel a running task.

CancellationToken

CancellationTokenSource tokenSource= new CancellationTokenSource();
CancellationToken token = tokenSource.Token;
Task t1 = Task.Factory.StartNew(() =>
{
while (!token.IsCancellationRequested)
{
Console.Write("* ");
Thread.Sleep(1000);
}
},
token);
Console.WriteLine("Press any key to stop the task\n");
Console.ReadKey();
tokenSource.Cancel();
Console.WriteLine("\n\nPress any key to stop the task");
Console.ReadKey();

A little explanation to the code above would make it clearer.

The CancellationToken is used in asynchronous task. The CancellationTokenSource token is used to signal that the Task should cancel itself.

In the above case, the operation will just end when cancellation is requested via Cancel() method. As the calling thread requests the cancellation on callee (i.e. Task running in another thread), outside users of the tasks won’t see anything different because the tasks will just have a RanToComplete state. The RanToComplete is the status of the task under consideration. If you want to signal to outside users that your task has been cancelled. You can do this by throwing an OperationCancelledException.

CancellationTokenSource tokenSource = new CancellationTokenSource();
CancellationToken token = tokenSource.Token;
Task t1 = Task.Factory.StartNew(() =>
{
while (!token.IsCancellationRequested)
{
Console.Write("* ");
Thread.Sleep(100); //Throw if cancelled
}
if (token.IsCancellationRequested)
{
token.ThrowIfCancellationRequested();
}
},
token);
try
{
Console.WriteLine("Press any key to stop the task\n");
Console.ReadKey();
tokenSource.Cancel();
t1.Wait();
}
catch (AggregateException ex)
{
Console.WriteLine(“Exception Occured {0}”, ex.InnerExceptions[0].Message);
}
Console.WriteLine("\n\nPress any key to stop the task");
Console.ReadKey();
}

Instead of canceling your running task and catching exception to show user what has really happened, you can continue with another task only if the previous task has been cancelled. Make use of Continue With method which takes the returned value from the cancelled task as a parameter. The same is shown in the following code snippet.

CancellationTokenSource tokenSource = new CancellationTokenSource();
CancellationToken token = tokenSource.Token;
Task t1 = Task.Factory.StartNew(() =>
{
while (!token.IsCancellationRequested)
{
Console.Write("* ");
Thread.Sleep(1000); //Throw if cancelled
}
},
token).ContinueWith((t) =>
{
t.Exception.Handle((e)=> true);
Console.WriteLine("The Task is interrupted");
},
TaskContinuationOptions.OnlyOnCanceled);
Console.WriteLine("Press key any key to cancel\n");
Console.ReadKey();
tokenSource.Cancel();
Console.WriteLine("\n\nEnd Application");
Console.ReadKey();