Tasks cancellation and continue with

Question

I'm perplex on the cancellation of several tasks and continue with a task to display result. According to what I understood, this program should display

Tasks canceled

CancellationTokenSource cts = new CancellationTokenSource();
List<Task> tasks = new List<Task>();
for(int i= 0; i<3; i++)
{
    tasks.Add(Task.Run(() =>{
        while(!cts.Token.IsCancellationRequested)
            Thread.Sleep(500);
    
        // Uncomment this to see 'Tasks canceled' in the result
        //if(cts.Token.IsCancellationRequested)
        //    cts.Token.ThrowIfCancellationRequested();
    },cts.Token));
}

Task.WhenAll(tasks).ContinueWith(task =>
{
    if(task.IsCanceled)
        Console.WriteLine("Tasks canceled");
    if(task.IsCompleted)
        Console.WriteLine("Tasks completed");
});
Thread.Sleep(2000);
cts.Cancel();

Unfortunately it display

Tasks completed

If i uncomment the throwing of cancel exception the program display

Tasks canceled

Tasks completed

Why? it seems that I missed something but I do not see what...

Completed is true because it ended.. not because it got all the way to the end without issue — BugFinder, Nov 28 '16 at 14:41
To "cancel" a cancellation token does not automatically cancel any task. If you don't `ThrowIfCancellationRequested` the task simply is not cancelled, so you don't see your expected output. — René Vogt, Nov 28 '16 at 14:42
See the second comment to the second accepted answer here : http://stackoverflow.com/questions/7343211/cancelling-a-task-is-throwing-an-exception — PaulF, Nov 28 '16 at 14:52

Kaspars Ozols · Accepted Answer · 2016-11-28T15:22:05.990

1

In first case this is what your code says it to do - stop looping when cancellation is requesting. After it exits loop, there is nothing more to execute hence task is complete.

By design to really cancel task, you need to throw exception to change execution flow. In fact, if statement is not needed in your code sample - that ThrowIfCancellationRequested method will handle this check by itself and will throw exception if cancellation has been requested.

Regarding last part, here is excerpt from MSDN:

IsCompleted will return true when the task is in one of the three final states: RanToCompletion, Faulted, or Canceled.

https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.iscompleted(v=vs.110).aspx

So the task is complete even if it is was cancelled.

edited Nov 28 '16 at 15:22

answered Nov 28 '16 at 14:40

Kaspars Ozols

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Ok for this. But why must i throw cancelation exception to see that task is canceled? – Troopers Nov 28 '16 at 14:42
1

@Troopers Because that's how they were designed to work. – Servy Nov 28 '16 at 14:43

Steve · Answer 2 · 2016-11-28T14:48:29.020

0

based on the doc

Task.WhenAll

Creates a task that will complete when all of the Task objects in an array have completed.

so the task is guarantee to be in a complete state after all of your 3 tasks ended

What you want is probably

Task.WhenAll(tasks).Wait();
Foreach(Task task in tasks)
{
    if(task.IsCanceled)
        Console.WriteLine("Tasks canceled");
    if(task.IsCompleted)
        Console.WriteLine("Tasks completed");
}

Edit: The reason why the task is not canceled is because you never throw that exception. Lets imagine this case: Your task is almost done when you call cts.Cancel(); Right after you call it the task completed. So nothing in your task is canceled and thus its not marked as canceled.

edited Nov 28 '16 at 14:48

answered Nov 28 '16 at 14:40

Steve

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The question is why the tasks aren't marked as canceled. When you wait on a task it can still be canceled. – Servy Nov 28 '16 at 14:42
1

Sorry, You are off topic – Troopers Nov 28 '16 at 14:46

Tasks cancellation and continue with

2 Answers2