Does Task.Wait(int) stop the task if the timeout elapses without the task finishing?

Question

I have a task and I expect it to take under a second to run but if it takes longer than a few seconds I want to cancel the task.

For example:

Task t = new Task(() =>
        {
            while (true)
            {
                Thread.Sleep(500);
            }
        });
t.Start();
t.Wait(3000);

Notice that after 3000 milliseconds the wait expires. Was the task canceled when the timeout expired or is the task still running?

Solution 1

If you want to cancel a Task, you should pass in a CancellationToken when you create the task. That will allow you to cancel the Task from the outside. You could tie cancellation to a timer if you want.

To create a Task with a Cancellation token see this example:

var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;

var t = Task.Factory.StartNew(() => {
    // do some work
    if (token.IsCancellationRequested) {
        // Clean up as needed here ....
    }
    token.ThrowIfCancellationRequested();
}, token);

To cancel the Task call Cancel() on the tokenSource.

Solution 2

Task.Wait() waits up to specified period for task completion and returns whether the task completed in the specified amount of time (or earlier) or not. The task itself is not modified and does not rely on waiting.

Read nice series: Parallelism in .NET, Parallelism in .NET – Part 10, Cancellation in PLINQ and the Parallel class by Reed Copsey

And: .NET 4 Cancellation Framework / Parallel Programming: Task Cancellation

Check following code:

var cts = new CancellationTokenSource();

var newTask = Task.Factory.StartNew(state =>
                           {
                              var token = (CancellationToken)state;
                              while (!token.IsCancellationRequested)
                              {
                              }
                              token.ThrowIfCancellationRequested();
                           }, cts.Token, cts.Token);


if (!newTask.Wait(3000, cts.Token)) cts.Cancel();

Solution 3

The task is still running until you explicitly tell it to stop or your loop finishes (which will never happen).

You can check the return value of Wait to see this:

(from http://msdn.microsoft.com/en-us/library/dd235606.aspx) Return Value

Type: System.Boolean true if the Task completed execution within the allotted time; otherwise, false.

Solution 4

Was the task canceled when the timeout expired or is the task still running?

No and Yes.

The timeout passed to Task.Wait is for the Wait, not the task.

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Comments

[BitMask777]

Doesn't this allow you to only request cancellation "from the outside"? Wouldn't it be up to the delegate whether it pays attention to the token-based request and actually throws or just continues working without cancellation?

[Brian Rasmussen]

Correct, the delegate decides whether it will act on the request by checking the IsCancellationRequest on the property.

[Michael R]

This is interesting, because the help text here seems to say the opposite: msdn.microsoft.com/en-us/library/dd270644(v=vs.110).aspx says, "Waits for the Task to complete execution within a specified number of milliseconds." Not sure if I understand this yet.

[Michael R]

I noticed that the cts.Token is also passed to the Wait() method. I found a note in Microsoft's documentation: "Passing the cancellationToken object to this method simply allows the wait to be canceled based on some condition." msdn.microsoft.com/en-us/library/dd321457%28v=vs.110%29.aspx

[Richard]

@Michael If the time passed to Wait expires it will return, even if the task has not completed. In this timeout case the task will continue to execute until it completes (and a later Wait would thus return as completed).

[bside]

Last row can look prettier, like this: cts.CancelAfter(3000); newTask.Wait(cts.Token);