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POWhich Task in a continuation chain was running when cancelled?
 

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    <p>I have a continuation chain of <code>Task</code>s that may be cancelled, say, after a timeout occurs. I want to identify which task was running when the cancellation happened. Here's what I'm talking about:</p> <pre><code>CancellationTokenSource cts = new CancellationTokenSource(); Task timeoutTask = Task.Delay(5000).ContinueWith((t) =&gt; cts.Cancel()); Task workChain = Task.Factory.StartNew((t) =&gt; { Console.WriteLine("Running task " + Task.CurrentId); Thread.Sleep(1000); }, -1, cts.Token); Task parent = workChain; for (int i = 0; i &lt; 10; i++) { parent = parent.ContinueWith((t, o) =&gt; { Console.WriteLine("Running task " + Task.CurrentId); Console.WriteLine("Last Task.AsyncState = " + t.AsyncState); Thread.Sleep(1000); }, i, cts.Token, TaskContinuationOptions.OnlyOnRanToCompletion, TaskScheduler.Current); } parent.ContinueWith((t) =&gt; { Console.WriteLine("Cancel task " + Task.CurrentId); Console.WriteLine("Last running Task.AsyncState = " + t.AsyncState); }, TaskContinuationOptions.OnlyOnCanceled); </code></pre> <p>When I run the above, the antecedent passed into the <code>OnlyOnCanceled</code> one isn't the task that was running when things were cancelled. I know why: the <code>OnlyOnCanceled</code> task <em>is</em> parented by the last task created in the loop, and when the timeout occurs, all tasks that aren't completed are marked as canceled without being started.</p> <p>Having each task check the state of the token and store something elsewhere works most of the time, but there's a small chance that the cancellation happens after one task completes before the next task begins. In that case, I don't find out anything about the first canceled task. I could always store something when a task starts and something else if it is canceled, but this starts to feel kludgy pretty quickly.</p>
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    1. USMike Van Vertloo
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    1. COSo, you say you want to know which task was running when cancellation happened, but as you said, it could be none. Cancellation can happen after one ends and before the next starts. Do you want to know the last task to be completed successfully, or the first task that *didn't* complete successfully?
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      1. USServy
    2. COSide note, if you want the CTS to cancel the task in 5000 ms just put 5000 in the CTS constructor; much simpler than what you're doing.
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      1. POWhich Task in a continuation chain was running when cancelled?
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    3. COWhy are you using `ContinueWith()` instead of `await`? And why are you using a chain of `Task`s instead of a single `Task` with a loop in it?
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