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POTask Parallel Library - How do you get a Continuation with TaskContinuationOptions.OnlyOnCanceled to Fire?
 

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    <p>I'm experimenting with the Task support in .NET 4.0 - specifically with the continuation support. What I'm perplexed about is that I can't figure out how to get a continuation with the <code>TaskContinuationOptions.OnlyOnCanceled</code> flag set to execute. If I do a <code>ThrowIfCancellationRequested</code> in my worker routine, it only seems to propagate out of the continuation as a fault instead of a cancel operation. For instance, given this code:</p> <pre><code>using System; using System.Threading; using System.Threading.Tasks; namespace TaskExp1 { class Program { static void Main() { var n = 10000; DumpThreadId("main method"); var cts = new CancellationTokenSource(); var task = Task.Factory.StartNew&lt;int&gt;(_ =&gt; Sum(cts.Token, n), cts.Token); task.ContinueWith(t =&gt; { DumpThreadId("ContinueWith Completed, ", newline:false); Console.WriteLine("The result is " + t.Result); }, TaskContinuationOptions.OnlyOnRanToCompletion); task.ContinueWith(t =&gt; { DumpThreadId("ContinueWith Faulted, ", newline: false); Console.WriteLine(t.Exception.InnerExceptions[0].Message); }, TaskContinuationOptions.OnlyOnFaulted); task.ContinueWith(_ =&gt; { DumpThreadId("ContinueWith Cancelled, "); }, TaskContinuationOptions.OnlyOnCanceled); Console.WriteLine("Computing sum of " + n + " ..."); Thread.SpinWait(100000); cts.Cancel(); Console.WriteLine("Done."); Console.ReadLine(); } static int Sum(CancellationToken cancelToken, int n) { DumpThreadId("from Sum method"); int sum = 0; for (; n &gt; 0; n--) { Thread.SpinWait(500000); if (n == 10000) cancelToken.ThrowIfCancellationRequested(); checked { sum += n; } } return sum; } static void DumpThreadId(string msg = "", bool newline = true) { var formattedMsg = String.Format("ThreadId: {0} {1}", Thread.CurrentThread.ManagedThreadId, msg); if (newline) formattedMsg += "\n"; Console.Write(formattedMsg); } } } </code></pre> <p>This outputs:</p> <pre><code>ThreadId: 9 main method Computing sum of 10000 ... Done. ThreadId: 10 from Sum method ThreadId: 10 ContinueWith Faulted, The operation was canceled. </code></pre> <p>How do I exit my worker (Sum) method such that the <code>OnlyOnCanceled</code> continuation gets fired?</p>
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