Task Parallel Library - How do you get a Continuation with TaskContinuationOptions.OnlyOnCanceled to Fire? - task-parallel-library

Question

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 TaskContinuationOptions.OnlyOnCanceled flag set to execute. If I do a ThrowIfCancellationRequested 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:
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<int>(_ => Sum(cts.Token, n),
cts.Token);
task.ContinueWith(t =>
{
DumpThreadId("ContinueWith Completed, ", newline:false);
Console.WriteLine("The result is " + t.Result);
}, TaskContinuationOptions.OnlyOnRanToCompletion);
task.ContinueWith(t =>
{
DumpThreadId("ContinueWith Faulted, ", newline: false);
Console.WriteLine(t.Exception.InnerExceptions[0].Message);
}, TaskContinuationOptions.OnlyOnFaulted);
task.ContinueWith(_ =>
{
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 > 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);
}
}
}
This outputs:
ThreadId: 9 main method
Computing sum of 10000 ...
Done.
ThreadId: 10 from Sum method
ThreadId: 10 ContinueWith Faulted, The operation was canceled.
How do I exit my worker (Sum) method such that the OnlyOnCanceled continuation gets fired?

Answer 1

When you are using the _ => lambda expression you are using the
Func<Object, TResult> function, Object state
overload. If you change the Factory.StartNew to
Task.Factory.StartNew<int>(() => Sum(cts.Token, n), cts.Token);
it will call the "ContinueWith Cancelled".