How StartNew responds to CancellationToken

UnitTests.cs

using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.VisualStudio.TestTools.UnitTesting;

[TestClass]
public class StartNewCancellationTokenUnitTests
{
    [TestMethod]
    public void CancellationTokenPassedToStartNew_CancelsTaskWithTaskCanceledException()
    {
        var cts = new CancellationTokenSource();
        cts.Cancel();
        var task = Task.Factory.StartNew(() => { }, cts.Token);
        TaskCanceledException exception = null;
        try
        {
            task.Wait();
        }
        catch (AggregateException ex)
        {
            // TaskCanceledException derives from OperationCanceledException
            exception = ex.InnerException as TaskCanceledException;
        }
        Assert.IsTrue(task.IsCanceled);
        Assert.IsNotNull(exception);
        Assert.AreEqual(cts.Token, exception.CancellationToken);
    }

    [TestMethod]
    public void CancellationTokenObservedByDelegate_FaultsTaskWithOperationCanceledException()
    {
        var cts = new CancellationTokenSource();
        cts.Cancel();
        var task = Task.Factory.StartNew(() => { cts.Token.ThrowIfCancellationRequested(); });
        OperationCanceledException exception = null;
        try
        {
            task.Wait();
        }
        catch (AggregateException ex)
        {
            exception = ex.InnerException as OperationCanceledException;
        }
        Assert.IsTrue(task.IsFaulted);
        Assert.IsNotNull(exception);
        Assert.AreEqual(cts.Token, exception.CancellationToken);
    }

    [TestMethod]
    public void CancellationTokenObservedAndPassed_CancelsTaskWithTaskCanceledException()
    {
        var cts = new CancellationTokenSource();
        var taskReady = new ManualResetEvent(false);
        var task = Task.Factory.StartNew(() =>
        {
            taskReady.Set();
            while (true)
                cts.Token.ThrowIfCancellationRequested();
        }, cts.Token);
        taskReady.WaitOne();
        cts.Cancel();
        OperationCanceledException exception = null;
        try
        {
            task.Wait();
        }
        catch (AggregateException ex)
        {
            // TaskCanceledException derives from OperationCanceledException
            exception = ex.InnerException as TaskCanceledException;
        }
        Assert.IsTrue(task.IsCanceled);
        Assert.IsNotNull(exception);
        Assert.AreEqual(cts.Token, exception.CancellationToken);
    }
}

So helpful. Would be nice if this were part of the Microsofts docs.

Might be interesting to add another test that demonstrates the user delegate never executes if the token is already in the cancellation requested state before the task is created.

👍 Good point. Yes, the delegate is never actually executed if it is cancelled before it starts. :)