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);
    }
}

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. :)