Task behaviour differs from Task behaviour when OperationCanceledException thrown

Question

Why does this test fail?
The only difference between t1 and t2, as far as I can tell, is that t1 is a Task, and t2 is a Task<int>. Yet for some reason t2 ends up in the Faulted status, as opposed to the Canceled status. Why would the behavior differ?

[Test]
public void Test_foo()
{
    var t1 = Task.Run(() =>
    {
        throw new OperationCanceledException();
    });
    try
    {
        t1.Wait();
    }
    catch (AggregateException e)
    {
        Assert.IsTrue(t1.IsCanceled);
    }

    var t2 = Task.Run(() =>
    {
        throw new OperationCanceledException();
        return 1;
    });
    try
    {
        t2.Wait();
    }
    catch (AggregateException e)
    {
        Assert.IsTrue(t2.IsCanceled); // fails, it's Faulted 
    }
}

score 2 · Accepted Answer · edited May 23 '17 at 12:33

The main difference between your tasks is the overload for the Task.Run method you're using:

task1 is created with Task.Run Method (Func<Task>), rather than task2 is created with Task.Run<TResult> Method (Func<TResult>). This overloads do create task with a little bit difference:

for task1 the Result property is set to System.Threading.Tasks.VoidTaskResult, and CreationOptions is set to None,
rather than task2 the CreationOptions is set to DenyChildAttach, and the result is a default(int), which is 0.

When you are waiting the task2, the Result property isn't being set to real value, because the exception is thrown. According MSDN:

When a task instance observes an OperationCanceledException thrown by user code, it compares the exception's token to its associated token (the one that was passed to the API that created the Task). If they are the same and the token's IsCancellationRequested property returns true, the task interprets this as acknowledging cancellation and transitions to the Canceled state. If you do not use a Wait or WaitAll method to wait for the task, then the task just sets its status to Canceled.

If you are waiting on a Task that transitions to the Canceled state, a System.Threading.Tasks.TaskCanceledException exception (wrapped in an AggregateException exception) is thrown. Note that this exception indicates successful cancellation instead of a faulty situation. Therefore, the task's Exception property returns null.

If the token's IsCancellationRequested property returns false or if the exception's token does not match the Task's token, the OperationCanceledException is treated like a normal exception, causing the Task to transition to the Faulted state. Also note that the presence of other exceptions will also cause the Task to transition to the Faulted state. You can get the status of the completed task in the Status property.

So, here we can find the reason for this behavior - the exception is treated like a normal exception because of the token mismatch. This is strange, because the token is definitely the same (I've checked that in Debug, the hash code is equal, Equals method and double equals operator returns true), but the comparison still returns false. So, the solution for your case is explicit usage of the cancellation tokens, something like this (I've added the Thread.Sleep to avoid the race condition):

var t1TokenSource = new CancellationTokenSource();
var t1 = Task.Run(() =>
{
    Thread.Sleep(1000);
    if (t1TokenSource.Token.IsCancellationRequested)
    {
        t1TokenSource.Token.ThrowIfCancellationRequested();
    }
    //throw new TaskCanceledException();
}, t1TokenSource.Token);
try
{
    t1TokenSource.Cancel();
    t1.Wait();
}
catch (AggregateException e)
{
    Debug.Assert(t1.IsCanceled);
}

var t2TokenSource = new CancellationTokenSource();
var t2 = Task.Run(() =>
{
    Thread.Sleep(1000);
    if (t2TokenSource.Token.IsCancellationRequested)
    {
        t2TokenSource.Token.ThrowIfCancellationRequested();
    }
    //throw new TaskCanceledException();
    return 1;
}, t2TokenSource.Token);
try
{
    t2TokenSource.Cancel();
    t2.Wait();
}
catch (AggregateException e)
{
    Debug.Assert(t2.IsCanceled);
}

Another quote from MSDN:

You can terminate the operation by using one of these options:

By simply returning from the delegate. In many scenarios this is sufficient; however, a task instance that is canceled in this way transitions to the TaskStatus.RanToCompletion state, not to the TaskStatus.Canceled state.

By throwing a OperationCanceledException and passing it the token on which cancellation was requested. The preferred way to do this is to use the ThrowIfCancellationRequested method. A task that is canceled in this way transitions to the Canceled state, which the calling code can use to verify that the task responded to its cancellation request.

As you can see, the preffered way is working predictably, the direct exception throw does not. Please also note that in case of usage task is created with DenyChildAttach too, and doesn't have a Result property, so there is some difference in constructors, which you've faced with.
Hope this helps.

Task behaviour differs from Task behaviour when OperationCanceledException thrown

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