What is the correct way to test this Task?

Question

I have a method which does repeating work using a task and async/await.

    public static Task ToRepeatingWork(this Action work, int delayInMilliseconds)
    {
        Action action = async () =>
        {                
            while (true)
            {
                try
                {
                    work();
                }
                catch (MyException ex)
                {
                    // Do Nothing
                }
                await TaskEx.Delay(new TimeSpan(0, 0, 0, 0, delayInMilliseconds));
            }
        };
        return new Task(action, SomeCt, TaskCreationOptions.LongRunning);
    }

I have written a corresponding test:

    [TestMethod, TestCategory("Unit")]
    public async Task Should_do_repeating_work_and_rethrow_exceptions()
    {
        Action work = () =>
        {
            throw new Exception("Some other exception.");
        };

        var task = work.ToRepeatingWork(1);
        task.Start();
        await task;
    }

I am expecting this test to fail, but it passes (and crashes the test-runner).

However if in the ToRepeatingWork method, I change the action from async to a normal action and use Wait instead of await, the test behaves as expected.

TaskEx.Delay(new TimeSpan(0, 0, 0, 0, delayInMilliseconds)).Wait();

What is wrong here?

`TaskEx`? `Ex`? Are you saying that in 2016 you're still using the async *preview*? — Damien_The_Unbeliever, Oct 18 '16 at 05:59
Can you do `await work.ToRepeatingWork(1);`, instead of explicitly starting also when does it exits the While loop, looks like never — Mrinal Kamboj, Oct 18 '16 at 06:01
`TaskEx`, we need the application to run in .Net 4.0. Using TaskEx so that Delay and Run are available. — SharePoint Newbie, Oct 18 '16 at 06:05
@MrinalKamboj The real code takes in a `CancellationToken` which is used in the while loop. Don't I have to start the task before I `await` it? — SharePoint Newbie, Oct 18 '16 at 06:11
@SharePointNewbie Check out my Truly `async` solution for your use case — Mrinal Kamboj, Oct 18 '16 at 09:21

score 1 · Accepted Answer · answered Oct 18 '16 at 12:58

You should never, ever use the task constructor. If you have work to place on the thread pool, then use Task.Run. This is a problem, but not what is causing the crash.

You should also avoid async void, so use Func<Task> instead of Action. This is what is causing the crash.

public static Task ToRepeatingWork(this Action work, int delayInMilliseconds)
{
  Func<Task> action = async () =>
  {                
    while (true)
    {
      try
      {
        work();
      }
      catch (MyException ex)
      {
        // Do Nothing
      }
      await TaskEx.Delay(new TimeSpan(0, 0, 0, 0, delayInMilliseconds));
    }
  };
  return Task.Run(() => action());
}

[TestMethod, TestCategory("Unit")]
public async Task Should_do_repeating_work_and_rethrow_exceptions()
{
  Action work = () =>
  {
    throw new Exception("Some other exception.");
  };

  var task = work.ToRepeatingWork(1);
  await task;
}

is there a reason why you have not marked the lambda passed to Task.Run as async and what is the difference between this and `Task.Run(async () => await action())`? Thanks for the help — SharePoint Newbie, Oct 18 '16 at 17:24
@SharePointNewbie: [It's the same thing](http://stackoverflow.com/questions/19098143/what-is-the-purpose-of-return-await-in-c). — Stephen Cleary, Oct 18 '16 at 18:13

What is the correct way to test this Task?

1 Answers1