TaskCreationOptions.AttachedToParent is not waiting for child task

Question

According to MSDN:

You can use the AttachedToParent option to express structured task parallelism, because the parent task implicitly waits for all child tasks to finish.

So I have this code:

public async Task<int> GetIntAsync()
{
    var childTask = Task.Factory.StartNew(async () =>
    {
        await Task.Delay(1000);
    },TaskCreationOptions.AttachedToParent);

    return 1;
}

public async Task<ActionResult> Index()
{
    var watch = Stopwatch.StartNew();
    var task = GetIntAsync();
    var result = await task;
    var time = watch.ElapsedMilliseconds;

    return View();  
}

I would like to know why the time is 0 and not 1000.

Answer 1

Code that uses the Task-based Asynchronous Pattern (TAP) does not normally use AttachedToParent. AttachedToParent was part of the design of the Task Parallel Library (TPL). Both the TPL and TAP share the same Task type, but there are many TPL members that should be avoided in TAP code.

In TAP, you can support the notion of "parent" and "child" async methods by having the "parent" async method await the task returned from the "child" async method:

public async Task<int> GetIntAsync()
{
  var childTask = Task.Run(() =>
  {
    ...
    await Task.Delay(1000);
    ...
  });
  ...

  await childTask;
  return 1;
}