When to return task over using async

Question

I am abit confused when it comes to how a method should look like when dealing with async / task.

From my understanding, a method that just creates a new task doesnt need to be async since this would produce overhead as it wraps the hole thing in a new task.

So this:

async Task _doStuff()
{
    await Task.Run(()=> _stuff());
}

Is better this way:

Task _doStuff()
{
    return Task.Run(()=> _stuff());
}

However, it gets a bit more complicated, if there are some preconditionchecks Which way is better then?

async Task _doStuff()
{
    if(stuffWasDone)
        return;
    await Task.Run(()=> _stuff());
}

or

Task _doStuff()
{
    if(stuffWasDone)
        return Task.Run(()=> {}); // this could be a static-readonly in some helper like AsyncHelper.Empty(); as there is no FromResult for Task.

    return Task.Run(()=> _stuff());
}

Yuval Itzchakov · Accepted Answer · 2014-12-22 10:42:33Z

4

You dont have to use Task.Run in order to generate a Task. A Task is a promise. If a Task doesn't need to execute, don't create one. Using Task.Run has the overhead of invoking an empty lambda on a threadpool thread, you don't need that. Simply returning should suffice.

You may also use Task.FromResult if no Task is actually needed:

Task DoStuffAsync()
{
    if(stuffWasDone)
    {
       return Task.FromResult(0);
    }

    return Task.Run(() => _stuff());
}

If you don't want to re-generate the Task over again, you can simply put it in a local variable:

private static Task _emptyTask = Task.FromResult(0);

As for your first example of await vs return await, you cannot say one is "better" then the other, as they serve different purposes. The former will asynchronously wait for the Task to complete before returning while the latter will return a hot task to the caller. This will matter in regards of exception handling. More so, if you're not going to be consuming the result of the task afterwards, using await will incur in a redundant generation of a state machine.

As another side note, wrapping synchronous methods in asynchronous wrappers is bad practice. Let the end user explicitly call Task.Run instead, don't fool them with async behavior.

edited Dec 22, 2014 at 10:42

answered Dec 22, 2014 at 10:28

Yuval Itzchakov

150k32 gold badges275 silver badges333 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Yuval Itzchakov Over a year ago

@CSharpie What do you by mean "simple return doesn't work"? Task.FromResult isn't a simple return statement.

Sriram Sakthivel Over a year ago

@PatrickHofman You should never return null from a method returning Task. Instead, return a completed task, faulted task, cancelled task or just throw the exception. But remember no null.

Patrick Hofman Over a year ago

@SriramSakthivel: Can you explain why?

Yuval Itzchakov Over a year ago

@PatrickHofman Because a Task is a promise which acts as a contract with calling code. A Task returning null is unexpected behavior. Look throughout BCL async API's, you always get some form of a Task which describes the behavior of what occured inside the Task. null will surprise callers and will result in undefined behavior most of the time.

Sriram Sakthivel Over a year ago

@PatrickHofman Because who cares about checking null for Task returned from a method? You can't await it(NRE will be thrown), or calling Wait will also result in NRE. NRE gives no information about what is null. Makes debugging worse.

Collectives™ on Stack Overflow

When to return task over using async

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related