C# Difference between Task.Run(() =>) and Task.Run(async() => await) [duplicate]

Question

Is there any scenario where writing method like this:

public async Task<SomeResult> DoSomethingAsync()
{
    // Some synchronous code might or might not be here... //
    return await DoAnotherThingAsync();
}

instead of this:

public Task<SomeResult> DoSomethingAsync()
{
    // Some synchronous code might or might not be here... //
    return DoAnotherThingAsync();
}

would make sense?

Why use return await construct when you can directly return Task<T> from the inner DoAnotherThingAsync() invocation?

I see code with return await in so many places, I think I might have missed something. But as far as I understand, not using async/await keywords in this case and directly returning the Task would be functionally equivalent. Why add additional overhead of additional await layer?

Answer 1

There is one sneaky case when return in normal method and return await in async method behave differently: when combined with using (or, more generally, any return await in a try block).

Consider these two versions of a method:

Task<SomeResult> DoSomethingAsync()
{
    using (var foo = new Foo())
    {
        return foo.DoAnotherThingAsync();
    }
}

async Task<SomeResult> DoSomethingAsync()
{
    using (var foo = new Foo())
    {
        return await foo.DoAnotherThingAsync();
    }
}

The first method will Dispose() the Foo object as soon as the DoAnotherThingAsync() method returns, which is likely long before it actually completes. This means the first version is probably buggy (because Foo is disposed too soon), while the second version will work fine.

Answer 2

If you don't need async (i.e., you can return the Task directly), then don't use async.

There are some situations where return await is useful, like if you have two asynchronous operations to do:

var intermediate = await FirstAsync();
return await SecondAwait(intermediate);

For more on async performance, see Stephen Toub's MSDN article and video on the topic.

Update: I've written a blog post that goes into much more detail.