What is the purpose of "return await" in C#?

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.

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.

The only reason you'd want to do it is if there is some other await in the earlier code, or if you're in some way manipulating the result before returning it. Another way in which that might be happening is through a try/catch that changes how exceptions are handled. If you aren't doing any of that then you're right, there's no reason to add the overhead of making the method async.

Another case you may need to await the result is this one:

async Task<IFoo> GetIFooAsync()
{
    return await GetFooAsync();
}

async Task<Foo> GetFooAsync()
{
    var foo = await CreateFooAsync();
    await foo.InitializeAsync();
    return foo;
}

In this case, GetIFooAsync() must await the result of GetFooAsync because the type of T is different between the two methods and Task<Foo> is not directly assignable to Task<IFoo>. But if you await the result, it just becomes Foo which is directly assignable to IFoo. Then the async method just repackages the result inside Task<IFoo> and away you go.

If you won't use return await you could ruin your stack trace while debugging or when it's printed in the logs on exceptions.

When you return the task, the method fulfilled its purpose and it's out of the call stack. When you use return await you're leaving it in the call stack.

For example:

Call stack when using await: A awaiting the task from B => B awaiting the task from C

Call stack when not using await: A awaiting the task from C, which B has returned.

Making the otherwise simple "thunk" method async creates an async state machine in memory whereas the non-async one doesn't. While that can often point folks at using the non-async version because it's more efficient (which is true) it also means that in the event of a hang, you have no evidence that that method is involved in the "return/continuation stack" which sometimes makes it more difficult to understand the hang.

So yes, when perf isn't critical (and it usually isn't) I'll throw async on all these thunk methods so that I have the async state machine to help me diagnose hangs later, and also to help ensure that if those thunk methods ever evolve over time, they'll be sure to return faulted tasks instead of throw.

This also confuses me and I feel that the previous answers overlooked your actual question:

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

Well sometimes you actually want a Task<SomeType>, but most time you actually want an instance of SomeType, that is, the result from the task.

From your code:

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

A person unfamiliar with the syntax (me, for example) might think that this method should return a Task<SomeResult>, but since it is marked with async, it means that its actual return type is SomeResult. If you just use return foo.DoAnotherThingAsync(), you'd be returning a Task, which wouldn't compile. The correct way is to return the result of the task, so the return await.

Another reason for why you may want to return await: The await syntax supports automatic conversion between Task<T> and ValueTask<T> return types. For example, the code below works even though SubTask method returns Task<T> but its caller returns ValueTask<T>.

async Task<T> SubTask()
{
...
}

async ValueTask<T> DoSomething()
{
  await UnimportantTask();
  return await SubTask();
}

If you skip await on the DoSomething() line, you'll get a compiler error CS0029:

Cannot implicitly convert type 'System.Threading.Tasks.Task' to 'System.Threading.Tasks.ValueTask'.

You'll get CS0030 if you try to explicitly typecast it. Don't bother.

This is .NET Framework, by the way. I can totally foresee a comment saying "that's fixed in .NET hypothetical_version", I haven't tested it. :)