What do async/await keywords really do?

[ShervanN]

Hi,

The snippet code is from Page 460, Please see the two images. What do async/await keywords really do?
With await keyword, the type of data variable is SomeData, without await keyword is ValueTask

ReadAsync signature is:

  public virtual ValueTask<T> ReadAsync(CancellationToken cancellationToken = default) 
{
//....
}

[christiannagel]

ValueTask

The await keyword works with every returned awaiter object that implements the GetAwaiter method. When async/await was introduced, the await keyword required a Task.

Like the Task class, the ValueTask implements the GetAwaiter method, and thus await can wait on ValueTask as well. The difference is that Task is a class and thus an object is created that needs cleanup from the garbage collector, whereas ValueTask is a struct. Compared to an asynchronous invocation, an object is a small overhead. However, if you get a lengthy list, e.g. with asynchronous streaming, not every returned item is really asynchronous, thus the ValueTask can reduce the overhead.

The Task class returns a TaskAwaiter, the ValueTask returns a ValueTaskAwaiter.

async and return values

With the sample code, reader.ReadAsync returns a ValueTask<SomeData>. As you can see from your first image, without using async, this is what's returned. You can use this ValueTask as you want and do some parallel processing before you check for the result.

Using the await keyword, await creates a code block with all the code lines that follow in the method - including the assignment to the variable data, and passes this code block to the OnCompleted(Action) method to the awaiter. When the awaiter completes, this code block is invoked, and first the result of the task is assigned to the data variable.

That's why you see a different result type with the await. If a method returns Task<string> or ValueTask<string>, you'll see a string variable assigned when using await. A library could return MyCustomTask<SomeData>. In case MyCustomTask implements the GetAwaiter method, you can do SomeData data = await MethodReturningCustomTask().

I hope this helps.

More:

Stephen Toub has a great blog post: How Async/Await Really Works in C#

[ShervanN]

Thank you so much.