c# - multiple awaits vs Task.WaitAll - equivalent?

Question

In terms of performance, will these 2 methods run GetAllWidgets() and GetAllFoos() in parallel?

Is there any reason to use one over the other? There seems to be a lot happening behind the scenes with the compiler so I don't find it clear.

============= MethodA: Using multiple awaits ======================

public async Task<IHttpActionResult> MethodA()
{
    var customer = new Customer();

    customer.Widgets = await _widgetService.GetAllWidgets();
    customer.Foos = await _fooService.GetAllFoos();

    return Ok(customer);
}

=============== MethodB: Using Task.WaitAll =====================

public async Task<IHttpActionResult> MethodB()
{
    var customer = new Customer();

    var getAllWidgetsTask = _widgetService.GetAllWidgets();
    var getAllFoosTask = _fooService.GetAllFos();

    Task.WaitAll(new List[] {getAllWidgetsTask, getAllFoosTask});

    customer.Widgets = getAllWidgetsTask.Result;
    customer.Foos = getAllFoosTask.Result;

    return Ok(customer);
}

=====================================

Answer 1

The first option will not execute the two operations concurrently. It will execute the first and await its completion, and only then the second.

The second option will execute both concurrently but will wait for them synchronously (i.e. while blocking a thread).

You shouldn't use both options since the first completes slower than the second and the second blocks a thread without need.

You should wait for both operations asynchronously with Task.WhenAll:

public async Task<IHttpActionResult> MethodB()
{
    var customer = new Customer();

    var getAllWidgetsTask = _widgetService.GetAllWidgets();
    var getAllFoosTask = _fooService.GetAllFos();

    await Task.WhenAll(getAllWidgetsTask, getAllFoosTask);

    customer.Widgets = await getAllWidgetsTask;
    customer.Foos = await getAllFoosTask;

    return Ok(customer);
}

Note that after Task.WhenAll completed both tasks already completed so awaiting them completes immediately.