Task.CompletedTask

Is Task.CompletedTask Expensive?

Consider the following code:

private Task OnButtonClick()
{
   //do some sync work
   return Task.CompletedTask;
}

Many will tell you this is expensive for one of two reasons:

The compiler has to build out an async state machine to handle the Task.
The compiler needs to build a Task to return.

Neither are true.

The method is not async - it has no Async/Await signature - and the low level code looks exactly the same as the high level code. There's no need for async state machines because there's no awaiters. You can check out the code at SharpLab.
Task.Completed is cached as a singleton. Everyone gets the same object returned. The Task code looks like this:

private static Task s_completedTask;
 
public static Task CompletedTask
{
    get
    {
        var completedTask = s_completedTask;
        if (completedTask == null)
            s_completedTask = completedTask = new Task(false, (TaskCreationOptions)InternalTaskOptions.DoNotDispose, default(CancellationToken)); // benign initialization ----
        return completedTask;
    }
}

You can view this code here

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Task.CompletedTask