If my interface must return Task what is the best way to have a no-operation implementation?

Question

In the code below, due to the interface, the class LazyBar must return a task from its method (and for argument\'s sake can\'t be changed). If LazyBar

Answer 1

Today, I would recommend using Task.CompletedTask to accomplish this.

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Pre .net 4.6:

Using Task.FromResult(0) or Task.FromResult<object>(null) will incur less overhead than creating a Task with a no-op expression. When creating a Task with a result pre-determined, there is no scheduling overhead involved.

Answer 2

return Task.CompletedTask; // this will make the compiler happy

Answer 3

To add to Reed Copsey's answer about using Task.FromResult, you can improve performance even more if you cache the already completed task since all instances of completed tasks are the same:

public static class TaskExtensions
{
    public static readonly Task CompletedTask = Task.FromResult(false);
}

With TaskExtensions.CompletedTask you can use the same instance throughout the entire app domain.

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The latest version of the .Net Framework (v4.6) adds just that with the Task.CompletedTask static property

Task completedTask = Task.CompletedTask;

Answer 4

When you must return specified type:

Task.FromResult<MyClass>(null);

Answer 5

I prefer the Task completedTask = Task.CompletedTask; solution of .Net 4.6, but another approach is to mark the method async and return void:

    public async Task WillBeLongRunningAsyncInTheMajorityOfImplementations()
    {
    }

You'll get a warning (CS1998 - Async function without await expression), but this is safe to ignore in this context.

Answer 6

return await Task.FromResult(new MyClass());