When to use Task in Interface Methods in C#?

Question

There are interface methods that you obviously know that will be async, implementations that will read from the disk for example, you can easily decide to use Task. There are also interface methods that might use async, you can make them return Task too.

I can't help but see Task as an implementation detail that I'm hesitant to put Task on my interfaces when I'm not sure if the implementation will be async / will use async methods. If I decide to not use Task, the implementation methods can't be async when needed.

Although it feels weird to adjust the contract based on the requirement of the implementation, this is what I will do. Is this the correct way?

Answer 1

Is this the correct way?

Yes. Just use Task if you know there will sometimes be opportunity to be asynchronous.

That still leaves you the opportunity to not, because the interface cannot require the use of async - it can only require the return type of Task or Task<T>.

If you end up not needing to do anything asynchronously, you can use Task.FromResult() to return a completed Task with a value. For example:

protected override Task<int> SomeMethod() {
    return Task.FromResult(1);
}

Or Task.CompletedTask if there is no return value.

Otherwise, if you don't specify Task as the return type, and you want to do something asynchronously, you're left with two options:

1. Use synchronous methods in your implementation,
2. If no synchronous methods are available for what you want to do (which is increasingly common), then you either have to:
   • Block synchronously on the asynchronous methods, or
   • Use async void (if the method returns void), which can cause other complications.

All of those are worse options than returning a completed Task in cases where you don't need to run asynchronously.