How do I create a C# style async member in F#?

Question

In C# you can annotate methods with async like this:

class Foo
{
    public async void Bar()  
    {  

    }  
}

This is different to an F# async; I believe that in F# these are called tasks.

So, how do I write a C#-style async member function in F#?

// Not real code
type Foo () = 
  member async this.Bar () = 
    ()

The solution must compile to IL with the same public interface as the C# above.

Answer 1

A C# async method is just a method returning a value of type Task<T>. The C# compiler uses the async keyword to determine that you are allowed to use await inside the code block, but as far as I know, it is not represented in any way in the compiled code.

You say "the solution must compile to the same IL" - that's not going to be easily possible, because F# implements asynchronous operations differently. However, you can get it to compile to IL that has the same public interface using something like this:

type Foo () = 
  member this.Bar () = Async.StartAsTask <| async {
    // yadda yadda 
    }

The asynchronous operation is implemented using standard F# async workflow, so under the cover, this creates an F# Async<T>, but the Async.StartAsTask operation turns that into a Task<T> type, which is what C# expects.

EDIT: Another alternative is to use the TaskBuilder computation expression, which lets you directly create .NET tasks using something like this:

type Foo () = 
  member this.Bar () = task {
    // yadda yadda 
    }