Question

Given the following fragment:

type Foo() =
    static member Test (act : unit -> unit) = act()
    static member Test (act : Action) = Foo.Test act.Invoke

I get an error on the final line stating: A unique overload for method ‘Test’ could not be determined based on type information prior to this program point. A type annotation may be needed.

Unfortunately, the type annotation (act.Invoke : unit -> unit) does not resolve the ambiguity, and I cannot find an annotation that does fix it. I would like the Action version of be a wrapper round the -> version. My particular use case is defining a class that will be called from both F# and C#, so I want it to work natively from both languages.

Answer 1

F# inserts the conversion to Action for you, so you could (in theory) get by with one method.

type Foo() =
    static member Test (act : Action) = act.Invoke()

Foo.Test (fun () -> ())

I can’t figure out how to force the compiler to choose between Action and unit -> unit. Two other options:

1. make the Action overload the primary and do Foo.Test(Action(act))
2. put the logic in a third function which is called from the public methods

But my recommendation would be a single method taking Action. Since the conversion is automatic, interop, in a sense, is free.