Implement F# interface member with Unit return type in C#

Question

Suppose I've defined the following interface in F#:

type IFoo<'T> = 
  abstract member DoStuff : 'T -> unit

If I implement this in C# I need the method signature to be:

public void DoStuff<T>(T arg) {...}

What I really want to do is reference FSharp.Core and then use:

public Unit DoStuff<T>(T arg) {...}

This would simplify other code because I wouldn't have to deal with Action vs Func. I'm guessing that there isn't any clean way to achieve this? How about some evil hacks?

Answer 1

Transformation of Unit to void is baked into the compiler. There's a FuncConvert class in F# Core for converting between FSharpFunc and Converter. What about defining a similar class to convert Action<T> to Func<T, Unit>?

static class ActionConvert {
    private static readonly Unit Unit = MakeUnit();

    public static Func<T, Unit> ToFunc<T>(Action<T> action) {
        return new Func<T, Unit>(x => { action(x); return Unit; });
    }

    private static Unit MakeUnit() {
        //using reflection because ctor is internal
        return (Unit)Activator.CreateInstance(typeof(Unit), true);
    }
}

Then you could do

var foo = new Foo<int>();
var func = ActionConvert.ToFunc<int>(foo.DoStuff);

You could probably even forego the Unit instance and return null instead.