Question

I’ve struggled with this problem for quite some time now, and can’t seem to find any solution. Let me simplify it for you.

I have a generic function that I want to call, but the type argument I want to call it with only as an instance. Example

let foo_a<'a> () = typeof<'a>
let foo_b (t : System.Type) = foo_a<t>() // of course this does not work

I would like the following statement to be true

foo_a<int>() = foo_b(typeof<int>)

In C# I would’ve reflected out foo_a’s MethodInfo and do MakeGenericMethod(t), but how do I do this in F#?

Just to clearify, flipping the dependency and making foo_a calling foo_b instead, is not an option for me.

Answer 1

As @svick said, there’s no special way to do this in F# — you need to use Reflection just like you would in C#.

Here’s a simple example you can paste into F# interactive:

open System.Reflection

type Blah =
    //
    static member Foo<'T> () =
        let argType = typeof<'T>
        printfn "You called Foo with the type parameter: %s" argType.FullName


let callFoo (ty : System.Type) =
    let genericFoo =
        typeof<Blah>.GetMethod "Foo"

    let concreteFoo =
        genericFoo.MakeGenericMethod [| ty |]

    concreteFoo.Invoke (null, Array.empty);;  // The ;; is only needed for F# interactive

Output:

> callFoo typeof<int>;;
You called Foo with the type parameter: System.Int32
val it : obj = null