F# results of https://gist.github.com/brandonbloom/4b8c3e7698959f2bf6bc9c75a9de4b01

dispatch.fs

type Base() =
    member x.Method() = "base method";

type Derived() =
    inherit Base()
    member x.Method() = "derived method"

type Base with
    member x.Extension() = "base extension"

type Derived with
    member x.Extension() = "derived extension"    
    
let a = Base()
let b = Derived()

printfn "%s" (a.Method())
printfn "%s" (b.Method())

printfn "%s" (a.Extension())
printfn "%s" (b.Extension())

(*
base method
derived method
base extension
derived extension
*)
    

Woops, saw that I misread - the let b should be upcast to Base.

Correction:

let a = Base()
let b = Derived() :> Base