open System

[<AbstractClass>]
type B() =
    abstract member dString: unit -> string

type T1() =
    inherit B()
    override this.dString() = "T1"

type T2() =
    inherit B()
    override this.dString() = "T2"

type PrinterG = | F of B
                | G of B with
    member self.GenericMethod() =
        match self with
        | F a ->
            printf "GENERIC %A" (a.dString())
        | G a ->
            printf "GENERIC %A" (a.dString())

[F(T1()); F(T2()); G(T1()); G(T2())]
|> List.iter (fun (f:PrinterG) -> f.GenericMethod())