F# call member function from another function of a type when implement interface

Question

This is the working example from here:

type MethodExample() = 

    // standalone method
    member this.AddOne x = 
        x + 1

    // calls another method
    member this.AddTwo x = 
        this.AddOne x |> this.AddOne

That is what I want to do:

type IMethod =   
    abstract member  AddOne: a:int -> int
    abstract member  AddTwo: a:int -> int

type MethodExample() =     
    interface IMethod with

        member this.AddOne x = 
            x + 1

        // calls another method
        member this.AddTwo x = 
            this.AddOne x |> this.AddOne

The AddOne function is not available, I also tried to downcast this to MethodExample but is horrible and does not work.

How can I do it?

When facing problem like this, I usually put AddOne to MethodExample, rename to _AddOne then I can call it inside the interface implementation. This may be ugly but it works. — Nghia Bui

Fyodor Soikin Fyodor Soikin · Accepted Answer · 2019-04-25T01:03:43

In F# all interface implementations are private - meaning interface methods do not also appear as public methods of the class, like they do in C#. They are only accessible via the interface.

Therefore, in order to access them, you need to cast your class to the interface first:

member this.AddTwo x = 
    let me = this :> IMethod
    me.AddOne x |> me.AddOne

F# call member function from another function of a type when implement interface

1 Answers