Question

I know this is possible in C#, which produces simple and efficient code. — Two objects of the same class can access each other’s private parts.

class c1
{
    private int A;

    public void test(c1 c)
    {
        c.A = 5;
    }
}

But It seems impossible in F#, is it true?

type c1()
     let A = 0
     member test (c: c1) = c.A

Answer 1

Interesting question. It seems to work with an explicit field but not with a let binding:

// Works
type c1 =
    val private A : int
    new(a) = { A = a }
    member m.test(c : c1) = c.A

let someC1 = new c1(1)
let someMoreC1 = new c1(42);
let theAnswer = someC1.test someMoreC1

// Doesn't work
type c2() =
    let mutable A = 42
    // Compiler error: The field, constructor or member 'A' is not defined
    member m.test(c : c2) = c.A