struct Mutable {

    private int x;

    public int Mutate() {

        this.x = this.x + 1;

        return this.x;

    }

}

​

class Test {

    public readonly Mutable m = new Mutable();

    static void Main(string[] args) {

        Test t = new Test();

        System.Console.WriteLine(t.m.Mutate());

        System.Console.WriteLine(t.m.Mutate());

        System.Console.WriteLine(t.m.Mutate());

    }

}