void Main()
{
    var x = new Outer();
    x.MyInner.Print();
    x.MyInner.Print();
}

class Outer
{
    private int _privateMember;

    public Inner MyInner;
    public Outer()
    {
        MyInner = new Inner(this);
    }

    public class Inner
    {
        Outer _outer;

        public Inner(Outer outer)
        {
            _outer = outer;
        }

        public void Print()
        {
            Console.WriteLine(_outer._privateMember++);
        }
    }
}