using System;

public class Program

{

    class ChildReference

    {

        public int Id {get; set;}

    }

    class ParentReference

    {

        public ChildReference Child {get; set;}

    }

    public static void Main()

    {

        ChildReference obj = null;

        ParentReference parent = null;

        Console.WriteLine($"ChildReference Id = {0}", obj?.Id);

        Console.WriteLine($"Parent - ChildReference Id = {0}", parent?.Child.Id);

    }

}