using System;

public class Program

{

    public static void Main()

    {

        A a = new A();//null  ;

            if((a?.b) is null)

            {

                Console.WriteLine("null");  

            }

        else

        {

                Console.WriteLine("not null");  

        }

    }

    class A

    {

        public A b {get;set;}

    }

}

​