using System;

public class Program

{

    public static void Main()

    {

        Student student = new Student

        {

            Name = "King",

            oAddress = null

        };

        int? doorNum = student.oAddress?.DoorNumber;

​

    }

}

​

public class Student

{

    public string Name;

    public Address oAddress= null;

    public Student()

    {

    }

}

​

public class Address

{

    public int DoorNumber;

    public Address()

    {

    }

}

​

​

​

​