using System;

​

#nullable enable 

public class Address

{

    public string? Building;

    public string Street = string.Empty;

    public string City = string.Empty;

    public string Region = string.Empty;

    public static void Main()

    {

        var Address = new Address();

        Address.Building = null;

        Address.Street = null;

        Address.City = "İstanbul";

        Address.Region = null;

        Console.WriteLine(Address.City);

    }

}