using System;

public class Foo {

    public int num;

}

​

public class Bar {

    public Foo foo;

}

​

public class Baz {

    public Bar bar;

}

​

public class Program

{

    static void PrintNumber(Baz baz) {

        var number = baz?.bar?.foo?.num;

        if( number == null ) {

            Console.WriteLine("null");

        } else {

            Console.WriteLine(number);

        }

    }

    public static void Main()

    {

        var baz = new Baz();

        PrintNumber(baz);

        baz.bar = new Bar();

        PrintNumber(baz);

        baz.bar.foo = new Foo();

        PrintNumber(baz);

    }

}