static void Main(string[] args)
        {
            var legacy = new Legacy();

            legacy.Foo(null); // passing null is ok

            var newer = new ThisIsNew();

            newer.Foo(null !); // passing null is not ok

            Book[] books = new Book[3];
            books[0] = new Book("Professional C# 7");
            Console.WriteLine(books[0].Title);
            string publisher = books[0].Publisher ?? string.Empty;

            Console.WriteLine(books[0].Publisher !);
        }