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 !);
}