using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

}

​

public class Book

{

    public int Id { get; set; }

    public string Title { get; set; }

    private static int _lastId = 0;

    public static List<Book> Books { get; }

    public static int NextId => ++_lastId;

    static Book()

    {

        Books = new List<Book>

        {

            new Book

            {

                Id = NextId,

                Title = "Some cool computer book"

            }

        };

    }

    public Book(int? id = null)

    {

        Id = id ?? default(int);    

    }

    public void Save()

    {

        // create the book if it does not exist,

        // otherwise, find its index and replace it

        // by the current object.

        if (Books.Any(x => x.Id == id))

        {

        }

    }

}