OOP model

Set up the class hierarchy and interfaces.

The Library class

A Library class holds books together. It stores an inner List<Book> and accepts new books via an Add method. A Count property tells how many books it has.

This design is an example of encapsulation: the outside cannot reach the inner list directly; it works only through controlled members like Add and Count. The data stays safe inside the class.

Add and Count members

public void Add(Book b) => _books.Add(b); adds a new book with a single expression body. public int Count => _books.Count; exposes the list's count to the outside.

These members are called from outside; the _books field is private and cannot be reached directly. This is the practical payoff of encapsulation: internal changes do not break external code.

Exercise Add the Library class

The Book class below is ready. Add a Library class: it holds an inner List<Book> _books and exposes Add(Book b) and Count. In Main create a Library, add a book and print the count.

Show example solution

Write class Library { private List<Book> _books = new List<Book>(); public void Add(Book b) => _books.Add(b); public int Count => _books.Count; }. In Main write var lib = new Library(); lib.Add(new Book { Title = "Crime and Punishment", Author = "Dostoevsky" }); Console.WriteLine(lib.Count);
Example solution:
using System;
using System.Collections.Generic;

class Book
{
    public string Title { get; set; }
    public string Author { get; set; }
}

class Library
{
    private List<Book> _books = new List<Book>();

    public void Add(Book b) => _books.Add(b);

    public int Count => _books.Count;
}

class Program
{
    static void Main()
    {
        var lib = new Library();
        lib.Add(new Book { Title = "Crime and Punishment", Author = "Dostoevsky" });
        Console.WriteLine(lib.Count);
    }
}

Test yourself

Complete the list declaration

private List<____> _books = new List<Book>();

Show answer

private List<Book> _books = new List<Book>();

Why is _books private?
- It runs faster
- It provides encapsulation
- The compiler requires it
- The garbage collector needs it
Show answer

It provides encapsulation — Correct. private access shields the inner list from outside tampering.
Match the member to its role

_books

Inner list
Add

Add a book
Count

Book tally
private

Hidden access

Next lesson: LINQ layer

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