Day 5: Classes, Objects & Properties

Welcome to Day 5! Today we dive into Object-Oriented Programming (OOP).

In C#, almost everything is an object. A class is simply the blueprint, and the object is the actual building created from that blueprint.

Creating a Class

Let’s model a basic User for a system. You create a class using the class keyword.

public class User 
{
    // Fields (Internal data storage)
    private string _passwordHash;

    // Constructors (How to build the object)
    public User(string email) 
    {
        Email = email; // Set the property using the passed-in value
    }

    // Properties (Public access points to data)
    public string Email { get; set; }
    
    // Methods (Behaviors)
    public void SetPassword(string newPassword) 
    {
        _passwordHash = "HASHED_" + newPassword;
    }
}

Instantiating Objects

Now that we have the User class (the blueprint), let’s create actual User objects in our Program.cs. We use the new keyword to call the class constructor.

// Create a new instance of User
User myUser = new User("alice@example.com");

// Using properties and methods
Console.WriteLine(myUser.Email); // "alice@example.com"
myUser.SetPassword("superSecret123");

Deep Dive: Properties

In many languages (like Java), you have to write getEmail() and setEmail() methods manually to protect internal variables. C# solves this elegantly with Properties.

Auto-Implemented Properties

If you don’t need custom logic, C# creates the hidden backing field for you automatically.

public int Age { get; set; }

Read-Only / Init-Only Properties

Often, you want someone to read a value, but not change it after the object is created.

// Only code inside the class can set the ID
public int Id { get; private set; }

// Using 'init' means it can only be set exactly during object creation
public string Username { get; init; }

Object Initialization Syntax

A beautiful shortcut for creating objects when setting multiple properties.

public class Product 
{
    public string Name { get; init; }
    public decimal Price { get; set; }
}

// Creating an object without needing a massive constructor!
var laptop = new Product 
{
    Name = "MacBook Pro",
    Price = 1999.99m
};

Challenge for Day 5

Create a Book class. Give it an init-only Title property, an Author property, and a Pages property. Create a list, or just manually instantiate three different Book objects using Object Initialization Syntax!

Tomorrow: Inheritance & Interfaces.