C# — Classes and Objects

Basic class

using System;

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }

    public void Greet()
    {
        Console.WriteLine($"Hi, I'm {Name}!");
    }
}

class Program
{
    static void Main()
    {
        var alice = new Person { Name = "Alice", Age = 30 };
        alice.Greet();
    }
}

Access modifiers

public class BankAccount
{
    public string Owner { get; set; }     // Accessible everywhere
    protected double Balance { get; set; } // Accessible in derived classes
    private double interestRate;           // Accessible only in this class

    public BankAccount(string owner, double initial)
    {
        Owner = owner;
        Balance = initial;
        interestRate = 0.05;
    }

    public void Deposit(double amount)
    {
        if (amount > 0) Balance += amount;
    }

    public double GetBalance() => Balance;
}

Properties

using System;

public class Person
{
    // Auto-property
    public string Name { get; set; }

    // Property with validation
    private int age;
    public int Age
    {
        get => age;
        set
        {
            if (value < 0)
                throw new ArgumentException("Age cannot be negative");
            age = value;
        }
    }

    // Read-only property
    public string Description => $"{Name} is {Age} years old";

    // Init-only property (C# 9)
    public string Id { get; init; }
}

class Program
{
    static void Main()
    {
        var p = new Person { Name = "Alice", Age = 30, Id = "P001" };
        Console.WriteLine(p.Description);
        // p.Id = "P002"; // Error: init-only
    }
}

Constructors

using System;

public class Rectangle
{
    public double Width { get; }
    public double Height { get; }

    // Primary constructor (C# 12)
    public Rectangle(double width, double height)
    {
        Width = width;
        Height = height;
    }

    // Constructor chaining
    public Rectangle(double side) : this(side, side) { }

    // Static factory method
    public static Rectangle CreateSquare(double side) => new(side, side);

    public double Area() => Width * Height;
}

class Program
{
    static void Main()
    {
        var r1 = new Rectangle(5, 3);
        var r2 = new Rectangle(4);      // Square
        var r3 = Rectangle.CreateSquare(6);

        Console.WriteLine($"Area 1: {r1.Area()}"); // 15
        Console.WriteLine($"Area 2: {r2.Area()}"); // 16
        Console.WriteLine($"Area 3: {r3.Area()}"); // 36
    }
}

Destructor / IDisposable

using System;

public class Resource : IDisposable
{
    private bool disposed = false;

    public Resource()
    {
        Console.WriteLine("Resource acquired");
    }

    public void Dispose()
    {
        if (!disposed)
        {
            Console.WriteLine("Resource released");
            disposed = true;
        }
        GC.SuppressFinalize(this);
    }

    ~Resource()
    {
        Dispose();
    }
}

class Program
{
    static void Main()
    {
        // Using statement ensures Dispose is called
        using (var res = new Resource())
        {
            Console.WriteLine("Using resource...");
        }
        // Resource released here

        // C# 8 using declaration
        using var res2 = new Resource();
        Console.WriteLine("Using resource 2...");
    }
}

Static members

using System;

public class User
{
    private static int count = 0;
    public string Name { get; }

    public User(string name)
    {
        Name = name;
        count++;
    }

    public static int GetCount() => count;

    // Static constructor
    static User()
    {
        Console.WriteLine("User class initialized");
    }
}

class Program
{
    static void Main()
    {
        new User("Alice");
        new User("Bob");
        Console.WriteLine($"Total users: {User.GetCount()}"); // 2
    }
}

Records (C# 9+)

using System;

// Immutable data class
public record Point(double X, double Y);

// Record with custom methods
public record Person(string Name, int Age)
{
    public string Greeting => $"Hello, I'm {Name}!";
}

class Program
{
    static void Main()
    {
        var p1 = new Point(3, 4);
        var p2 = new Point(3, 4);

        Console.WriteLine(p1 == p2);  // True (value equality)
        Console.WriteLine(p1);        // Point { X = 3, Y = 4 }

        // Non-destructive mutation
        var p3 = p1 with { X = 10 };
        Console.WriteLine(p3);        // Point { X = 10, Y = 4 }
    }
}

Mini Practice

Write C# code that:

1. Creates a Car class with properties and methods
2. Implements IDisposable for cleanup
3. Uses a primary constructor
4. Demonstrates record equality

Up Next

In the next lesson, you'll learn about Constructors — initialization patterns in C#.