Classes and Objects

Key Distinction

Class = Blueprint or template (defines structure).

Object = Specific instance created from the class (stores actual data).

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Class Example (Blueprint)

A class defines what attributes and behaviors objects will have.

public class Student
{
    // Attributes (data)
    public string Name { get; set; } = "";
    public int Score { get; set; }

    // Behaviour (method)
    public void PrintDetails()
    {
        Console.WriteLine($"{Name}: {Score}");
    }
}

Important

A class does not store real data by itself - it's just a blueprint.

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Object Example (Instance)

An object is created from a class and stores actual values.

// Create an object from the Student class
Student student1 = new Student();
student1.Name = "Alex";
student1.Score = 85;

// Call the object's method
student1.PrintDetails();  // Output: Alex: 85

What This Shows

• student1 is an object
• It is created from the Student class
• It stores actual values
• It can call the class's methods

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Multiple Objects from One Class

You can create many objects from the same class, each with different data.

// Create first student
Student student1 = new Student();
student1.Name = "Alex";
student1.Score = 85;

// Create second student
Student student2 = new Student();
student2.Name = "Jordan";
student2.Score = 92;

// Create third student
Student student3 = new Student();
student3.Name = "Taylor";
student3.Score = 78;

// Each object stores different data
student1.PrintDetails();  // Alex: 85
student2.PrintDetails();  // Jordan: 92
student3.PrintDetails();  // Taylor: 78

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More Complex Class Example

public class Book
{
    // Attributes
    public string Title { get; set; } = "";
    public string Author { get; set; } = "";
    public int Year { get; set; }
    public bool IsAvailable { get; set; } = true;

    // Methods
    public void BorrowBook()
    {
        if (IsAvailable)
        {
            IsAvailable = false;
            Console.WriteLine($"{Title} has been borrowed.");
        }
        else
        {
            Console.WriteLine($"{Title} is not available.");
        }
    }

    public void ReturnBook()
    {
        IsAvailable = true;
        Console.WriteLine($"{Title} has been returned.");
    }

    public void PrintDetails()
    {
        string status = IsAvailable ? "Available" : "Borrowed";
        Console.WriteLine($"{Title} by {Author} ({Year}) - {status}");
    }
}

Using the Book class

Book book1 = new Book();
book1.Title = "Clean Code";
book1.Author = "Robert Martin";
book1.Year = 2008;

book1.PrintDetails();  // Clean Code by Robert Martin (2008) - Available
book1.BorrowBook();    // Clean Code has been borrowed.
book1.PrintDetails();  // Clean Code by Robert Martin (2008) - Borrowed
book1.ReturnBook();    // Clean Code has been returned.

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Complete Example

using System;

public class Student
{
    public string Name { get; set; } = "";
    public int Score { get; set; }

    public void PrintDetails()
    {
        Console.WriteLine($"{Name}: {Score}");
    }

    public string GetGrade()
    {
        if (Score >= 80) return "A";
        if (Score >= 60) return "B";
        if (Score >= 40) return "C";
        return "F";
    }
}

public class Program
{
    public static void Main()
    {
        // Create objects from Student class
        Student s1 = new Student();
        s1.Name = "Alex";
        s1.Score = 85;

        Student s2 = new Student();
        s2.Name = "Jordan";
        s2.Score = 72;

        // Use object methods
        s1.PrintDetails();
        Console.WriteLine($"Grade: {s1.GetGrade()}");

        s2.PrintDetails();
        Console.WriteLine($"Grade: {s2.GetGrade()}");
    }
}

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Another Example: Car Class

public class Car
{
    // Attributes
    public string Make { get; set; } = "";
    public string Model { get; set; } = "";
    public int Year { get; set; }
    public double FuelLevel { get; set; } = 100.0;

    // Methods
    public void Drive(double distance)
    {
        double fuelUsed = distance * 0.08;  // 8L per 100km
        if (fuelUsed <= FuelLevel)
        {
            FuelLevel -= fuelUsed;
            Console.WriteLine($"Drove {distance}km. Fuel remaining: {FuelLevel:F1}L");
        }
        else
        {
            Console.WriteLine("Not enough fuel!");
        }
    }

    public void Refuel(double amount)
    {
        FuelLevel += amount;
        if (FuelLevel > 100) FuelLevel = 100;
        Console.WriteLine($"Refueled. Fuel level: {FuelLevel:F1}L");
    }

    public void PrintDetails()
    {
        Console.WriteLine($"{Year} {Make} {Model} - Fuel: {FuelLevel:F1}L");
    }
}

Using the Car class

Car myCar = new Car();
myCar.Make = "Toyota";
myCar.Model = "Camry";
myCar.Year = 2020;

myCar.PrintDetails();      // 2020 Toyota Camry - Fuel: 100.0L
myCar.Drive(200);          // Drove 200km. Fuel remaining: 84.0L
myCar.Drive(500);          // Drove 500km. Fuel remaining: 44.0L
myCar.Refuel(30);          // Refueled. Fuel level: 74.0L

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Quick Reference

Term	Meaning	Example
Class	Blueprint/template	Student
Object	Specific instance	student1
Attributes	Data/properties	Name, Score
Methods	Behaviors/actions	PrintDetails()

Exam Language

"A class is a blueprint; an object is a specific instance created from that blueprint."