Classes and Objects in C#

# CHAPTER 13

Classes and Objects #

1. Introduction #

2. Learning Objectives #

• Define a custom Class.

• Use the new keyword to instantiate an Object.

• Understand Access Modifiers (public, private).

• Differentiate between Fields and Properties.

• Create Methods inside a class.

3. Creating a Class #

// The Blueprint
class Car
{
    // Fields
    public string brand;
    public string color;

    // Method
    public void Drive()
    {
        Console.WriteLine($"The {color} {brand} is driving!");
    }
}

4. Creating an Object #

class Program
{
    static void Main()
    {
        // Object 1
        Car myCar = new Car();
        myCar.brand = "Toyota";
        myCar.color = "Red";
        myCar.Drive(); // "The Red Toyota is driving!"

        // Object 2
        Car friendCar = new Car();
        friendCar.brand = "BMW";
        friendCar.color = "Black";
        friendCar.Drive(); // "The Black BMW is driving!"
    }
}

5. Access Modifiers (public vs private) #

• public: Any other code can access and change this variable.

• private: ONLY the code inside the class itself can see or change this variable. It is hidden from the outside world.

If you don't write an access modifier, C# defaults variables to private.

class BankAccount
{
    private decimal balance = 1000; // Hidden!

    public void CheckBalance()
    {
        // The method is inside the class, so it can read the private variable
        Console.WriteLine($"Balance: ${balance}"); 
    }
}

class Program
{
    static void Main()
    {
        BankAccount myAccount = new BankAccount();
        // myAccount.balance = 0; // ERROR! Cannot access private variable
        myAccount.CheckBalance(); // OK! The method is public.
    }
}

*This is the core of Encapsulation!*

6. Fields vs. Properties #

class Employee
{
    // A modern C# Auto-Implemented Property
    public string Name { get; set; }
    
    // A Property with custom logic
    private int _age; // Private backing field (conventionally starts with underscore)
    
    public int Age
    {
        get { return _age; }
        set 
        { 
            if (value >= 18) { _age = value; } // Validation!
            else { Console.WriteLine("Invalid Age!"); }
        }
    }
}

7. Mini Project: Employee Management System #

using System;

namespace EnterpriseApp
{
    class Employee
    {
        public string Name { get; set; }
        public string Department { get; set; }

        public void DisplayDetails()
        {
            Console.WriteLine($"Employee: {Name} | Dept: {Department}");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Employee emp1 = new Employee();
            emp1.Name = "Alice";
            emp1.Department = "IT";

            Employee emp2 = new Employee();
            emp2.Name = "Bob";
            emp2.Department = "HR";

            emp1.DisplayDetails();
            emp2.DisplayDetails();
        }
    }
}

8. OOP and Memory Explanation #

9. Common Mistakes #

• Forgetting new: Car c1; c1.Drive(); will crash with a NullReferenceException. You declared the variable, but you never built the actual object!

• Using public fields: Don't write public int health;. Always use properties: public int Health { get; set; }.

10. Best Practices #

• Properties should use PascalCase (Name), while private backing fields should use camelCase with an underscore (_name).

• One Class per File: In real projects, you don't put all classes in Program.cs. You create a new file (e.g., Car.cs) for the Car class.

11. Exercises #

1. 1. Create a Book class with auto-implemented properties for Title, Author, and Pages.

1. 2. In Main(), create two Book objects, assign values to them, and print their details.

12. MCQs with Answers #

13. Interview Questions #

• Q: Differentiate between a Field and a Property in C#. Why are properties preferred?

• Q: Explain what the NullReferenceException is and what causes it when dealing with Objects.

14. Summary #

15. Next Chapter Recommendation #