Classes & Objects in C#

1. The Design Sheet = Class

A class defines what data and actions every car will have. No real car exists yet.

public class Car
{
    public string Owner; // Data: Every car has an owner
    private int engineNumber; // Data: Hidden from outside
    
    public void Start() // Action: Every car can start
    { 
        Console.WriteLine($"{Owner}'s car started"); 
    }
}

2. Build the Real Car = Object

new takes the design and builds 1 real car in memory.

Car kunalCar = new Car(); 
kunalCar.Owner = "Kunal"; // OK: Owner is public
// kunalCar.engineNumber = 1; // Error: engineNumber is private
kunalCar.Start();

3. Access Control: public vs private

public = Anyone can use it. private = Only code inside the class can use it. Default is private for fields.

1. Why Do We Need Classes?

Without classes, managing 3 cars looks like this:

string kunalCarOwner = "Kunal"; int kunalCarSpeed = 0;
string prernaCarOwner = "Prerna"; int prernaCarSpeed = 0;

Classes group related data into one unit:

public class Car
{
    public string Owner;
    public int Speed;
}

2. Fields & Methods

Fields = Data the object stores. Methods = Actions the object can do.

public class Car
{
    public string Owner; // Field
    private int Speed; // Field - private
    
    public void Accelerate() // Method
    {
        Speed += 10; // OK: Can access private Speed inside class
    }
}

3. Constructors: The Factory Setup

A constructor runs automatically when you use new. Used to set initial values.

public class Car
{
    public string Owner;
    
    public Car(string ownerName) 
    {
        Owner = ownerName;
        Console.WriteLine($"New car for {Owner} rolled out");
    }
}
Car sanjuCar = new Car("Sanju");

If you do not write one, C# gives a free empty constructor: public Car(){}

4. All Access Modifiers: Who Gets the Keys?

Access modifiers control visibility. Here are all 6:

Most used:

public class Car
{
    public string Owner; // Everyone can see
    private int engineNumber; // Only Car code can see. Default for fields.
    protected string VinNumber; // Car + child classes like SportsCar can see
    internal string FactoryCode; // Any code in same project can see
}

5. static: Shared by All Cars

Instance fields: Each car has its own Owner.
static fields: One value shared by all cars.

public class Car
{
    public string Owner; // Instance
    public static int TotalCars; // Static
    
    public Car() { TotalCars++; }
}
Car a = new Car(); 
Car b = new Car();
Console.WriteLine(Car.TotalCars); // 2. Use ClassName for static.

6. How Memory Works: Parking Lot Analogy

Car kunalCar = new Car();

1. Heap = Parking Lot: The real Car object is built and parked here.
2. Stack = Valet Ticket: The variable kunalCar is just a ticket holding the parking spot number.

Car anotherCar = kunalCar; copies the ticket. Both point to same car. This is why objects are "reference types".