Class

A class is like a blueprint or template for creating objects.

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Creating a Class

public class Car
{
}

Adding State and Behavior

A class usually has two main parts:

• Fields → store data or state

• Methods → perform actions or define behavior

Defining a Field

A field is simply a variable declared inside a class that holds data.

public class Car
{
    public string brand;
    public int speed;
}

Defining a Method

A method represents an action that an object can perform — for example, driving or stopping.

public class Car
{
    public string brand;
    public int speed;

    public void Drive()
    {
        Console.WriteLine(brand + " is driving at " + speed + " km/h");
    }

    public void Stop()
    {
        Console.WriteLine(brand + " stopped!");
    }
}

The keyword void means this method doesn’t return any value.

Creating an Object from a Class

To use a class, you need to create an object (an instance of that class)

Car myCar = new Car();

myCar.brand = "BMW";
myCar.speed = 120;
myCar.Drive();

Adding a Constructor

A constructor is a special method that runs automatically when an object is created.

public class Car
{
    public string brand;
    public int speed;

    // Constructor
    public Car()
    {
        brand = "Unknown";
        speed = 0;
    }
}

Car myCar = new Car();
Console.WriteLine(myCar.brand); // Unknown

Constructor with Parameters

You can create a constructor that takes values from the user when an object is created

public class Car
{
    public string brand;
    public int speed;

    // Constructor with parameters
    public Car(string carBrand, int carSpeed)
    {
        brand = carBrand;
        speed = carSpeed;
    }

    public void Drive()
    {
        Console.WriteLine(brand + " is driving at " + speed + " km/h");
    }
}

Car car1 = new Car("Toyota", 100);
Car car2 = new Car("BMW", 180);

car1.Drive();       // Toyota is driving at 100 km/h
car2.Drive();       // BMW is driving at 180 km/h

Working with Static Members

Static members belong to the class itself, not to any specific object.

public class Car
{
    public string brand;
    public int speed;
    public static int count = 0; // static field

    public Car(string carBrand, int carSpeed)
    {
        brand = carBrand;
        speed = carSpeed;
        count++; // increases each time a new car is created
    }
}

Car car1 = new Car("BMW", 120);
Car car2 = new Car("Audi", 140);
Console.WriteLine("Total cars: " + Car.count);      // Total cars: 2

Using Getters and Setters

In C#, properties let you control how fields are read or updated.
They use getters (for reading values) and setters (for assigning values).

Instead of using public fields like this:

public class Car
{
    public string brand;
    public int speed;
}

You can use properties to protect the data:

public class Car
{
    private string brand;  // private field

    public string Brand    // Property
    {
        get { return brand; }              // Getter
        set { brand = value; }             // Setter
    }
}

Now you can access the property safely:

Car myCar = new Car();
myCar.Brand = "BMW";          // calls the setter
Console.WriteLine(myCar.Brand); // calls the getter

Adding Validation

You can add logic inside the setter to validate data before saving it.

public class Car
{
    private int speed;

    public int Speed
    {
        get { return speed; }
        set
        {
            if (value < 0)
                throw new ArgumentException("Speed cannot be negative!");
            speed = value;
        }
    }
}

Auto-Implemented Properties

If you don’t need any extra logic, you can let C# automatically create a hidden field:

public class Car
{
    public string Brand { get; set; }
    public int Speed { get; set; }
}

This is the same as manually writing a field with a basic getter and setter.

Read-Only and Write-Only Properties

You can also make a property read-only (getter only) or write-only (setter only):

public class Car
{
    public string Model { get; } = "Sedan";  // read-only
    public string Password { private get; set; }  // write-only from outside
}