C# Classes and Objects

C# is an object-oriented language. A class describes a kind of thing, and an object is one real thing made from that class.

Classes and Objects

Class	Objects
Student	Ali, Ayesha, Bilal
Course	Web Development, Graphic Designing
Product	Rice bag, cooking oil

In a top-level program, write your classes after the main code.

Creating a Class

Example

csharp

Student s1 = new Student();
s1.Name = "Ali";
s1.Marks = 76;
s1.Introduce();

Student s2 = new Student { Name = "Ayesha", Marks = 93 };
s2.Introduce();

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

    public void Introduce()
    {
        Console.WriteLine($"I am {Name}, my marks are {Marks}");
    }
}

Output

I am Ali, my marks are 76
I am Ayesha, my marks are 93

Name and Marks are properties. { get; set; } means other code can read and change them. The second object is made with an object initializer, which sets properties inside { }.

Constructors

A constructor runs when the object is created. It has the class name and no return type:

Example

csharp

Product oil = new Product("Cooking oil 1L", 620);
Product rice = new Product("Rice 5kg", 1850);
oil.Show();
rice.Show();

class Product
{
    public string Name { get; }
    public int Price { get; }

    public Product(string name, int price)
    {
        Name = name;
        Price = price;
    }

    public void Show() => Console.WriteLine($"{Name}: Rs {Price}");
}

Output

Cooking oil 1L: Rs 620
Rice 5kg: Rs 1850

Properties with only get can be set in the constructor but not changed later. This keeps the object safe.

Encapsulation

Keep data private and allow changes only through methods that check the values:

Example

csharp

BankAccount acc = new BankAccount();
acc.Deposit(10000);
acc.Withdraw(3500);
acc.Withdraw(50000);   // refused
Console.WriteLine($"Balance: Rs {acc.Balance}");

class BankAccount
{
    private int balance = 0;

    public int Balance => balance;

    public void Deposit(int amount)
    {
        if (amount > 0) balance += amount;
    }

    public void Withdraw(int amount)
    {
        if (amount <= balance) balance -= amount;
        else Console.WriteLine("Not enough balance");
    }
}

Output

Not enough balance
Balance: Rs 6500

Access modifier	Who can use the member
`public`	Any code
`private`	Only code inside the same class (the default)
`protected`	The class and classes that inherit from it
`internal`	Any code in the same project

Inheritance

A class can inherit from another with :. Mark a method virtual to let child classes change it with override:

Example

csharp

Animal[] pets = { new Cat(), new Dog() };
foreach (Animal pet in pets)
{
    pet.Speak();
}

class Animal
{
    public virtual void Speak() => Console.WriteLine("...");
}

class Cat : Animal
{
    public override void Speak() => Console.WriteLine("Meow");
}

class Dog : Animal
{
    public override void Speak() => Console.WriteLine("Woof");
}

Output

Meow
Woof

Each object uses its own version of Speak(). This is called polymorphism.

Start every new class small: a few properties, a constructor and one or two methods. Add more only when your program needs it.

Exercise

Which keyword lets a child class replace a virtual method from its parent?

newbaseoverridestatic