Classes, objects and constructors

Why classes?

So far you have stored data in separate variables: name, balance, accountNo. In a real bank there are millions of accounts, and each one has the same kinds of data and the same actions (deposit, withdraw). A class groups the data and the actions together under one name. This style is called object-oriented programming (OOP).

Your first class

class BankAccount
{
    // fields: the data each object holds
    public string Owner = "";
    public decimal Balance;

    // a method: something the object can do
    public void Deposit(decimal amount)
    {
        Balance += amount;
    }
}

Creating objects with new

var asha = new BankAccount();
asha.Owner = "Asha";
asha.Deposit(5000m);

var ravi = new BankAccount();
ravi.Owner = "Ravi";
ravi.Deposit(800m);

Console.WriteLine($"{asha.Owner}: {asha.Balance}");   // Asha: 5000
Console.WriteLine($"{ravi.Owner}: {ravi.Balance}");   // Ravi: 800

Constructors

A constructor is a special method with the same name as the class and no return type. It runs automatically when you call new, and its job is to put the object into a valid starting state. If you write no constructor, C# gives you an empty one.

var acc = new BankAccount("Meena", 1000m);
Console.WriteLine(acc.Balance);   // 1000

class BankAccount
{
    public string Owner;
    public decimal Balance;

    public BankAccount(string owner, decimal openingBalance)
    {
        Owner = owner;
        Balance = openingBalance;
    }
}

The this keyword

class Student
{
    public string name;
    public int rollNo;

    public Student(string name, int rollNo)
    {
        this.name = name;      // this.name = the field; name = the parameter
        this.rollNo = rollNo;
    }
}

Several constructors

class Product
{
    public string Name;
    public decimal Price;

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

    public Product(string name) : this(name, 0m) { }   // calls the other constructor
}

Objects are references

A class is a reference type. The variable holds a reference (an address) to the object, not the object itself. Assigning one variable to another copies the reference, so both names point to the same object.

var a = new BankAccount("Asha", 100m);
var b = a;            // same object
b.Balance = 999m;
Console.WriteLine(a.Balance);   // 999

Object initializers and ToString

var c = new City { Name = "Bhopal", Pin = 462001 };
Console.WriteLine(c);    // Bhopal - 462001

class City
{
    public string Name = "";
    public int Pin;
    public override string ToString() => $"{Name} - {Pin}";
}

Since C# 12 you may also see primary constructors on classes: class Product(string name, decimal price) { ... }. The parameters can be used anywhere in the class body. They are handy for short classes; the classic constructor shown above works everywhere and is what most code uses.

Common mistakes and fixes

• Declaring a variable but never calling new: BankAccount acc; acc.Deposit(5) does not compile (unassigned), and a field left null throws NullReferenceException.
• Calling an instance method from static code without an object: error CS0120, an object reference is required. Create an object first, or make the method static if it uses no object data.
• Giving a constructor a return type (public void BankAccount()): that makes an ordinary method, not a constructor.
• Forgetting this. when names clash: name = name; assigns the parameter to itself and the field stays empty. The compiler warns you; use this.name = name.