Open/Closed Principle

Add new behavior by adding code, not by changing code that already works.

What is Open/Closed Principle?

The Open/Closed Principle says code should be open for extension but closed for modification: you add new behavior by adding new classes, not by editing working ones.

public interface IDiscount { decimal Apply(decimal price); }

// New discount? Add a class; the checkout code stays untouched.
public class StudentDiscount : IDiscount
{
    public decimal Apply(decimal price) => price * 0.9m;
}

Level: Freelancer · More OOP terms at this level

Updated September 27, 2026