Software entities (classes, modules, functions) should be open for extension but closed for modification.

This means:
Open for Extension: You should be able to add new functionality to an existing module or class without altering its existing code.

Closed for Modification: Once a class is written and tested, you should not need to modify it to add new functionality.

Why is OCP Important?

• Preserves Stability: Existing code remains untouched, reducing the risk of introducing new bugs.

• Enhances Flexibility: Makes the system adaptable to new requirements without rewriting or breaking existing logic.

• Supports Scalability: New behaviors can be added with minimal effort.

• Aligns with Agile Practices: Easily adapts to changing requirements, a common scenario in software development.

How Violates OCP?

OCP is violated when:

• Adding new behavior requires modifying existing classes or methods.
• This leads to tight coupling, making it harder to maintain or extend the system.

Example of OCP Violation:
Imagine a PaymentProcessorclass that handles different payment methods:

public class PaymentProcessor
{
    public void ProcessPayment(string paymentType, decimal amount)
    {
        if (paymentType == "CreditCard")
        {
            Console.WriteLine($"Processing credit card payment of {amount:C}.");
        }
        else if (paymentType == "Cash")
        {
            Console.WriteLine($"Processing cash payment of {amount:C}.");
        }
    }
}

Problems:

• Adding a new payment type requires modifying the ProcessPayment method.
• Every change increases the likelihood of bugs in existing functionality.
• This violates OCP because the class is not closed for modification.

How to Implement Class-Level OCP

OCP can be implemented using techniques like polymorphism, composition, and strategy patterns. Below are steps for implementation using class-level OCP with execution flow.

Step 1: Define an Abstraction
Create an abstract class or interface to define common behavior.

public interface IPaymentProcessor
{
    void ProcessPayment(decimal amount);
}

Step 2: Create Concrete Implementations
Implement the interface for each payment type.

public class CreditCardProcessor : IPaymentProcessor
{
    public void ProcessPayment(decimal amount)
    {
        Console.WriteLine($"Processing credit card payment of {amount:C}.");
    }
}

public class CashPaymentProcessor : IPaymentProcessor
{
    public void ProcessPayment(decimal amount)
    {
        if (amount > 1000)
        {
            Console.WriteLine("Cash payment cannot exceed $1000.");
            return;
        }
        Console.WriteLine($"Processing cash payment of {amount:C}.");
    }
}

Step 3: Use Abstraction in the Client
The client depends on the abstraction (IPaymentProcessor) and can dynamically work with any implementation.

public class PaymentHandler
{
    private readonly IPaymentProcessor _paymentProcessor;

    public PaymentHandler(IPaymentProcessor paymentProcessor)
    {
        _paymentProcessor = paymentProcessor;
    }

    public void HandlePayment(decimal amount)
    {
        _paymentProcessor.ProcessPayment(amount);
    }
}

Step 4: Call PaymentHandler
Create and use the appropriate payment processor at runtime.

var creditCardProcessor = new CreditCardProcessor();
var paymentHandler = new PaymentHandler(creditCardProcessor);
paymentHandler.HandlePayment(500);

var cashProcessor = new CashPaymentProcessor();
var cashPaymentHandler = new PaymentHandler(cashProcessor);
cashPaymentHandler.HandlePayment(1200);

Execution Flow

• The PaymentHandlerrelies on the IPaymentProcessorinterface.
• Adding a new payment type (e.g., DigitalWalletProcessor) requires creating a new class that implements IPaymentProcessor.
• No changes are made to the existing PaymentHandleror other classes.

How to Implement Method-Level OCP

For finer-grained control (specific method behavior), we can use strategies or composition to inject new logic into a method.

Example: Discount Calculation

public interface IDiscountStrategy
{
    decimal CalculateDiscount(decimal price);
}

public class RegularDiscount : IDiscountStrategy
{
    public decimal CalculateDiscount(decimal price) => price * 0.1m; // 10% discount
}

public class VIPDiscount : IDiscountStrategy
{
    public decimal CalculateDiscount(decimal price) => price * 0.2m; // 20% discount
}

public class DiscountCalculator
{
    public decimal Calculate(decimal price, IDiscountStrategy discountStrategy)
    {
        return discountStrategy.CalculateDiscount(price);
    }
}

Usage:

var calculator = new DiscountCalculator();
decimal discountedPrice = calculator.Calculate(1000, new VIPDiscount());
Console.WriteLine($"Discounted Price: {discountedPrice:C}");

Flow:
New discount types can be added without modifying the DiscountCalculator.

By following OCP, you build systems that are easier to extend and maintain while reducing risks of breaking existing functionality.