Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types without altering the correctness of the program.

This means, if you have a class B that extends a class A, you should be able to replace A with B without breaking the functionality of your application. Apply LSP in inheritance scenarios and when using polymorphism.

Why is LSP Important?

• Ensures Reliable Substitution: Promotes a design where derived classes work seamlessly in place of their base class.

• Improves Maintainability: Ensures that new derived classes won’t break the existing code.

• Enhances Reusability: Enables the use of polymorphism effectively, as derived classes adhere to the behavior of their base class.

• Prevents Violations of Expected Behavior: Protects against surprising behaviors caused by derived classes altering functionality unexpectedly.

• Extensibility: Adding a new payment method (e.g., DigitalWalletProcessor) requires implementing the same base class, ensuring consistency.

How to Implement SRP

Scenario: Payment System
You’re developing a system to process payments. Initially, there’s a base class PaymentProcessorand a derived class CreditCardProcessor. Later, a new requirement arises to handle cash payments.

Without LSP (Violation):

public class PaymentProcessor
{
    public virtual void ProcessPayment(decimal amount)
    {
        Console.WriteLine($"Processing payment of {amount:C}.");
    }
}

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

public class CashPaymentProcessor : PaymentProcessor
{
    public override void ProcessPayment(decimal amount)
    {
        if (amount > 1000)
        {
            throw new InvalidOperationException("Cash payments cannot exceed $1000.");
        }
        Console.WriteLine($"Processing cash payment of {amount:C}.");
    }
}

Here’s the issue:

• The base class PaymentProcessorassumes any amount can be processed, but the CashPaymentProcessoradds a restriction (amount <= 1000), violating the principle.

• If you use the CashPaymentProcessorwhere a PaymentProcessoris expected, the client might encounter unexpected exceptions.

With LSP (Solution):
Refactor the code to separate concerns and ensure substitutability. Use interfaces or base classes that clearly define the responsibilities of each payment type.

public abstract class PaymentProcessor
{
    public abstract void ProcessPayment(decimal amount);
}

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

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

Here’s how it works with LSP:

public void ProcessTransaction(PaymentProcessor paymentProcessor, decimal amount)
{
    paymentProcessor.ProcessPayment(amount);
}

• You can pass either CreditCardProcessor or CashPaymentProcessorto ProcessTransaction, and the behavior will always align with the expectations of the base class.

• The CashPaymentProcessor does not throw unexpected exceptions; it gracefully handles cases exceeding its limit.