SOLID principles

Question 1

S — Single Responsibility Principle

Answer 1

• A class should have only one reason to change.
• Analogy: hire accountant, cleaner, developer instead of one person who does all of that
• Prevents: “Everything breaks when one thing changes.”
• A class should only be responsible for validation, not validation, data access, and email sending

Question 1

What Is SOLID

Answer 1

5 design principles that make code easier to change, test, extend

• S - Single Responsibility Principle (SRP)
• O - Open/Closed Principle (OCP)
• L - Liskov Principle (LSP)
• I - Interface Segregation Principle (ISP)
• D - Dependency Inversion Princinple (DIP)

Question 2

O — Open/Closed Principle (OCP)

Answer 2

• Code should be open for extension, closed for modification
• Analogy: don’t open phone to add features, instead use cases, attachments, snap-on lenses etc
• Prevents: “We keep modifying the same fragile file.”
• DiscountCalculator -> Calculate()
• vs.
• IDiscountStrategy -> RegularDiscount : IDiscountStrategy -> GetDiscount()

Question 3

L — Liskov Substitution Principle

Answer 3

• Subtypes must be substitutable for their base types
• Analogy: if renting a car, instead I get a scooter
• Meaning: even it’s a vehicle, it doesn’t behave correctly
• Prevents: “Swapping implementations crashes production.”
• Example: swapping any PaymentProcessor should work for any IPaymentProcessor implementation

Question 4

I — Interface Segregation Principle (ISP)

Answer 4

• Don’t force classes to implement methods they don’t use
• If you see NotImplementedException, something’s wrong.
• Analogy: restaurant where every customer has to order steak, sushi, dessert, wine even if they’re vegetarian
• Prevents: “Why does this class implement 12 methods it doesn’t need?”

Question 5

D — Dependency Inversion Principle

Answer 5

• Depend on abstractions, not concrete implementations
• If you see this inside business logic: “new SomeConcreteClass()” -> you’re probably breaking DIP.
• Analogy: an appliance that connects directly to power plants is wrong. An appliance that connects to a wall outlet is correct. Allows the power plant to be able to change
• Prevents: “We can’t test this without hitting the database.”
• Example: call IPaymentProcessor instead of PayPalProcessor

Question 6

Abstract vs. Interface

Answer 6

Use Abstract when:
- you need base type inheritance
- Is a true “is-a” relationship
- CreditCardProcessor IS A PaymentProcessor
- PayPalProcessor IS A PaymentProcessor

Use Interface when:
- you need interchangeable implementations
- you need to define a contract

public class OrderService
{
private StripeProcessor _processor;
}

vs

public class OrderService
{
private readonly IPaymentProcessor _processor;
}

Question 7

Why SOLID is important for coding?

Answer 7

• More maintanable
• Separation of concerns and loose coupling
• Easier to add new features, change implementations, and test without affecting other parts of system

Question 8

How have you implemented SOLID at current position?

Answer 8

• We intentionally don’t use direct SQL queries in the UI layer
• Instead the UI layer calls a business logic layer
• Business logic layer calls data logic layer