Design Principals of Object Oriented Programming

Question 1

What is the shorthand for the Design Principals of OOP?

Answer 1

SOLID

Question 2

What is the purpose of SOLID?

Answer 2

To make OOP code more understandable, flexible, and maintainable

Question 3

What does SOLID stand for?

Answer 3

• Single Responsibility
• Open/Closed
• Liskov Substitution
• Interface Segregation
• Dependency Inversion

Question 4

What is the Single Responsibility principle?

Answer 4

Every class should have only one responsibility

Question 5

What is the main benefit of the Single Responsibility principle?

Answer 5

Code is modular and focussed

Question 6

Can you describe an example of Single Responsibility principle in C#

Answer 6

Customer class with customer details and calculateDiscount() method.

The method is not performed by Customer class so should be put into it’s own class Discount

Question 7

What is the Open/Closed principle?

Answer 7

A class should be open for extension but closed for modification

Question 8

What is the main benefit of the Open/Closed principle?

Answer 8

Less impact on changes made in the code

Question 9

Can you describe an example of Open/Closed principle in C#

Answer 9

We now have a class called Discount with out **calculateDiscount() **method. We may have different types of customers, so make **abstract **class with virtual function

Question 10

What is the Liskov Substitution principal?

Answer 10

Every child class should be able to replace it’s parent class easily

Question 11

What is the main benefit of the Liskov Subsitution principle?

Answer 11

Helps us create good inheritance hierarchies

Question 12

Can you describe an example of Liskov Substitution principle in C#

Answer 12

What if we now have a Referral Customer? They aren’t actually a customer yet so won’t have a discount! So calculateDiscount() function does nothing.

Solution - create **CustomerBase **class without **calculateDiscount() **method and it in derived class.

Question 13

What is the Interface Segregation principal?

Answer 13

No code should depend on methods it does not use

Question 14

What is the main benefit of the Interface Segregation principle?

Answer 14

Looser coupling

Question 15

Can you describe an example of Interface Segregation principle in C#

Answer 15

A Data Access Layer has CRUD methods defined and SQLServer, Oracle implementations.

However, one our implementations is just for a Reporting System so just needs Read method.

So we create interfaces IRead(Read method) and IDal : IRead(Add method, Update method, Delete method)

Question 16

What is the Dependency Inversion principal?

Answer 16

Higher level modules should not depend on lower level modules directly but through abstraction

Question 17

What is the main benefit of the Dependency Inversion principle?

Answer 17

Helps to decouple

Great for testing

Question 18

Can you describe an example of Dependency Inversion principle in C#

Answer 18

Back to our Customer example. If we need to change logic in the Discount class we have to use IF statements. We should instead have an IDiscount interface with different implementations. We can use interface instead.

Question 19

Does Dependency Inversion solve making code loosely coupled?

Answer 19

No, as you still need to create instances of the objects

Question 20

What Design Patterns in C# can we use to decouple code?

Answer 20

• Factory
• Dependency Injection

See upcoming cards on Design Patterns