C# course

Lecture 5.1

Object-oriented programming in C# part 2

OOP principles

  • Coupling/Cohesion
  • Abstration
  • S.O.L.I.D.


"Abstraction - is the process of separating ideas from specific instances of those ideas" (Wikipedia)

Key ideas of abstraction:

  • ignore irrelevant features, properties and methods
  • emphasizing on really important features
  • keep only those details about an object that are relevant to the current perspective

Abstraction goals

  • simplify complex representation and expose only important properties of an object
  • build simplified model of an object
  • hide unimportant properties of an object

Abstraction in C#

Abstraction in C# might be achieved in several ways:

  • with Interfaces
  • with abstract classes
  • with inheritance

Coupling and Cohesion

Coupling and Cohesion are characteristic of code. How tightly code is written.


is a measure of interdependence between software modules


is degree to which the elements of a module belong together

Coupling is usually contrasted with cohesion e.g. low coupling and high cohesion


Cohesion describes how closely all the methods in a class support a central purpose of the class

Good module should have high (or strong) cohesion

Cohesion is high (or strong) if:

  • methods carry out a small number of related activities
  • classes aimed for single purpose
  • advantages of high cohesion:
  • reduced module complexity
  • increased system maintainability
  • increased module reusability

High cohesion demo
Low cohesion demo


Coupling describes how tightly a classes or modules are related to other classes or modules

Good class or method should have low (loose) coupling:

  • modules should have a very little dependency on other modules
  • a module must be easily re-used by other modules (avoid direct dependencies among modules)
Loose coupling demo
Tight cohesion demo


S.O.L.I.D. - is an abbreviation of 5 principles of object oriented design

  • S - Single responsibility principle
  • O - Openness principle
  • L - Liskov substitution principle
  • I - Interface segregation principle
  • D - Dependency inversion principle