The document discusses the Single Responsibility Principle (SRP) as one of the five SOLID principles for object-oriented programming, emphasizing that a class should have only one responsibility and reason to change. It highlights the advantages of adhering to SRP, such as ease of maintenance and testing, while also noting potential downsides like increased class count and complexity in understanding the overall system. The document includes code samples and a summary of key points related to SRP and its implications in software design.

1. The Single Responsibility Principle
Lars-Erik Kindblad
Senior Consultant
Blog: kindblad.com

2. Agenda
What is the Single Responsibility Principle?
Code Sample
Summary

3. Single Responsibility Principle
Single Responsibility Principle = SRP
1 of the 5 SOLID principles for creating better object-oriented code
 The Single Responsibility Principle
 The Open Closed Principle
 The Liskov Substitution Principle
 The Interface Segregation Principle
 The Dependency Inversion Principle
SRP states that ”a class or module should have one, and only one, reason
to change”
 A class should do one thing
 A class should have only one responsibility
The benefit: smaller classes that are easier to read, maintain and unit-test

4. The God Object
A God object is an object that knows too much or does too much
The opposite of the SRP

5. Single Responsibility Objects

6. CODE SAMPLE

7. Database code
Send mail code

8. 1. Extract mail sending code

9. 2. Extract database code

10. Repositories are potential God-objects

11. 3. Replace Repository.GetUser with a DbCommand

12. 4. Replace Repository.AddUser with a DbCommand

13. 5. Replace Repository.DeleteUser with a DbCommand

14. UserManager looks like
a facade class
... Extract

15. 6. Extract RegisterUser

16. 7. Rename UserManager
... to UserFacade

17. Not every dependency
are needed at all times

18. 8. Change to a Service Locator

19. 9. Validation
Could be extracted but not
needed now since the class
is small...

20. Overview
AddUserDbCommand
DeleteUserDbCommand
GetUserDbCommand
SendEmailConfirmationRegisterUser
UserFacade
Remove?

21. Summary
A class following the single responsibility principle is a class that does only
one thing and has only one reason to change
The opposite of SRP is a God-object
Benefits
 Easy to give the class a good name
 Less code per class means reduced complexity, less errors, easier to maintain, extend
and test
Arguments against
 Too many classes
 Difficult to understand the big picture
The pros outweights the cons – SRP is a requirement for
writing maintainable unit-tests

22. QUESTIONS?

23. The information contained in this presentation is proprietary.
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