The document discusses the SOLID principles of object-oriented design, which are a set of five design principles aimed at making software designs more understandable, flexible and maintainable. It defines each principle - Single Responsibility Principle (SRP), Open-Closed Principle (OCP), Liskov Substitution Principle (LSP), Interface Segregation Principle (ISP), and Dependency Inversion Principle (DIP). For each principle, it provides an example of how to apply the principle correctly and how to avoid violating it. The document emphasizes that following these principles helps produce code that is easier to maintain and extend over time.

1. SOLID Principles
Every developer
should know
TOAN NGUYEN
TO A N - H U U . N G U Y E N @ KO L E K TO . N L
NGUYENTOANUIT

2. Agenda
What are SOLID?
Why should we care?
How can we apply SOLID principles?
◦ S – Single responsibility principle
◦ O – Open–closed principle
◦ L – Liskov substitution principle
◦ I – Interface segregation principle
◦ D – Dependency inversion principle
SOLID - TOAN NGUYEN

3. About me
Nguyen Huu Toan
Senior Dev – skype nguyentoanuit@gmail.com
SWGLayer team
https://app.kolekto.nl
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4. What is SOLID?
“It is not enough for code to work.”
- Robert C. Martin
SOLID is five basic principles which help us write clean code
◦ Understandable
◦ Easier to maintain
◦ Easier to extend
The abbreviation SOLID was introduced by Michael Feathers
But the concept of SOLID principles were first introduced by Robert C. Martin
in his book, Design Principles and Design Patterns.
So let see how SOLID can help us be stronger
SOLID - TOAN NGUYEN

5. Why should
we care?
BUT WAIT A MINUTE!!!
WHY SHOULD WE NEED
TO APPLY IT?
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6. Why should we care?
Can we just make it work without following these principles?
Finish the business rules/PBI and delivery functionality to
client are our main task, why do we need to care about these
annoying rules/principles?
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7. Why should we care?
Yes, of-course.
BUT..... the functionality that you create today has a good
chance of changing in the next release.
The more you care about good code today, the more you are
loved by your teammate later
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8. Why should we care?
So I will make it work first and refactor later on?
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9. Why should we care?
Yes, sure! But it will be very expensive.
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10. Why should we care?
“People will forget how fast you did a job, but they remember
how well you did it!”
- iDev
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11. How can we
apply SOLID
principles?
S – Single responsibility principle (SRP)
O – Open–closed principle (OCP)
L – Liskov substitution principle (LSP)
I – Interface segregation principle (ISP)
D – Dependency inversion principle (DIP)
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12. S – Single responsibility principle
“A class should only have a single responsibility”
So that, a class should have one and only one reason to change.
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13. S – Single responsibility principle
public class Production
{
public Guid Id { get; set; }
public string Title { get; set; }
public string ShortDescription { get; set; }
public string ImageUrl { get; set; }
public int RemainItems { get; set; }
public string CheckStatus()
{
if (RemainItems > 500)
{
return "There are a lot of remaining items";
}
else if (RemainItems <= 500 && RemainItems > 200)
{
return "Warning: We should buy more";
}
else
{
return "Red warning: We need by more";
}
}
}
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Inventory
business

14. S – Single responsibility principle
public void SendEmail(EmailData emailData)
{
var email = new
{
from = emailData.From,
to = emailData.To,
message = emailData.Message
};
graph.sendEmail(email);
}
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15. S – Single responsibility principle
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public void SendEmail(EmailData emailData, bool generateAttachment)
{
var email = new GraphEmail
{
from = emailData.From,
to = emailData.To,
message = emailData.Message
};
if (generateAttachment)
{
var document = new
{
title = emailData.AttachmentName,
content = pdfGenerator.GeneratePDF(emailData.Message)
};
email.attachment = document;
}
graph.sendEmail(email);
}
Generate attachment is not
a job of send email function

16. Avoid violate SRP
1. Long method
2. Boolean flag
3. Class (service) serve many operations
4. Always ask why is the responsibility of class/service/module when extending
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17. Benefits of SRP
1. Easy to follow/understand
2. Maintainable
3. Test easily
4. Easy to extend
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18. Open–closed principle
Entities should be open for extension, but closed for modification.
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19. Open–closed principle
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20. Open–closed principle
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21. Open–closed principle
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User form

22. Open–closed principle
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Is it fully follow OCP?

23. Open–closed principle
As application grow up, a lot of new behaviors of the form will be added such as
◦ Handle dirty form
◦ Confirmed/warning message
◦ User only can view data, can’t make a change in the form
◦ Show overlay when data is loading
◦ more and more …..
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24. Open–closed principle
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25. Open–closed principle
Composition instead of inheritance
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<form>
<FormContext.Provider value={setupContextForm()}>
<Header />
<DirtyCheck />
<BaseWarningMessage />
<ConfirmMessage />
<Messages />
<BaseValidation />
<UserFormFields />
<FormButtons />
....
</FormContext.Provider>
</form>

26. Benefits of OCP
1. Reusable and maintainable
2. Avoid regression bugs
3. Extend easily and quickly
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27. Avoid violate OCP
1. Using inheritance
2. Using Composition
3. Using interface
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28. Liskov substitution principle
Let Φ(x) be a property provable about objects x of type T. Then Φ(y)should be true for
objects y of type S where S is a subtype of T.
Developer language
"Objects in a program should be replaceable with instances of their subtypes without
altering the correctness of that program.“
Related to???
Open – close principle
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29. Liskov substitution principle
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public class Airplane
{
public string Name { get; set; }
public virtual void TakeOff()
{
Console.WriteLine("Hello madam and gentleman, we are taking off");
}
public virtual void Landdown()
{
Console.WriteLine("Hello madam and gentleman, we are landing");
}
public virtual void Up()
{
Console.WriteLine("Air plane go up");
}
public virtual void Down()
{
Console.WriteLine("Air plane go down");
}
}
public class TraditionalAirPlane : Airplane
{ public TraditionalAirPlane()
{
Name = “Traditional one";
}
}

30. Liskov substitution principle
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static void Main(string[] args)
{
var airPlane = new BoeingB52();
var pilot = new Pilot(airPlane);
pilot.HandleAirePlane();
}

31. Liskov substitution principle
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public class SmartAirplane : Airplane
{
public bool AirPlaneUp { get; set; }
public bool OffAutoDetection { get; set; }
public SmartAirplane()
{
Name = "I am smart air plane" ;
}
public override void Up()
{
if (AirPlaneUp && !OffAutoDetection)
{
throw new System. Exception ("No worry, I will control it");
}
base.Up();
}
public void TurnOffAutoDetect()
{
OffAutoDetection = true;
}
}

32. Liskov substitution principle
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static void Main(string[] args)
{
var airPlane = new SmartAirePlane ();
var pilot = new Pilot(airPlane);
airPlane.AirPlaneUp = true;
pilot.HandleAirePlane();
}

33. Benefits of LSP
Avoid unexpected behavior of software
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34. Avoid violate LSP
1. No easy way to detect the violation
2. Review code
3. Shouldn’t implement stricter validation than implementation of superclass
4. Shouldn’t break superclass behavior
5. Should run test case (unit test) for all subclass instances
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35. Interface segregation principle
“Many client-specific interfaces are better than one general-purpose interface.”
Similar??
SRP
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36. Interface segregation principle
public interface IEmailService
{
void SendEmail(EmailData emailData);
}
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public class GraphEmailService : IEmailService
{
dynamic graph = new { };
public void SendEmail(EmailData emailData)
{
var email = new
{
from = emailData.From,
to = emailData.To,
message = emailData.Message
};
graph.sendEmail(email);
}
}
public class SendGridEmailService : IEmailService
{
dynamic sendGrid = new { };
public void SendEmail(EmailData emailData)
{
var email = new
{
from = emailData.From,
to = emailData.To,
message = emailData.Message
};
sendGrid.sendEmail(email);
}
}

37. Interface segregation principle
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38. Interface segregation principle
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39. Benefits of ISP
Similar Single responsibility
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40. Avoid violate ISP
1. Large interface (flat interface)
2. throw new NotImplementedException();
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41. Dependency inversion principle
1. High-level modules should not depend on low-level modules. Both should depend on
abstractions.
2. Abstractions should not depend on details. Details should depend on abstractions.
Related to???
OCP & LSP
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42. Benefits of DIP
Decoupling between
1. Abstractions and details
2. High level and low level
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43. Distinguish between DIP, IoC and DI
1. DIP: Dependency inversion principle is about decoupling
2. IoC: Inversion of control is a design principle used by framework libraries that allow the
framework to regain and instance of class during program execution. Or could be referred by
"Hollywood Principle: Don't call us, we'll call you"
3. DI: Dependency Injection is an implementation of IoC. Another implementation of IoC is
service locator
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44. Q&A
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45. Thanks you!
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