Dependency Inversion Principle(DIP)

Dependency Inversion Principle(DIP)

What Is Dependency Inversion Principle(DIP)?

In object-oriented design, the dependency inversion principle(DIP) refers to a specific form of decoupling software modules.
Dependency Inversion Principle or DIP has two key points:
1.Abstractions should not depend upon details;
2.Details should depend upon abstractions.
The principle could be rephrased as use the same level of abstraction at a given level. Interfaces should depend on other interfaces. Don’t add concrete classes to method signatures of an interface. However, use interfaces in your class methods.

Simple Example Of Dependency Inversion Principle

Consider the case of the Button object and the Door object.
The Button object senses the external environment. On receiving the Poll message, the Button object determines whether a user has “pressed” it. It doesn’t matter what the sensing mechanism is.
It could be a button icon on a GUI, a physical button being pressed by a human finger, or even a motion detector in a home security system. The Button object detects that a user has either activated or deactivated it.
The Door object affects the external environment. On receiving a TurnOn message, the Door object opens the door. On receiving a Close message, it closes that door.
How can we design a system such that the Button object controls the Door object?  The Button object receives Poll messages, determines whether the button has been pressed, and then simply sends the Open or Close message to the Door.

Consider the C# code implied by this model (above). Note that the Button class depends directly on the Door class. This dependency implies that Button will be affected by changes to Door. Moreover, it will not be possible to reuse Button to control a Motor object. In this model, Button objects control Door objects and only Door objects.


 public class Button  
  {  
   private Door door;  
   public void Poll()  
   {  
    if (/*some condition*/)  
     door.Open();  
   }  
  }  
The above solution violates DIP.The abstractions have not been separated from the details.Without such a separation, the high-level policy automatically depends on the low-level modules, and the abstractions automatically depend on the details.

Finding the Underlying Abstraction

What is the high-level policy? It is the abstraction that underlies the application, the truths that do not vary when the details are changed. It is the system inside the systemit is the metaphor. In the
Button/Door example, the underlying abstraction is to detect an open/close gesture from a user and relay that gesture to a target object.
What mechanism is used to detect the user gesture? Irrelevant! What is the target object? Irrelevant!
These are details that do not impact the abstraction.
The model in above can be improved by inverting the dependency upon the Lamp object. In model below, we see that the Button now holds an association to something called a ButtonServer,which provides the interfaces that Button can use to turn something on or off. Door implements the ButtonServer interface.
Thus, Door is now doing the depending rather than being depended on.
The design above allows a Button to control any device that is willing to implement the ButtonServer interface. This gives us a great deal of flexibility. It also means that Button objects will be able to control objects that have not yet been invented.
However, this solution also puts a constraint on any object that needs to be controlled by a Button.
Such an object must implement the ButtonServer interface. This is unfortunate, because these objects may also want to be controlled by a Switch object or some kind of object other than a Button.
By inverting the direction of the dependency and making the Door do the depending instead of being depended on, we have made Door depend on a different detail: Button. Or have we?
Door certainly depends on ButtonServer, but ButtonServer does not depend on Button. Any kind of object that knows how to manipulate the ButtonServer interface will be able to control a Door .
Thus,the dependency is in name only. And we can fix that by changing the name of ButtonServer to something a bit more generic, such as SwitchableDevice. We can also ensure that Button and SwitchableDevice are kept in separate libraries, so that the use of SwitchableDevice does not imply the use of Button.
In this case, nobody owns the interface. We have the interesting situation whereby the interface can be used by lots of different clients, and implemented by lots of different servers. Thus, the interface needs to stand alone without belonging to either group. In C#, we would put it in a separate namespace and library.
Further Reading

1.Agile Software Development, Principles, Patterns, and Practices By Robert C.Martin

2. Head First Design Patterns by Freeman, Eric; Freeman, Elisabeth; Kathy, Sierra; Bert, Bates

3.Object Solutions: Managing the Object-Oriented Project by Grady Booch,

Liskov Substitution Principle

If you’ve been following my blog,you’ll know that we have been covering the SOLID Principles,in my previous posts i covered The Single Responsibility Principle,the “S” in the SOLID Acronym,then we covered The Open Closed Principle,today  my focus will be on The Liskov Substitution Principle,the “L” in the SOLID Principles acronym.

What is LSP?

This principle is credited to Barbara Liskov,she wrote this principle in 1988 and she said:

“What is wanted here is something like the following substitution property: If for each object o1
of type S there is an object o2 of type T such that for all programs P defined in terms of T, the
behaviour of P is unchanged when o1 is substituted for o2 then S is a sub-type of T.”

In simple terms the above statement means:

“Subtypes must be substitutable for their base types.”

or more simply:

“It means that we must make sure that new derived classes are extending the base classes without changing their behaviour” 

A Simple Example Using C#

We’ll use the classic Rectangle-Square problem to demonstrate this principle. Let’s imagine that we need to find the area of any rectangle.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;


namespace SOLIDPrinciplesDemo
{
  // If any module is using a Base class then the reference to that Base class can be replaced with a Derived class without affecting the functionality of the module.
  // Or
  // While implementing derived classes, one needs to ensure that, derived classes just extend the functionality of base classes without replacing the functionality of base classes.

class Rectangle
{
 protected int mWidth = 0 ;
 protected int mHeight = 0;

public virtual void SetWidth(int width)
{
  mWidth = width;
}

public virtual void SetHeight(int height)
{
  mHeight = height;
}


public virtual int GetArea()
{
  return mWidth * mHeight;
}
}


  // While implementing derived class if one replaces the functionality of base class then,
  // it might result into undesired side effects when such derived classes are used in existing program modules.
class Square : Rectangle
{
  // This class modifies the base class functionality instead of extending the base class functionality
  // Now below methods implementation will impact base class functionality.
public override void SetWidth(int width)
{
  mWidth = width;
  mHeight = width;
}

public override void SetHeight(int height)
{
  mWidth = height;
  mHeight = height;
}

}


class LiskovSubstitutionPrincipleDemo
{

private static Rectangle CreateInstance()
{
  // As per Liskov Substitution Principle "Derived types must be completely substitutable for their base types".
  bool SomeCondition = false;

if (SomeCondition == true)
{
  return new Rectangle();
}
else
{
  return new Square();
}
}

public static void LSPDemo()
{
  Console.WriteLine("\n\nLiskov Substitution Principle Demo ");
  Rectangle RectangleObject = CreateInstance();

  // User assumes that RectangleObject is a rectangle and (s)he is able to set the width and height as for the base class
  RectangleObject.SetWidth(5);
  RectangleObject.SetHeight(10);
  // Now this results into the area 100 (10 * 10 ) instead of 50 (10 * 5).
  Console.WriteLine("Liskov Substitution Principle has been violated and returned wrong result : " + RectangleObject.GetArea());
  // So once again I repeat that sub classes should extend the functionality, sub classes functionality should not impact base class functionality.
}
}
}

I know the example above doesn’t exhaust the LSP principle,but im sure it gave you an idea what the principle stands for.

Benefits

This principle aims to keep functionality intact. The main purpose is to guarantee that objects lower in a relational hierarchy can be treated as though they are objects higher in the hierarchy. Basically, any child class should be able to do anything the parent can do.

Further Reading

1.Data Abstraction and Hierarchy,” Barbara Liskov, SIGPLAN Notices, 23(5) (May 1988)

2.Bertrand Meyer, Object-Oriented Software Construction, 2d. ed., Prentice Hall, 1997

3.Rebecca Wirfs-Brock et al. , Designing Object-Oriented Software, Prentice Hall,
1990.

4.Agile.Principles.Patterns.and Practices.In.C#[Robert.C.Martin]

That’s it for this week,next i will be covering Interface Segregation Principle(ISP),the letter “I” in the SOLID acronym.

****Happy Holidays and be safe****