SOLID : SRP + ISP

Question

I am trying to understand SOLID principles. I think I understand well the Liskov Substitution Principle, but I have some problems for other ones.

One example. I have two interfaces : Engine and car stereo. The problem is : For a car, we have 6 glasses.

So, my car implements interfaces Engine and Car Stereo.

But for the 6 glasses, should I implement them, or should I put them on an array of glasses knowing there are 4 laterals glasses which are able to be up or down and 2 windshields (glasses inherited by both).

The problem with the first one is, I can't implement 4 times the same glasses.

So the second one seems to me good but I am afraid that break SRP, I am not sure to understand what "Responsibilities" exactly are.

Answer 1

The correct design practise is to

code to interface rather than class.

In light of this principle I'd recommend to use a interfaces like IWindow, ISurface, IWindShield with a hierarchy like below

interface ISurface
{
  //surface specific properties which ideally should be DI-ed
  public SurfaceType SurfaceType {get; set;}
  public decimal Opacity {get;set;}
  public decimal Thickness {get; set;}
}

and

interface IWindow:ISurface
{
  //Window behavior specific properties and methods
  public void lowerWindow();
  public WindowHeight WindowLevel(){get;set;}
  public void shutWindow();
  // ...and more such window specific behavior
}

and

interface IWindShield:ISurface
{
  //WindShield behavior specific properties and methods
  public bool IsFogged(){get;set;}
  public bool IsClean(){get;set;}
  // ...and more such window specific behavior
}

Finally when assembling a car with all its functionality (presumably using a Builder pattern), you can have an array in the Car class of type ISurface type like below

class Car
{
   string carName;
   Engine carEngine;
   Stereo carStereo;
   List<ISurface> carGlasses;
   .....
   // other Car parts
   // this is a typical builder pattern example
}

this is build using a CarBuilder type class say MyCarBuilder where

interface CarBuilder
{
  public Car MyCar {get; set;}
  void AddEngine();
  void AddStereo();
  void AddGlasses();// this is what works on adding ISurface List items
  // and other car building methods
}

and the real builder class

class MyCarBuilder:CarBuilder
{
    //... implements all methods of building your custom car
}