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I have the following simplified requirement of parsing an inventory file that contains the letter of the brand of car and the corresponding spec string on each line. For example:

A Sedan
B Blue

And below I provide a simplified version of the code:

 class StockManager
 
 List<ICar> cars = new List<ICar>();
 public StockManager(List<string> inventoryFileLines)
 
 foreach(var inventoryFileLine in inventoryFileLines)
 
 string parts = inventoryFileLine.Split(' ');
 cars.Add(CreateCar(parts[0], parts[1]));
 
 

 public decimal CalculateTotal()
 
 decimal total = 0;
 foreach(var car in cars)
 
 total += car.GetPrice();
 
 return total;
 

 public ICar CreateCar(string brand, string spec)
 
 if(brand == "A")
 
 return new CarA(spec);
 else if(brand == "B")
 
 return new CarB(spec);
 
 throw new Exception();
 
 


 interface ICar
 
 decimal GetPrice();
 

 class CarA : ICar
 
 string type;

 public CarA(string type)
 
 this.type = type;
 
 public decimal GetPrice()
 
 if(type == "Sedan")
 
 return 30000;
 
 else if (type == "SUV")
 
 return 50000;
 
 throw new Exception();
 
 

 class CarB : ICar
 
 string color;

 public CarB(string color)
 
 this.color = color;
 
 public decimal GetPrice()
 
 if (color == "Orange")
 
 return 20000;
 else if (color == "Red")
 
 return 25000;
 
 throw new Exception();
 
 

In the future, new brands and specs might be added. This is the change that I should anticipate and provide flexibility for.
Now I want to apply the right design patterns but apply them for the right reasons, not just for the sake of having applied a design pattern. (As stated by GoF: “A design pattern should only be applied when the flexibility it affords is actually needed.”)

The first thing that came to my mind is the factory method or abstract factory pattern. So when there is a new car brand C is added in the future:

Factory Method

Make CreateCar virtual and override it in the new StockManager class that I will be using:

class StockManager2 : StockManager

 public StockManager2(List<string> inventoryFileLines) : base(inventoryFileLines) 
 public override ICar CreateCar(string brand, string spec)
 
 if (brand == "C")
 
 ...
 
 return base.CreateCar(brand, spec);
 

Abstract Factory

Make CreateCar method into its own abstract factory class and provide it to the StockManager class.

Both these refactorings look great if I want to use different alternative creation options at run-time, such as multiple valid CreateCar factories. And the Maze example is given by GoF also expands on this idea.

But as a matter of fact, the change I anticipate is not an alternative factory but a modified factory. So it seems much more logical to me to modify the CreateCar method instead of creating a new factory class and leaving the old one obsolete (speaking for the Abstract Factory method here). And the same holds true for creating a second StockManager2 class in the case of Factory method.
I know that Open/Closed principle (O of SOLID by Robert Martin) says not to modify the class but extend it, and the factory pattern does exactly that but does the above example justify its use, given the extensibility requirement I mentioned in the beginning? It seems like the requirement is not an extension in the sense explained in GoF but a true modification instead. But I would like to be corrected if I am wrong.

You can use the runtime object creation mechanism. So the c# will be created the class based on the string. The only expectation the class name and the given string must be the same.

But you can use a static dictionary if the values are not the same.
Here is the code:

public ICar CreateCar(string brand, string spec)

 System.Type type = typeof( ICar ).Assembly.GetTypes().Where( t => t.Name == brand ).FirstOrDefault();

 object instance = Activator.CreateInstance( type, new object[ 1 ] specs );
 return (ICar)instance;

of course this function does not handle any error, but it is the easy part.
By the way, in your code please use the NotImplementedException instead of Exception base class, because this is what you want :) implement the new brands.