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Factory Pattern

The Factory Design Pattern is one of the most used design pattern in modern programming languages. You will find references as "Factory Method" and "Abstract Factory" in the well-known "Gang of Four" patterns that describe how to solve recurring design problems.

These two variations aims towards the same goal: avoid direct object construction.

Factory Method​

Factory method UML Diagram

Abstract Factory​

Abstract Factory UML Diagram

Why?​

Avoiding direct object construction allows us to abstract the decision-making process from the calling class. The benefits are numerous:

  • The calling class do not need to know how to construct a component. The factory holds this information.
  • Open/Closed principle.
  • Segregation of concerns.
  • Reusability.
  • Reduces coupling.
  • Reduces duplication.

Problems​

How to avoid direct object construction?

Examples​

Considering an IShape interface with several implementations such as Circle, Rectangle and Square.

public interface IShape
{
    void Draw();
}

public class Circle : IShape
{
    ...
}

public class Rectangle : IShape
{
    ...
}

public class Square : IShape
{
    ...
}

Without a factory​

  • The code contains strong coupling to each implementation.
  • Adding/altering behaviors will require to change the calling class.
public static void main(string[] args)
{
    IShape circle = new Circle();
    circle.Draw();
    IShape rectangle = new Rectangle();
    rectangle.Draw();
    IShape square = new Square();
    square.Draw();
}

Applying the Factory pattern​

  • The code does not have a direct coupling to any IShape implementation, thanks to Abstraction.
  • Adding/altering behaviors will only require to change the factory. Our code is open for extension but closed for modification (OCP).
  • The factory is reusable, reducing duplication in the codebase.
public class ShapeFactory
{
    public IShape Create(string type)
        => switch (type)
            {
                case "CIRCLE":
                    return new Circle();
                case "RECTANGLE":
                    return new Rectangle();
                case "SQUARE":
                    return new Square();
                default:
                    return null;
            };
}

public static void main(string[] args)
{
    ShapeFactory factory = new ShapeFactory();
    IShape circle = factory.Create("CIRCLE");
    circle.Draw();
    IShape rectangle = factory.Create("RECTANGLE");
    rectangle.Draw();
    IShape square = factory.Create("SQUARE");
    square.Draw();
}

Online resources​

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