This is one of the creational patterns. Factory is used to create an object from instance of another class.  In Factory pattern, we create object without exposing the creation logic. This helps to deal with large and more complex programs. In this pattern, an interface is used for creating an object, but let subclass decide which class to instantiate. Creation process is done in run time.

I’ll use example from typical video game where Enemy object is created, and its type is based on random number. Object is created in run time.

UML diagram:

First let’s create Interface that will be shared between all enemies that can be crated with Factory. This will be used by client.

public interface IFactory
{
  void Demage(int dmg);
}

Now we need to create Classes that will inherit this interface. In my example this will be enemyType1 and 2.

public class EnemyType1 : IFactory
{
  int _health=100;
  public void Demage(int dmg)
  {
     _health -= dmg;
     Console.WriteLine("EnemyType1 health:" + _health.ToString());
  }
}

public class EnemyType2 : IFactory
{
  int _health = 200;
  public void Demage(int dmg)
  {
     _health -= dmg;
     Console.WriteLine("EnemyType2 health:" + _health.ToString());
  }
}

Both classes also contains health count, just to make it more interesting.

Next we need to create the Interface for factory itself.

public abstract class EnemyFactory
{
  public abstract IFactory InstantiateEnemy(int type);
}

As a return type we use interface IFactory, this way we can return any object that inherits from it.

In The factory Class we override the method with switch statement that will return the objects based on the input number.

public class InstantiateEnemyFactory : EnemyFactory
{
  public override IFactory InstantiateEnemy(int type)
  {
     switch (type)
     {
        case 1:
            return new EnemyType1();
        case 2:
           return new EnemyType2();
         default:
           throw new ApplicationException(string.Format("Wrong number"));
     }
  }
}

Last step is to create client that will use the factory class.

class Program
{
  static void Main(string[] args)
  {
  //random number
  int type = new Random().Next(1, 1000)% 2 + 1;
  //Instance of the class
  EnemyFactory factory = new InstantiateEnemyFactory();

  IFactory newEnemy= factory.InstantiateEnemy(type);
  newEnemy.Demage(20);

  type = new Random().Next(1, 2000)%2 + 1;

  IFactory newEnemy2 = factory.InstantiateEnemy(type);
  newEnemy2.Demage(20);

  Console.ReadKey();

  }
}

Numbers don’t have to be random. I’m using random numbers to demonstrate the example. I real world we would rather create a list of enemies rather than choosing name for each of them.

This patters is very simple and effective. This makes it commonly used across all languages. To conclude, Factory defines an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. I hope you found it useful and you will implement this pattern to your next project. Stay Awesome!

Link to full source code