Factory Pattern

After reading this article, a reader recently requested that I show some sample code for the "factory" mentioned in the article.

Before I give you some sample code, however, let me just write a few words about the Factory Pattern: As the pattern's name suggests, the Factory Pattern creates stuff. Specifically, "factories" create classes without coupling you to a specific instance of the classes it creates.

Here is some sample code:

public class CreditCardBase
{
  //this is the base class that all credit cards implement:
  //it could be an abstract class, or it could have some functionality;
  //it could also be an interface... whatever floats your boat.
}

public static class CreditCardFactory
{
  public static CreditCardBase GetCreditCard(object someObject)
  {
       //business logic that returns correct credit cart type goes in here.
       if(someObject == "something")
            return new Visa(); //visa of course implements CreditCardBase
       else
            return new Amex(); //also implements CreditCardBase
  }
}

public class CreditClassClient
{
   //this class consumes a credit card
   public void DoWork()
   {
        CreditCardBase myCreditCard = CreditCardFactory.GetCreditCard(new bizObject());
        //do stuff with myCreditCard
        myCreditCard.Run();
   }
}

And there you have it. That's pretty much all there's to it, and the code is pretty self explanatory.

There's one more thing that's worth mentioning: usually, classes returned by factories have private constructors - to avoid people instantiating the class without calling the factory. This, however is problematic for several reasons:

1. You may have clients that already instantiate your class.
2. People usually make private constructors singletons (please don't).
3. How do you extend a class that has a private constructor? :)

Finally, to the anonymous reader that asked for the factory code: let me know if this was useful.