• Class constructors exist so that clients can create an instance of a class.
  • There are situations where the client does not, or should not, know which one of several candidate classes to instantiate.
• The Factory Method defines an interface for creating an object (IFactory), but lets subclasses decide which class to instantiate.
  • It allows the client to use an interface for creating an object while still retaining control over which class to instantiate.
• The key objective of the Factory Method is extensibility.
• Factory Methods are frequently used in “manager” type components, such as, document managers, account managers, permission managers.
• Not all methods return “new object” are Factory methods. The Factory rules are:
  1. The method creates a new object.
  2. The method returns an abstract class or interface.
  3. The abstract class or interface is implemented by several classes.
• In .NET, the factory method is typically implemented as a static method which creates an instance of a particular type determined at compile time.
• System.Convert class is one example of factory method which exposes many static methods that, given an instance of a type, returns another new type.
  • These methods don’t return base classes or interface types of which the true type is only known at runtime.
• This is exactly where Abstract Factory and Factory Method differ:
  • "Factory Methods" are abstract and return class types.
  • "Abstract Factory" methods are virtual or abstract and return abstract classes or interfaces.

Participants

• Product (IAuto)
  • Defines the interface of objects the factory method creates.
• ConcreteProduct (Bmw)
  • Implements the Product interface.
• Creator (IAutoFactory)
  • It declares the factory method, which returns an object of type Product.
  • It may also define a default implementation of the factory method that returns a default ConcreteProduct.
  • It may call the factory method to create a Product object.
• ConcreteCreator (BmwFactory)
  • It overrides the factory method to return an instance of a ConcreteProduct.

IAutoFactory autoFactory = new BmwFactory();
IAuto auto = autoFactory.GetInstance();
    
interface IAuto
{
    string GetName();
}

class Bmw : IAuto
{
    public string GetName() => "Bmw";
}

interface IAutoFactory
{
    IAuto GetInstance();
}

class BmwFactory : IAutoFactory
{
    public IAuto GetInstance() => new Bmw();
}

Rules of Thumb

• Factory Methods are usually called within Template Methods.
• Designs start using Factory Method (less complicated, more customizable) and evolve toward Abstract Factory, Prototype, or Builder (more flexible and complex).
• The advantage of a Factory Method is that it can return the same instance multiple times, or can return a subclass rather than an object of that exact type.
• Some Factory Method advocates recommend that absolutely all constructors should be private or protected.
• The new operator considered harmful.
  • There is a difference between requesting an object and creating one.
  • The new operator always creates an object, and fails to encapsulate object creation.
  • A Factory Method enforces that encapsulation, and allows an object to be requested without inextricable coupling to the act of creation.