Factory Patterns

• The job of the Factory design pattern is to create concrete sub classes. You can see the Factory design pattern used throughout the .NET Framework.
• The essence of the Factory Pattern is to "Define an interface for creating an object, but let the subclasses decide which class to instantiate. 
• The Factory method lets a class defer instantiation to subclasses." Factory methods encapsulate the creation of objects. 
• This can be useful if the creation process is very complex, for example if it depends on settings in configuration files or on user input.

A C# example of the Factory Pattern

// A Simple Interface

public interface IVehicle

{

void Drive(int miles);

}

// The Vehicle Factory

public class VehicleFactory

{

public static IVehicle getVehicle(string Vehicle)

{

switch (Vehicle) {

case "Car":

return new Car();

case "Lorry":

return new Lorry();

default:

throw new ApplicationException(string.Format("Vehicle '{0}' cannot be created", Vehicle));

break;

}

}

}

// A Car Class that Implements the IVehicle Interface

public class Car : IVehicle

{

public void IVehicle.Drive(int miles)

{

// Drive the Car

}

}

// A Lorry Class that Implements the IVehicle Interface

public class Lorry : IVehicle

{

public void IVehicle.Drive(int miles)

{

// Drive the Lorry

}

}