Factory Method

Description

A Factory is a class (or it can be just a method) that creates objects derived from a common super-class or interface. It is usually used in a situation where you don't want your client class to have to figure out what the specific class of an object is, because it could be of several different types. You create a super-class or interface and code the client to call methods of that super-class or interface. Now you have the problem of how to provide the different objects to the client without the client having to know which class is being created. The answer is to create a factory class that has a method that takes a parameter which it uses to figure out which type of object to return.

Example

Here is an example in VB.NET of a factory pattern: First the factory class. (Note: this could be implemented as a method in the client class rather than as a class.)

Public Class ProcessControllerFactory

    Public Shared Function GetController( process As String) As ProcessController
        Dim result As ProcessController

        Select Case process.ToUpper()
            Case "EXTRACT"
                result = New ExtractController()
            Case "IMPORT"
                result = New ImportController()
            Case "MATCHING"
                result = New MatchingController()
            Case "CONFIRMATION"
                result = New ConfirmationController()
            Case "DISTRIBUTION"
                result = New DistributionController()
            Case Else
                result = Nothing
        End Select

        Return result
    End Function

End Class

The client class gets the process controller it wants like this:

    Public Sub Execute( action as String)
        'declare the variable using the superclass as the type
        Dim processCtrl As ProcessController
        'now get the controller
        processCtrl = ProcessControllerFactory.GetController(action)
        'now call the method common to all classes derived from the superclass
        processCtrl.runProcesses()
        'other code here
    End Sub

If the factory were implemented as a method then "Execute" would look like this:

    Public Sub Execute( action as String)
        Dim processCtrl As ProcessController
        'calls its own method
        processCtrl = GetController(action)
        processCtrl.runProcesses()
        'other code here
    End Sub

The classes of the objects produced by the factory look like this:

Public Class ImportController
    Inherits ProcessController

    Public Overrides Sub runProcesses()
        'code goes here
    End Sub

End Class

And the super-class of all process controller classes looks like this:

Public MustInherit Class ProcessController

    Public Overridable Sub runProcesses()
        'default handling code goes here or it could be an abstract method
    End Sub

End Class

Resources

Data & Object Factory: Factory Method

Factory pattern example

The Factory Method Design Pattern by Gopalan Suresh Raj

Solve application issues with the factory pattern

Implementing the factory pattern using attributes and activation - C# Programming

The Simple Factory Pattern in C#