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Factory

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20 Oct 2013

FactoryThe job of the Factory design pattern is to create concrete sub classes. You can see the Factory design pattern used throughout the .NET

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Introduction

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# and VB.NET 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
    }
}

' A Simple Interface
Public Interface IVehicle
    Sub Drive(ByVal miles As Integer)
End Interface

' The Vehicle Factory
Public Class VehicleFactory
    Public Shared Function getVehicle(ByVal Vehicle As String) As IVehicle
        Select Case Vehicle
            Case "Car"
                Return New Car

            Case "Lorry"
                Return New Lorry

            Case Else
                Throw New ApplicationException(String.Format("Vehicle '{0}' cannot be created", Vehicle))
        End Select
    End Function
End Class

' A Car Class that Implements the IVehicle Interface
Public Class Car
    Implements IVehicle
    Public Sub Drive(ByVal miles As Integer) Implements IVehicle.Drive
        ' Drive the Car
    End Sub
End Class

' A Lorry Class that Implements the IVehicle Interface
Public Class Lorry
    Implements IVehicle
    Public Sub Drive(ByVal miles As Integer) Implements IVehicle.Drive
        ' Drive the Lorry
    End Sub
End Class

UML Diagram

Factory Design Pattern

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