Factory pattern

A factory creates objects.

The factory design pattern relies on a type hierarchy. The classes must all implement an interface or derive from a base class. We use an abstract class as the base.

 

Factory pattern example: C#

using System;

class Program
{
    abstract class Position
    {
 public abstract string Title { get; }
    }

    class Manager : Position
    {
 public override string Title
 {
     get
     {
  return "Manager";
     }
 }
    }

    class Clerk : Position
    {
 public override string Title
 {
     get
     {
  return "Clerk";
     }
 }
    }

    class Programmer : Position
    {
 public override string Title
 {
     get
     {
  return "Programmer";
     }
 }
    }

    static class Factory
    {
 /// <summary>
 /// Decides which class to instantiate.
 /// </summary>
 public static Position Get(int id)
 {
     switch (id)
     {
  case 0:
      return new Manager();
  case 1:
  case 2:
      return new Clerk();
  case 3:
  default:
      return new Programmer();
     }
 }
    }

    static void Main()
    {
 for (int i = 0; i <= 3; i++)
 {
     var position = Factory.Get(i);
     Console.WriteLine("Where id = {0}, position = {1} ", i, position.Title);
 }
    }
}

Output

Where id = 0, position = Manager
Where id = 1, position = Clerk
Where id = 2, position = Clerk
Where id = 3, position = Programmer