Factory Pattern

Published 11/10/2005 by Henry in C# | Patterns
Tags: ,

The challenge:
We got a lot of classes derived from one baseclass. Like a TableRow with controls in it.
Every control got another control in the third row, like:

  • A Textbox
  • A DropdownListBox 
  • A RadioButtonList
  • Multiple Textboxes and a Dropdownlist (Date)
  • Etc.

In our client (A Webb App) we do not want to be bothered with which control we have to instantiate. We want to instantiate and get the right kind of object returned.
For this kind of scenario we can use the Factory Pattern.

A Factory Pattern returns an instance of one of the several classes depending upon the parameters passed to the shared/non-shared factory method. Typically, all these classes are inherited from a common parent class.
Each of them has common methods. But each of them has their own optimization task operation on data. For simplicity I decided to give an example that is more simple than the real life implementation I described above.
I will use a very simple example.

We got an abstract Employee class (must be inherited), which will act as our base class.
The Employee class has two properties:

  • Name (string)
  • Salary (double)

We derive from the Employee class two classes:

  • Developer
  • Manager

Because we do not want the hassle to instantiate another class everytime we want to create a developer instance, or a Manager instance and because there will be more Employee derived objects in the future (Secretary, Architect, etc.), we create a Factory class that does the job for us.
We call it the EmployeeFactory. 

UMLFactory.jpg

 Here is the code for the Employee class, with the derived classes:

   1:  using System;
   2:  namespace FactoryExample
   3:  {
   4:      public abstract class Employee
   5:      {
   6:          #region Private Members
   7:          private string m_Name = string.Empty;
   8:          private double m_Salary = 0;
   9:          #endregion
  10:          #region Public Properties
  11:          public string Name
  12:          {
  13:              get { return m_Name; }
  14:              set { m_Name = value; }
  15:          }
  16:          public double Salary
  17:          {
  18:              get { return m_Salary; }
  19:              set { m_Salary = value; }
  20:          }
  21:          #endregion
  22:      }
  23:   
  24:      public class Developer:  Employee
  25:      {
  26:          public Developer()
  27:          {
  28:              Name = "John Refactorer";
  29:              Salary = 2000;
  30:          }
  31:      }
  32:   
  33:      public class Manager: Employee
  34:      {
  35:          public Manager()
  36:          {
  37:              Name = "Peter PeopleManager";
  38:              Salary = 5000;
  39:          }
  40:      }
  41:  }

Next we have the Factory:

   1:  using System;
   2:  namespace FactoryExample
   3:  {
   4:      public class EmployeeFactory
   5:      {
   6:          public Employee CreateEmployee(int employeeKind)
   7:          {
   8:              Employee EmployeeObject;
   9:              switch (employeeKind)
  10:              {
  11:                  case 0:
  12:                      EmployeeObject = new Developer();
  13:                      break;
  14:                  case 1:
  15:                      EmployeeObject = new Manager();
  16:                      break;
  17:                  default:
  18:                      EmployeeObject = new Developer();
  19:                      break;
  20:              }
  21:              return EmployeeObject;
  22:          }
  23:      }
  24:  }

And last but not least the console app that calls the factory to instantiate the Developer and manager objects.

   1:  using System.Collections;
   2:  using System.Text;
   3:  namespace FactoryExample
   4:  {
   5:      class Program
   6:      {
   7:          static void Main(string[] args)
   8:          {
   9:              EmployeeFactory EmpFactory = new EmployeeFactory();
  10:              Employee emp;
  11:              for (int i = 0; i < 2; i++)
  12:              {
  13:                  emp = EmpFactory.CreateEmployee(i);
  14:                  Console.WriteLine("{0} - ({1}) Name: {2} Salary: {3}",
  15:                                      i,
  16:                                      emp.GetType().Name,
  17:                                      emp.Name,
  18:                                      emp.Salary);
  19:                  emp = null;
  20:              }
  21:              Console.Read();
  22:          }
  23:      }
  24:  }

When we run this code the output in the console looks like this:

 Console_fact.gif

I hope you get the idea, I have some real use for this pattern in the situaton described at the beginning of this post.

Henry Cordes
My thoughts exactly...


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