Design patterns c#

Singleton pattern

When we want only one instance of the object to be created and shared between clients (global variables of project)

To achieve/implement singleton pattern

1. Define the constructor as private, so no instance/object can be created
2. Define instance (variables) and methods as static. This will cause one global instance to be created and shared across.

E.g.

Public class clsSingleton
{
  public static int intCounter;
  private clsSingleton()
    {
       //This is a private constructor
     }

   public static void Hits()
    {
         intCounter++;
     }
    public static int getTotalHits()
     {
        return intCounter;
      }
}

Client Code
clsSingleton.Hit();                 //Everytime counter increments by 1

Factory Pattern

public class clsFactoryInvoice

{

  public static IInvoice getInvoice(int intInvoiceType)

  {

    IInvoice objInv;

    if(intInvoiceType == 1)

    {

      objInv = new clsInvoiceWithHeader();

    }

    else if (intInvoiceType ==2)

    {

        objInv = new clsInvoiceWithoutHeader();

     }

     else

      { return null;}

    return objInv;

  }

}

public interface IInvoice

{

  public void print();

}

public class clsInvoiceWithHeader : IInvoice

{

  public void print()

  {

     console.writelinne("Invoice will be printed with header");

   }

}

public class clsInvoiceWithoutHeader : IInvoice

{

  public void print()

  {

     console.writelinne("Invoice will be printed without header");

   }

}

Benefits : Client no need to know which all concrete classes available (loosely coupled) client will have object created just by passing parameter.