Creating Extensible and Flexible Hosted Applications: Part 2

The Problem

For this article I will use the same requirements and goals defined in part 1. You are hosting your bill processing application for Company A and the new Customer B wants added functionality.

Possible Solution

In the previous version I used the factory pattern and inheritance to extend the base functionality for Company B. In this article I will show how to use the Decorator pattern for extending the functionality.  

using System;
using myBusinessObjects;
namespace ConsoleSample
{
class MySample
{
[STAThread]
static void Main(string[] args)
{
// test harness for company B
Bill myBill = new Bill();
myBill.CostofProduct = 10.25;
myBill.NumberOfProducts = 3;
CommissionBill CompanyBBill = new CommissionBill(myBill);
CompanyBBill.Commission = 9.2;
Console.WriteLine(CompanyBBill.calculateBill().ToString());
/*
// test harness for company A
Bill myBill = new Bill();
myBill.CostofProduct = 10.25;
myBill.NumberOfProducts = 3;
Console.WriteLine(myBill.calculateBill().ToString());
*/
}
}
}
namespace myBusinessObjects
{ 
class Bill
{
public double CostofProduct;
public int NumberOfProducts;
public virtual double calculateBill()
{
return CostofProduct * NumberOfProducts;
}
public virtual double getDiscountPrice()
{
return CostofProduct * 0.8;
}
}
// This is an abstract class representing any
// future decorators of the base class Bill
// It must conform to the interface of the base class as
// well and maintain a reference to the instance class that
// it will decorate
abstract class BillDecorator : Bill
{
//maintain the instance
protected Bill BillItem;
public BillDecorator ( Bill BillItem )
{
this.BillItem = BillItem;
}
//direct the bill interface commands to the instance
public override double calculateBill()
{
return BillItem.calculateBill();
}
public override double getDiscountPrice()
{
return BillItem.getDiscountPrice();
}
}
class CommissionBill : BillDecorator
{
public CommissionBill( Bill BillItem ) : base( BillItem ) {}
public double Commission;
public override double calculateBill()
{
return base.calculateBill() + Commission;
}
}
}