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resource management。
In the sample code； the TaxableAmount property is the result of multiplying the data members
_amount and _taxableRate。 For example； when calculating the taxable ine of a capital gain
in Canada； you multiply the total amount by 50％。 Thus； the _taxableRate data member would
have a value of 0。50。
Another item to note is that the declaration of TaxIne is lacking a Public scope； but has
a NotInheritable keyword prefixed to the identifier。 When a class is prefixed with NotInheritable；
it means that the class cannot be subclassed。 From a design perspective； the code is saying
TaxIne is a shared class that should not be subclassed lest it affect the shared behavior。
The NotInheritable keyword can also be applied to a method； which means that the method
cannot be overloaded or overridden。 You would use NotOverridable on a method when you do
not want a derived class to change the behavior of the method。 Though to be able to use the
NotOverridable keyword； you first need to define a method that overrides another method。
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180 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
The implementation of ITaxDeduction is similar to ITaxIne：
NotInheritable Class TaxDeduction
Implements ITaxDeduction
Private _amount As Double
Public Sub New（ByVal amount As Double）
_amount = amount
End Sub
Public ReadOnly Property Amount（） As Double _
Implements ITaxDeduction。Amount
Get
Return _amount
End Get
End Property
End Class
TaxIne and TaxDeduction are sealed classes because the functionality will be shared
among implementations。 But is it wise to expose the interfaces and not the classes themselves？
Interfaces are used to separate implementation from ideas。 Interfaces change very little； whereas
implementations can and will change more often。 But if an implementation behaves like an
interface in terms of changing interface signature； why not just expose the class itself？ The
answer is that sometimes you will expose the class； and sometimes you will expose the interface。
For the tax engine； exposing the sealed classes TaxIne and TaxDeduction would probably
have been fine。 The rule of thumb is that you expose classes only when you are sure that the
interface signatures of methods and properties will not change very often。
Implementing a Base Tax Account
The ITaxAccount interface is also a candidate for base class functionality。 The implementation
of this interface looks like this：
MustInherit Class BaseTaxAccount
Implements ITaxAccount
Private _deductions（100） As ITaxDeduction
Private _ines（100） As ITaxIne
Public Sub AddDeduction（ByVal deduction As ITaxDeduction） _
Implements ITaxAccount。AddDeduction
For c1 As Integer = 0 To 100
If _deductions（c1） Is Nothing Then
_deductions（c1） = deduction
Exit For
End If
Next
End Sub
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CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 181
Public Sub AddIne（ByVal ine As ITaxIne） _
Implements ITaxAccount。AddIne
For c1 As Integer = 0 to 100
If _ines（c1） Is Nothing Then
_ines（c1） = ine
Exit For
End If
Next
End Sub
Public ReadOnly Property Deductions（） As ITaxDeduction（） _
Implements ITaxAccount。Deductions
Get
Return _deductions
End Get
End Property
Public ReadOnly Property Ine（） As ITaxIne（） _
Implements ITaxAccount。Ine
Get
Return _ines
End Get
End Property
Public MustOverride Function GetTaxRate（ByVal ine As Double） As Double _
Implements ITaxAccount。GetTaxRate
End Class
Let’s take stock of what has been acplished and decide if the tax engine is plete
from a base functionality perspective。
o Ideas have been defined for a plete tax engine。
o Some interfaces have been implemented in the form of base classes。
o Some interfaces have been implemented as sealed default implementations。
The tax engine can be considered plete because all of the interfaces have been defined
and accounted for from a base functionality perspective as either base classes or default imple
mentations。 It is does not mean that interfaces will always be implemented。 Sometimes some
interfaces will not have a base functionality or base classes。
The important aspect to remember when defining the base functionality is to account for
all interfaces to serve a purpose。 Do not define an interface as a placeholder for potential future
functionality。 When seeing an interface； a user expects it to serve some type of purpose。
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182 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
■Note The rule of thumb for interfaces is that once defined and put into production； they are not changed。
This rule of thumb is almost written in concrete。 You never change interfaces once they are in production
because doing so would wreak havoc—all the code that uses the interface would need to be updated。 In
general； if you feel the need to change an interface； create a new one。
With base functionality plete； we are ready to implement a tax system for a particular
country。
Using the Base Functionality of the Tax Engine
to Calculate Taxes
We will use the base functionality of the tax engine to calculate Canadian taxes。 I chose the
Canadian tax system because that is the system that I happen to know and understand； and
there is plenty of online documentation on how to calculate the tax rate。
SOME BACKGROUND ON CANADIAN TAXES
The first thing I would like to say about Canadian taxes is that Canadians pay too much—way too much。 The
second ment is that Canadian taxes are simple； in that they don’t let you write off many deductions （for
example； interest payments on your house）； which brings us back to the first point that Canadians pay way too
much。 Of course； people in other countries would probably say the exact same thing。
Canadian taxes are based on calculating the tax rate at the federal level and at the provincial level。 Thus；
to calculate your taxes； you need to know in which province and in which year you are paying your taxes。 Canadian
taxes change quite a bit depending on the year。 From an implementation perspective； it means the tax engine
must know about provincial taxes； federal taxes； and which year。
Capital gains in Canada are calculated on a 50％ basis。 This means if I made a capital gain of 200 Canadian
dollars； then only 100 Canadian dollars are declared as taxable monies。
Implementin