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OR I E N TE D P R O G R AM M IN G 157
Public Function ConvertFrom（ByVal value As Double） As Double
Dim realExchange As Double = ExchangeRate
ExchangeRate = realExchange ＋ _spread
Dim retval As Double = ConvertValueInverse（value）
ExchangeRate = realExchange
Return retval
End Function
End Class
The ConvertTo（） and ConvertFrom（） methods have the added logic in that the spread will
be added or subtracted from the exchange rate。 The methods read the current exchange rate；
assign it to a temporary variable； define a new exchange rate taking into account the spread；
calculate the currency； and then reassign the exchange rate。
The ConvertTo（） and ConvertFrom（） methods swap values to achieve the desired calcula
tions。 There is absolutely nothing wrong with doing this； and you will often swap values in your
own code。 What is important is to restrict access on what classes can do this。 Because ExchangeRate
is a property with protected access； it means only those classes that subclass can assign and
change the value of ExchangeRate。 And this implies that the class doing the subclassing knows
what it is doing to the data。 It is a valid assumption and one that you can use to your benefit。
The caller is not aware of this swapping； because HotelCurrencyTrader is taking advantage of
the object…oriented technique to not expose the state of the type。
That’s it—we’re finished with the currency exchange application。 For the remainder of
this chapter； I will fill in some extra details that are important。
Learning More About Preprocessor Directives；
Properties； and the MustOverride Keyword
Some details about preprocessor directives； properties； and the MustOverride keyword that did
not e up in this chapter’s example are worth mentioning。 These are things you should
know about when writing your own code。
More Preprocessor Directive Details
Earlier in the chapter； you saw how you can use the hash character （#） and conditional state
ments to conditionally include code during pilation。 The technical term for this jargon is
called preprocessing the code； and the statements are called preprocessor directives。 Table 6…1
shows the preprocessor directives you can use。
Table 6…1。 Available Preprocessor Directives
Hash Code Description
#Const Used to define the pilation identifier； such as the INTEGRATE_TESTS
identifier used in this chapter’s example。 You would define this at the top
of a source code file to activate conditional preprocessor statements
used throughout the source code file。 Using a #Const directive does not
span multiple source code files。
#If and #End If Used to conditionally include or not include a piece of source code。 You
would use a conditional directive whenever you wanted to define source
code configurations； such as for debug or production builds。
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158 CH AP T E R 6 ■ L E A R N IN G T HE B AS IC S O F OB J E CT OR I E N T E D P R O G R AM M IN G
Table 6…1。 Available Preprocessor Directives （Continued）
Hash Code Description
#ElseIf Instead of a single #If block； you could have multiple tests for code
inclusion。 You would use #ElseIf when you have more than a single
preprocessor directive； such as for debug； production； and performance
builds。
#Else A default code block that is included if the other #If statements did
not trigger。
#Region and These have absolutely nothing to do with the pilation of the source
#EndRegion code。 They are used by Visual Studio to create regions of source code that
can be “folded。” Folding means to collapse a block code out of sight
temporarily。 Source code files can be bee long； and constantly paging
up and down bees tedious。 By folding the code； you are reducing the
amount of times that you need to page up and down； even though the
code still exists。
The following example demonstrates how to use the preprocessor directives。
#Const ACTIVATE_1 = True
Namespace TestDefine
Class Example
#If ACTIVATE_1 Then
Private _dataMember As Integer
#ElseIf Not NO_ACTIVATE_10 Then
Private _dataMember3 As Integer
#Else
Private _defaultValue As Integer
#End If
End Class
End Namespace
In general； you will use preprocessor directives when you are building debug mode or
release mode code。 Debug mode means that the code is piled such that it can be debugged
and analyzed using pretty symbols。 Release mode means when the code is piled； the code
can still be debugged； but the pretty symbols are missing。
■Note Visual Basic Express does not give you the flexibility to choose debug mode or release mode for
development。 To get this feature； you need the full version of Visual Studio。
Think of the pretty symbols as milestones on the road。 Every time your code executes in
debug mode； the code is saying； “You are now calling this method in this source code file；” or
“Oops； that property code in the source code file is not working properly。” In release mode； the
pretty symbols are more like road signs saying； “Place X y Miles/Kilometers。” The signs help
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CH A PT E R 6 ■ L E A R N I N G T HE B AS IC S O F O B J E CT OR I E N TE D P R O G R AM M IN G 159
guide you； but the information you get from them is very limited； that is； you don’t know which
towns you’re passing。
More Property Scope Details
In this chapter’s examples； the Get and Set parts of the properties always had the same scope。
However； properties can have mixed scope； that is； the Get and Set parts can have different
scopes。 Scope splitting works only if you specify both the Get and Set parts of the property。 The
idea behind splitting the scope is to enable the implementation of the logic where classes in the
inheritance chain are allowed to assign a property； and classes external to the inheritance
chain are allowed only to read the property。 The following is an example where a property is
declared as Public and the assignment of the property is Protected。
Class PropertyScopeExample
Private _value As Integer
Public Property Value（） As Integer
Protected Set（ByVal value As Integer）
_value = value
End Set
Get
Return _value
End Get
End Property
End Class
The MustOverride Keyword
In this chapter’s example； the keyword MustInherit was used to declare a class that could be
referenced； but not instantiated。 It is also possible to define methods that need to be imple
mented。 In Visual Basic； to require a derived class to implement the method； you use MustOverride。
The idea behind declaring methods that need to be overridden is that it allows a developer to
define an intention in a base class that can then be implemented in the subclass。
In the implementation of HotelCurrencyTrader and ActiveCurrencyTrader； two methods
were defined： ConvertTo（） and ConvertFrom（）。 A developer could e to the conclusion that
the methods are mon to both classes； and thus they could be defined in the base class
CurrencyTrader。 That is not a bad idea。 Let’s revisit the CurrencyTrader class and see how these
two methods could be added to CurrencyTrader as MustOverride methods。
Public MustInherit Class CurrencyTrader
Private _exchangeRate As Double
Protected Property ExchangeRate（） As Double
Get
Return _exchangeRate
End Get
Set （ByVal Value As Double）
_exchangeRate = Value
End Set
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End Property
Protected Function ConvertValue（ByVal input As Double） As Double
Return _exchangeRate * input
End Function
Protected Function ConvertValueInverse（ByVal input As Double） As Double
Return input / _exchangeRate
End Function
Public MustOverride Function ConvertTo（ByVal value As Double） As Double
Public MustOverride Function ConvertFrom（ByVal value As Double） As Double
End Class
A MustOverride method is declared without an implementation。 Any class that subclasses
CurrencyTrader is required to implement ConvertTo（） and ConvertFrom（）。 In the case of
HotelCurrencyTrader and ActiveCurrencyTrader； this is not a problem； because the methods
are already impleme
