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Inherits CurrencyTrader
Public Sub VerifyExchangeRate（ByVal value As Double）
If ExchangeRate value Then
Throw New Exception（〃ExchangeRate verification failed〃）
End If
End Sub
End Class
#End If
The keyword Partial prefixes the Class keyword。 The first implementation of
TestCurrencyTrader is an example of not exposing state。 The second implementation of
TestCurrencyTrader； which is declared in the context of a conditional pilation statement；
contains the method VerifyExchangeRate（）。 This is a verification method that tests the
ExchangeRate property for a particular value。
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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 151
■Note You can use partial classes only in the context of a single assembly； as the Partial keyword cannot
be used across assembly boundaries。 When I say “single assembly；” I am referring to the piled pieces of
source code illustrated in Chapter 1。 In other words； if you define a partial class in a library； then all
pieces of the partial class need to be defined in the library。
Partial classes make it simple to separate functionality into various source code files。 This
example demonstrates using partial classes to manipulate internal state of a class without
violating the do…not…expose…internal…state rule。 Another use of partial classes is in the context
of code generators； where one source code file contains the custom code； and the other source
code file contains the generator code。
■Note Only one part of a partial class needs to be explicitly qualified as Partial。 The piler is smart
enough to realize that if it sees one part as Partial； then all the other parts must also be partial （even if they
aren’t explicitly qualified with the Partial keyword）。
Finishing the Base Class
The ExchangeRate property is one of the pieces of shared functionality。 Another piece of shared
functionality we want to implement is the calculation of the exchange rate。 We’ll do this with
ConvertValue（） and ConvertValueInverse（） methods； which convert a currency from one value
to another using multiplication or division。 The following shows the methods in the pleted
base class implementation of CurrencyTrader。
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
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
End Class
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The bolded code highlights the methods that convert the currency from one unit to another。
Notice that there is no declaration of specific currency units； because the base class is a utility
class used to help us realize an active trader or hotel trader implementation。
■Note Base class functionality； even when appearing trivial； is defined to ensure consistency in implemen
tation。 Without consistency； you encounter the problem where one implementation does one thing and another
implementation does something pletely different。
This pletes our test code。 Now we will implement the active trader and hotel trader
ponents of the currency exchange application。
Writing the Active Trader and Hotel Trader
Currency Converters
With the TestCurrencyTrader test solution pleted； it’s time to turn our attention to the
CurrencyTrader solution。 As mentioned earlier； this consists of the active trader and hotel
trader currency converter ponents。 Here； you’ll see more clearly what it means to use
inheritance。
Implementing ActiveCurrencyTrader
The ActiveCurrencyTrader class implements the logic of the active currency trader。 To begin；
we’ll add its constructor。
Adding a Constructor to ActiveCurrencyTrader
To give ActiveCurrencyTrader some default state； we use a constructor。 However； the constructor
will serve another purpose； in that any class that instantiates ActiveCurrencyTrader will consider
the instance as immutable。 Immutable means that once data has been assigned to the instance； it
cannot be altered。 In other words； it is unchangeable。
■Note The String type is immutable because once a string variable has been assigned； it cannot be
changed。 Take a look at the methods associated with String； and you will see nothing that allows you to
modify the contents。 An immutable type is good because it allows you to implement a set…it…and…forget…it
object； and it prevents other classes from accidentally changing the contents。 Overall； an immutable type is
robust and predictable （though they are the exception； not the rule； most types allow you to modify state）。
The following shows the constructor code added to ActiveCurrencyTrader。
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