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ideas。 ing up with ideas and then implementing them is part of the development process
called test…driven architecture。
To start test…driven architecting； you think of the requirements and then e up with a
general solution。 In the case of our example； the requirement is to create a tax engine to calcu
late ine tax。 Generally speaking； it means figuring out what the total taxable ine is；
subtracting the total deductions； and applying a tax rate to the remaining sum to calculate the
total tax。
In programming terms； the general ideas are converted into source using Visual Basic
interfaces。
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166 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
Representing Ideas Using Visual Basic Interfaces
Think of Visual Basic interfaces as programmatic constructs you can use to jot down ideas。 When
you jot down ideas to solve a problem； you usually implement a bread…crumb trail approach—
you start with one idea； and then follow it to another idea； which leads to another idea； until
you have jotted down all of your ideas。
Your first idea is the central idea。 In our tax application； the central idea is the tax engine
itself。 The tax engine is used to perform a tax calculation and to pull together all of the other
pieces。 The other pieces are some nebulous things that you need to plete the tax engine。 I
like to call these other pieces dependencies。 The dependencies are ideas that you need to finish
the previous idea； hence the metaphor of following bread crumbs to your solution。
Ideas by themselves are designs that solve a particular problem or give an inkling of how
to solve the particular problem。 When an idea is converted into source code； it bees a blue
print that forces the implementation to take a certain shape。 Ideas when coded are Visual Basic
interfaces； which cannot be executed by themselves。 From a programmatic point of view； an
interface is like a MustInherit base class in that you can reference an interface； but you cannot
instantiate an interface。 To get a working idea； you write an implementation； or implement an
interface。
The following is an example of a Visual Basic interface。
Interface IExample
End Interface
The keyword Interface is associated with an identifier IExample； and from a syntax perspec
tive is used like the Class keyword。 You can associate the public scope with an interface。 The
interface contains methods and properties that determine the behavior of the classes that
implement the interface。 Consider the following source code； which defines an interface with
a single method and single property。
Interface IExample
Sub Method（）
Property ExampleProperty As Integer
End Interface
When you define an interface； the included methods and properties do not have an imple
mentation because you are defining a signature。 A class that implements an interface will play
a game called “match the properties and method signatures。”
■Note Generally speaking； the naming convention used to define a type is verbose and self…explanatory。
When I define interfaces； I use identifiers prefixed with a capital I； like IExample。 The capital I is a mon
convention used to identify interfaces。 You should use this notation to be consistent with other code。
The following code is a rudimentary implementation of the defined interface。
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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 167
Class ExampleImplementation
Implements IExample
Public Sub Method（） Implements IExample。Method
End Sub
Public Property ExampleProperty（） As Integer Implements IExample。ExampleProperty
Get
Return 0
End Get
Set（ByVal value As Integer）
End Set
End Property
End Class
ExampleImplementation implements the interface IExample using an explicit cross…reference
of the method or property signature。 The cross…reference of interface to implementation in the
sample method is prefixed with the keyword Implements； and the appropriate interface and
method。 The method name doesn’t need to be the same as the interface’s method name。 All
that matters is the signature of the method； or property。
■Note Visual Basic and the CLR are single…inheritance models in that a class can subclass only a single class。
But a Visual Basic class can implement as many interfaces as necessary。 If a class subclasses another class； it uses
Inherits； if it implements interfaces； it uses Implements。 The class is the first identifier after the colon。
When implementing an interface； all of the methods and properties of the interface must be
implemented in the class。 If they are not； then when the source code is piled； the piler will
plain about missing methods or properties and consider the implementation as inplete。
As inheritance is used； you can think of IExample as a base class that has no implementa
tion。 The following code illustrates how to instantiate the implementation and assign the
instance to a variable that has the interface as its type。
Dim cls As IExample = New ExampleImplementation（）
cls。Method（）
In the example； the class ExampleImplementation is instantiated and assigned to a variable
cls of type IExample。 Th