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C H AP TE R 8 ■ L E AR N IN G AB O U T CO M P O N E N T O R IE N TE D A R CH I TE C TU R E 225
Public Function CreatePrivateRoom（） As IRoom
Return New PrivateRoom（）
End Function
Public Function CreatePublicRoom（） As IRoom
Return New PublicRoom（）
End Function
End Module
The implementation has three methods： CreateBuilding（）； CreatePrivateRoom（）； and
CreatePublicRoom（）；。 The fact that CreatePrivateRoom（） and PrivateRoom（） have similar naming is
purely coincidental。 The method could just as well have been called CreateNonControlledRoom（）。
The CreatePrivateRoom（） and CreatePublicRoom（） methods are intended to define method
identifiers that users can understand。 What is instantiated in the method must return an IRoom
instance。
The CreateBuilding（） method is a builder method； and it returns a LightingController
instance。 It is fine to return a LightingController instance； because it is a globally defined type
and can serve as a basis for the builder method。 In the implementation of the builder method；
the room groupings and rooms are instantiated and added to the LightingController instance。
This is the work that the builder should save the end user。 Additionally； by providing a builder
method； you avoid creating museum structures that have glaring errors in their structures。
■Note Factory types serve to instantiate types and define builder methods； but can also be used to perform
generic structural operations。 Let’s say that in your museum there is a wing； which contains three public
rooms and a private room。 You could define a builder method that creates a wing； and the wing is added to
an already created building。 The general idea behind the factory type is to avoid errors and centralize repeated
instantiations。
Learning More About Private Classes and
Object Initialization
In this chapter； you learned how to apply interfaces； implementations； and ponents in a
kernel…type situation。 This is very much the type of programming that you will encounter as
you continue using Visual Basic。 Here； I will provide a few more details about using private
classes and initializing objects with nested data types。
Private Classes
The RoomGrouping and Room classes are defined in the LibLightingController project and are
private to the library。 This is because RoomGrouping and Room are classes only LightingController
needs to support its functionality。 The declaration of each class is internal to the assembly；
which is good； but it still means that some developers could use the classes within the kernel
assembly for their own purposes。 Sometimes that is a desirable feature； sometimes it is not。
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226 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E
In the case of LightingController； another approach is to declare the classes in the
context of LightingController； as follows：
Public Class LightingController
Private Class RoomGrouping
End Class
Private Class Room
End Class
End Class
The Room and RoomGrouping classes are declared within the class； and their declarations are
Private。 This means that only LightingController can instantiate and use the classes； and
there can never be the situation where another class will instantiate the types。 In the case of
LightingController； this would have been a better solution。
Private classes are also used in the factory context。 For example； imagine the situation
where you don’t ever want anyone but the factory to instantiate a room。 A possible IRoom decla
ration and factory could be as follows：
Public Module Factory
Private Class MyRoom ： Implements IRoom
End Class
Public Function CreateMyRoom（） As IRoom
Return New MyRoom（）
End Function
End Module
In this implementation of MyRoom； you can be sure that only Factory can ever instantiate
MyRoom； and you can always be sure that the only way to manipulate MyRoom is through the IRoom
interface。 All too often； developers bee lazy and instantiate types within the assembly； and
switch to the implementation type whenever the interface does not have the methods or prop
erties that they want。
Object Initialization with Nested Data Types
In this chapter； you saw how to use object initialization to assign data members in lieu of a
constructor。 Object initialization also works using nested data types。 Consider the situation
where a type references another type。 Using object initialization； you can instantiate and
assign multiple levels of objects。
Suppose you had this source code：
Class MyType
Private _dataMember As Integer
Public Property DataMember（） As Integer
Get
Return _dataMember
End Get
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C H AP TE R 8 ■ L E AR N IN G AB O U T CO M P O N E N T O R IE N TE D A R CH I TE C TU R E 227
Set （ByVal value as Integer）
_dataMember = value
End Set
End Property
End Class
Class EmbeddedMyType
Private _embedded As MyType
Public Property MyTypeProperty（） As MyType
Get
Return _embedded
End Get
Set （ByVal value As MyType）
_embedded = value
End Get
End Property
End Class
The type EmbeddedMyType has a property that references MyType。 If you were to instantiate
EmbeddedMyType； you would probably also want to instantiate and assign the property MyType。
You can do that with object initialization； like this：
Dim cls As EmbeddedMyType = _
New EmbeddedMyType（） With ｛ _
。MyTypeProperty = _
New MyType（） With ｛。DataMember = 10｝｝
The Important Stuff to Remember
In this chapter； you learned about writing a kernel； using Visual Basic default properties； and
the implementing enumeration functionality。 The main items to remember are as follows：
o A kernel is a ponent…oriented architecture where you are not in control of certain
implementations。 ponents make it possible to modularize a development process
so that separate teams have their own tasks。
o Interfaces are contracts between modules； and you test against the interface； not the
implementation。
o Placeholder interfaces are used to make it simpler to group object instances。
o Default properties are structural； and thus they serve as a tool to help you acplish a
task quicker。
o Default properties make it possible for your type to behave like an array。
o To iterate something that does not support iteration； you add enumeration function
ality； which implies implementing the IEnumerable and IEnumerator interfaces。
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228 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E
Some Things for You to Do
The following are some exercises that will help you apply the concepts y