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previous examples； we had control of every class； interface； and definition。 This time； we are
not in control； and thus we need to use a defensive style of programming。 This means we need
to write many tests and keep certain information private。
Defining the Interfaces
The core of the lighting controller is to control the lighting in a room that is part of the building。
We can define and organize the rooms by using interfaces。 We need four interfaces：
o IRoom： A placeholder interface for the idea of a room
o INoRemoteControlRoom： An interface for rooms that should not be controlled by the
lighting controller
o IRemoteControlRoom： An interface for rooms that should be pletely controlled by the
lighting controller
o ISensorRoom： An interface for rooms whose control is based on state （whether or not a
person is in the room）
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The interfaces for the rooms where lighting may be controlled； IRemoteControlRoom and
ISensorRoom； will depend on certain pieces of logic。 The interfaces will need to provide input
data and accept output data。 The logic might also seek input from other sources； such as time
of day or amount of sunlight outdoors。 It boils down to defining some type of logic that the kernel
implements。 This is key； and it relates to the children…and…parent issue。 While you accept your
children as being intelligent beings that can make decisions； at the end of the day； it is usually
the parent who makes the final decisions。 Likewise； while your kernel might accept input and
potential decisions； the kernel makes the final decisions。
Defining IRoom； a Placeholder Interface
For design purposes； the simplest and base idea is the room itself； which can be defined as
follows （in the controller library LibLightingSystem）：
Public Interface IRoom
End Interface
The interface does not have any methods or properties。 It is called a placeholder interface。
A placeholder type serves no other purpose than identifying that the implementation is of a
certain type。 Placeholder interfaces make it simpler to group objects that have certain capabilities。
Imagine defining objects without a placeholder interface； something like this：
Class Type1
End Class
Class Type2
End Class
Looking at Type1 and Type2； you cannot see any correlation between the two types； there
is no way to say that Type1 and Type2 have anything in mon。 （Well； technically there is a
correlation in that both types are derived from Object； but that type of correlation is like saying
that all people are humans。） Using a placeholder interface； Type1 and Type2 can be correlated；
as follows：
Class Type1
Implements IRoom
End Class
Class Type2
Implements IRoom
End Class
。 。 。
Dim rooms As IRoom（） = New IRoom（） ｛New Type1（）； New Type2（）｝
Having Type1 and Type2 implement the IRoom interface； which means do nothing other
than subclass IRoom； establishes a correlation between Type1 and Type2。 The correlation is that
both Type1 and Type2 are rooms。 We have no idea what kind of rooms； and we have no idea if
the rooms are in the same building。 We only know that they are rooms。
The use of placeholder interfaces is very important in kernel design。 Placeholders estab
lish that a type wants to be part of a grouping。 The kernel can use that grouping to define a list
of elements that are all similar。 It is like knowing the age of people to determine whether they
are eligible to drive。 The age does not indicate the sex or intelligence； nor if they are good or bad
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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 201
drivers。 The age is a placeholder that says； “Yes you are part of a grouping that is allowed to take
a driving test to give you the right to drive。”
In the case of our lighting control； defining the IRoom placeholder interface is saying that
whatever instance is associated with IRoom is indicating its interest in being part of the lighting
controller kernel。 When you have identified a type using a placeholder interface； you are saying
your type can be used in a certain context。 The context is determined by the placeholder
interface。
Defining the INoRemoteControlRoom Interface
Although the purpose of the lighting system is to control the lighting； some rooms should not
be controlled by the system。 Perhaps the room is private； or controlling its lighting would cause
problems。
For example； should a bedroom in a house be controlled by the lighting controller？ If the
lighting controller controls the lighting in the bedroom； it might turn off the lights while a person is
reading。 Or maybe it will turn on the lights when the person has decided to sleep in。 Of course；
the person could just switch the light on or off manually； but that is disruptive。 The inconve
nience of the controller getting it wrong outweighs the benefit of the controller getting it right；
so the controller should not deal with this room。
The definition of an interface that indicates that the controller should do nothing is as
follows （in the controller library LibLightingSystem）：
Public Interface INoRemoteControlRoom
Inherits IRoom
End Interface
As you can see； INoRemoteControlRoom lacks methods and properties； like our placeholder
interface IRoom。 However； in this case； there are no methods or properties because the kernel
system does not require them。 The idea behind the INoRemoteControlRoom interface is to indicate
that the type implementing the interface is a room； but a room that should not be managed by
the controller。 Using the bedroom as an example； the implementation is as follows （defined in
the Home project）：
Imports LibLightingSystem
Public Class Bedroom
Implements INoRemoteControlRoom
End Class
The definition of the bedroom allows the kernel to use an instance of a room； as follows：
Dim rooms As IRoom（）= New IRoom（10） ｛ ｝
rooms（0） = New Bedroom（）
。 。 。
If TypeOf（rooms（0）） Is INoRemoteControlRoom Then
' Take appropriate action
End If
This code creates an array of rooms and assigns the index 0 to an instance of Bedroom。 The
If statement asks if the IRoom instance in index 0 is of type INoRemoteControlRoom。
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■Note Using placeholder interfaces and inheritance sets up a very powerful architecture that allows you to
create groupings。 You can then filter individual instances based on refinements of the grouping。 All of this is
possible in the Visual Basic language using TryCast（） and Is； which allow queries of subclassed types of
an instance。 The queries are noninvasive and do not cause exceptions to be thrown。 The queries give you the
ability to make decisions based on whether an instance would like to be associated with a particular group