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Some Things for You to Do
The following are some exercises that will help you apply the concepts you learned in this chapter。
1。 The LightingController。AddRoomGrouping（） method has a mistake。 Write some tests to
find the error； and then fix the code and rerun your tests to verify that the error has
been fixed。
2。 The TestInsert（） test method is one example of an insertion test； but not all variations
have been tested。 Write another test method that implements the remaining variation（s）
that need to be tested。
3。 The declarations of RoomGrouping and Room are not optimal。 Fix the declarations。
4。 Implement a general collection class based on the experience of using the class in the
class LightingController。 Hint： look at how the linked list for Room is declared and figure
out a way to abstract that into some general collection class。
5。 When the method LightingController。AddRoom（） is called； the method is tested internally
to see if the handle is of type RoomGrouping。 Can you think of a more defensive pro
gramming technique to make sure that the code someone else passes to the kernel will
not cause the kernel to fail？ Hint： think about the methods to turn on or off the lights
and think of what could go wrong。
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C H A P T E R 9
■ ■ ■
Learning About Lists； Delegates；
and Lambda Expressions
One of the most mon pieces of code that you will write is code that manages many object
instances。 In the previous examples； many object instances were managed using an array。 In
Chapter 8； you learned that a linked list used in conjunction with a default property could make a
plain…vanilla object look like a collection。 This chapter introduces the collection classes；
which provide an easy way to manage a set of object instances。 Think of a collection object as
an infinite sack where things can be added； iterated through； and retrieved。
The chapter begins with a discussion of how to manage collections。 Then we will look at
different ways of iterating the data using a delegate。 Finally； we will look at lambda expressions
and closures to illustrate another way of processing data。
The project structure used in this chapter is a single console application。 As we will not be
building an overall application； but rather a set of sample code snippets； no tests or libraries
are involved。
Managing Collections
When you have a collection； what you actually have is an object that happens to point to many
other objects。
Visual Basic provides collection classes for managing collections。 Visual Basic 2005 intro
duced a different approach to collections； which solved many of the problems that came up
in earlier Visual Basic versions。 Here； we’ll look at managing collections both before and after
Visual Basic 2005； which will help you to understand how collections are used。
Managing a Collection Before Visual Basic 2005
Before Visual Basic 2005； the main collection classes were stored in the namespace System。
Collections （and； of course； these classes are still available for you to use）。 The following are
some of the classes and interfaces in that namespace：
229
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230 CH AP T E R 9 ■ L E A R N IN G AB OU T L I ST S； D E L E G A T E S； A N D L A M B DA E X P R E S SI ON S
o ArrayList： A general collection that manages all of the referenced objects using an
internal array。 This class manages the problem of increasing the size of an array。
o Hashtable： A collection class where the individual objects are stored using key/value
pairs。 In the previous chapter； the indexer was used to retrieve a room grouping based
on its identifier。 You could use a Hashtable to acplish the same thing。
o ICollection： An interface implement by ArrayList that provides basic functionality that
copies the references to another array。
o IDictionary： An interface implemented by Hashtable that allows a programmer to asso
ciate a key with a value。
o IList： An interface implemented by ArrayList that provides a general…access mechanism
for manipulating a collection of items。
o Queue： A collection that implements the first in； first out （FIFO） mechanism。 You could
use a queue when you are processing a set of instructions。 The first instruction to process
would be the first instruction added to the collection。
o Stack： A collection that implements the last in； first out （LIFO） mechanism。 Think of it as
a stack of papers。 When one piece of paper is laid on top of another； the first piece of
paper that is processed is the last piece of paper added to the stack of papers。
All of the collection types—ArrayList； Hashtable； Queue； and Stack—implement a way to
store a set of types。 The difference in the collection types lies in how the individual objects are
stored and retrieved from the collection。 For examples of using these collection types； see the
“Learning More About Collection Types” section later in this chapter。
Let’s walk through an example of using these collection classes。 Begin by creating a console
application and call it OneToManySamples。 Then add a new class （right…click your console appli
cation project and select Add Class）。 Call it Example。vb and add all of the following code to it：
Class Example
Public Property Value（） As Integer
Get
Return _value
End Get
Set（ByVal value As Integer）
_value = value
End Set
End Property
Private _value As Integer
End Class
Friend Module Tests
Private Sub PlainVanillaObjects（）
Dim objects As IList = New ArrayList（）
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C HA P TE R 9 ■ L E AR N I N G A B O U T L I ST S； DE L E G AT E S ； AN D L A M B D A E X PR E SSI O N S 231
objects。Add（New Example With ｛。Value = 10｝）
objects。Add（New Example With ｛。Value = 20｝）
For Each obj As Example In objects
Console。WriteLine（〃Object value （〃 & obj。Value & 〃）〃）
Next
End Sub
Public Sub RunAll（）
PlainVanillaObjects（）
End Sub
End Module
This is the type of code written before Visual Basic 2005； and it follows a standard set of steps：
1。 You define a custom type （ Example in this example）。
2。 You instantiate the custom type and add the instances to a collection。 In the example；
two instances of Example are added to the collection type ArrayList。
3。 The collection is manipulated to allow you to access and manipulate the instances of
the custom types。 In the example； the collection ArrayList is an interface instance of IList。
The bolded code in the example is where the action