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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 takes place。 Instantiating the type
ArrayList is the instantiation of a collection manager。 The ArrayList instance is then assigned
to the variable objects； which is of type IList。 IList is an interface making it possible to use the
collection in the context of a ponent…oriented development environment。 To add two objects
to the collection； we call the Add（） method twice。 To iterate the elements in the collection； we
use the For Each statement。
■Note The fact that the collection classes can be used in the context of a ponent…oriented application
is no coincidence。 When Microsoft created its library； ponents were an essential part of the library。
To run the tests； open Module1。vb in your console application and edit it as follows：
Module Module1
Sub Main（）
BeforeVisualBasic8。Tests。RunAll（）
Console。ReadKey（）
End Sub
End Module
Press Ctrl＋F5 to run the application and see the results。
■Note We did not need to import the System。Collections namespace because it is imported by default
in Visual Basic 2008。
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232 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
The Problem of Mixed Types
What is unique about the sample code is that the For Each statement works and happens to
know that the objects in the collection are of type Example。 However； the following code adds a
different object to the collection； which will cause the iteration to fail。
Class Another
End Class
Dim objects As IList = New ArrayList（）
objects。Add（New Example With ｛。Value = 10｝）
objects。Add（New Example With ｛。Value = 20｝）
objects。Add（New Another（））
For Each obj As Example In objects
Console。WriteLine（〃Object value （〃 & obj。Value & 〃）〃）