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（） …………………………………………………………Page 253…………………………………………………………… 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。 …………………………………………………………Page 254…………………………………………………………… 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 & 〃）〃）