…………………………………………………………Page 238…………………………………………………………… 216 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 other properties。 However； the default property is unique in that you use it like an array refer ence。 You could use the default property as follows： Dim foundHandle As Object = controller（〃description〃） In the example； the array index is a string and behaves more like a property bag。 However； if description were of type Integer； then the default property would behave like an index。 ■Note Default properties are utility…based and best added to classes that manage collections。 In the case of the LightingController； which manages a collection of room groupings； use of a default property is appropriate。 To find a particular room grouping； you would use the methods or default property of LightingController。 However； sometimes a user would like to work with all of the room group ings that are available and iterate over them。 The previous indexer example and the FindRoomGrouping（） method returned an object。 When you iterate the room groupings； you don’t want an object； because you don’t know what the object represents。 If you call the FindRoomGrouping（） method and you search based on a description； the object that is returned is cross…referenced with the description。 However； if you iterate using a numeric default property； the returned object means nothing to you other than being associated with a specific index。 What you really want to know is the description of each object。 You could define a default property with a numeric value； as in the following code； and iterate the individual elements to obtain descriptions。 Default Public ReadOnly Property Item（ByVal index As Integer） As String Get 。 。 。 End Get End Property ■Note A type can have multiple default property definitions； but each default property definition must have different array parameters。 Suppose we implemented a numeric default property。 To iterate the individual room groupings； we would need to use the following code。 For c1 As Integer = 0 To controller。Length 1 Dim description As String = controller（c1） Next …………………………………………………………Page 239…………………………………………………………… 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 217 This iteration code is acceptable and something that we could use； but it involves adding the property Length to the LightingController class。 Another approach is to use For Each； as follows： For Each rg As RoomGrouping In controller。RoomGroupingIterator（） " Do something with the rg Console。WriteLine（rg。Description） Next The For Each syntax is simpler。 It doesn’t matter that we’ve lost the information about which offset is which description； because that information is useless。 Remember we are dealing with a linked list that can change its order however it pleases。 Thus； having a numeric identifier is pletely meaningless。 The only reliable way to find a room grouping is to know its description or hold a specific index for the collection。 ■Note Unless you are absolutely sure that the collection you are manipulating does not move elements around； holding an index as a unique description of the object can be dangerous； and potentially could corrupt the state of an application。 In this chapter； I have illustrated two other techniques that can be used to refer ence a particular object： a handle and a default property。 We’ll e to the RoomGroupingIterator（） method in a bit； but first we need to see the enumerable that it returns。 The LightingController class has no For Each functionality built in。 To give it this function ality； you need to create a class that can be enumerated and a class that can be an enumerator； which is a class that can perform the iteration。 The plete implementation of a class that can be enumerated for BaseLinkedListItem is as follows （it is also the enumerator class； as you’ll see）。 Public Class LinkedListEnumerable Implements IEnumerable； IEnumerator Private _currNode As BaseLinkedListItem Private _firstNode As BaseLinkedListItem Public Sub New（ByVal linkedList As BaseLinkedListItem） _currNode = linkedList _firstNode = linkedList End Sub Public Function GetEnumerator（） As IEnumerator _ Implements IEnumerable。GetEnumerator Return Me End Function …………………………………………………………Page 240…………………………………………………………… 218 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 Public ReadOnly Property Current（） As Object _ Implements IEnumerator。Current Get Return _currNode End Get End Property Public Function MoveNext（） As Boolean _ Implements IEnumerator。MoveNext If _currNode Is Nothing Then Return False End If _currNode = _currNode。NextItem If _currNode Is Nothing Then Return False Else Return True End If End Function Public Sub Reset（） _ Implements IEnumerat