ASP.NET Creating Custom Collection

A custom collection class allows the use of the

For Each syntax (foreach in C#) to

loop through a set of custom typed items.The custom collection class must internally

keep references to the items with an array or other type of collection (ArrayList,

HashTable).The .NET Framework then requires the following two interfaces to be

implemented in order to support the For Each syntax:

• 
  
  System.Collections.IEnumerable
  
  


• 
  
  Function GetEnumerator() As IEnumerator
  
  


• 
  
  System.Collections.IEnumerator
  
  


• 
  
  Sub Reset()
  
  


• 
  
  Function MoveNext() As Boolean
  
  


• 
  
  ReadOnly Property Current As Object
  
  

In <script runat="server" /> block or codebehind:

Sub Page_Load()

Dim _customCollection As CustomCollection

Dim _customItem As CustomItem

Dim _index As Integer

_customCollection = New CustomCollection(10)

For Each _customItem In _customCollection

_index = _customItem.Index

Next

End Sub

Custom Collection defined in CustomCollection.vb

Public Class CustomCollection

Implements IEnumerable, IEnumerator

Private customItems() As CustomItem

Private currentIndex As Integer = -1

Public Sub New(ByVal Count As Integer)

Dim index As Integer

ReDim customItems(Count - 1)

For index = 0 To Count - 1

customItems(index) = New CustomItem(index)

Next

End Sub

#Region "Implementation of IEnumerable"

Public Function GetEnumerator() As Ienumerator

➥Implements IEnumerable.GetEnumerator

Return CType(Me, IEnumerator)

End Function

#End Region

#Region "Implementation of IEnumerator"

Public Sub Reset() Implements IEnumerator.Reset

currentIndex = -1

End Sub

Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext

If currentIndex < customItems.Length - 1 Then

currentIndex = currentIndex + 1

Return True

Else

Return False

End If

End Function

Public ReadOnly Property Current() As Object

➥Implements IEnumerator.Current

Get

Return customItems(currentIndex)

End Get

End Property

#End Region

End Class

Custom Item defined in CustomItem.vb

Public Class CustomItem

Private _index As Integer

Public ReadOnly Property Index() As Integer

Get

Return _index

End Get

End Property

Public Sub New(ByVal Index As Integer)

_index = Index

End Sub

End Class

This implementation uses a simple array of the CustomItem type and offers nothing

beyond the For Each support. It can be easily extended to support additional array or

ArrayList methods, such as Count/Length and Item.The base collection type can also

be changed to ArrayList or HashTable for further functionality.

This implementation of the CustomItem type is no more than a contrived example

with a single property of Index. It should be extended to include the various properties

and methods relevant to the actual custom item.The construction of the item, or set of

items, can also come from an external data store.