how to get the Index of object in collection - vb.net

Question

I'm trying to make a application, in this application I have a List(of T) collection that holds an object.
When processing the object I need to know it's Index from the list.
Example:
Public Class
Public oList as New List(of TestObject)
Private Sub Test()
Dim NewObject As New TestObject
oList.add(NewObject)
Index(NewObject)
End Sub
Private Sub Index(Byval TestObject As TestObject)
debug.print(Testobject.index)
End Sub
End Class
Is something like this possible? Ive seen it available in a reference file I used some time ago, but now I would like to make this available within my own class.
Can someone provide a sample?
PS: I know I can get the index using the List(Of T).IndexOf Method (T) but for future possibilities I would like to make the call from the object itself.

Answer 1

What usually happen is that they have a custom list, they don't directly used List(Of T) and store the list inside the object when they add that item to the list.
Module Module1
Sub Main()
Dim someList As New CustomList
someList.Add(New CustomItem())
someList.Add(New CustomItem())
someList.Add(New CustomItem())
Console.WriteLine(someList(1).Index)
Console.ReadLine()
End Sub
End Module
Class CustomItem
' Friend since we don't want anyone else to see/change it.
Friend IncludedInList As CustomList
Public ReadOnly Property Index
Get
If IncludedInList Is Nothing Then
Return -1
End If
Return IncludedInList.IndexOf(Me)
End Get
End Property
End Class
Class CustomList
Inherits System.Collections.ObjectModel.Collection(Of CustomItem)
Protected Overrides Sub InsertItem(index As Integer, item As CustomItem)
If item.IncludedInList IsNot Nothing Then
Throw New ArgumentException("Item already in a list")
End If
item.IncludedInList = Me
MyBase.InsertItem(index, item)
End Sub
Protected Overrides Sub RemoveItem(index As Integer)
Me(index).IncludedInList = Nothing
MyBase.RemoveItem(index)
End Sub
End Class

Answer 2

It looks like this
Public oList As New List(Of TestObject)
Private Sub Test()
Dim NewObject As New TestObject(oList.Count)
oList.add(NewObject)
End Sub
Public Class TestObject
Public index As Integer
Public Sub New(IndxOfObj As Integer)
Me.index = IndxOfObj
End Sub
End Class

Answer 3

If you necessarily need to have it as a property on the object I would suggest the following:
Public Class Main
Public oList As New List(Of TestObject)
Public Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
Dim NewObject As New TestObject(Me)
oList.Add(NewObject)
Dim NewObject2 As New TestObject(Me)
oList.Add(NewObject2)
MsgBox(NewObject2.Index)
End Sub
Public Function Index(ByVal TestObject As TestObject) As Integer
Return oList.IndexOf(TestObject)
End Function
End Class
Public Class TestObject
Private _main As Main
Public ReadOnly Property Index() As Integer
Get
Return _main.Index(Me)
End Get
End Property
Public Sub New(RootClass As Main)
_main = RootClass
End Sub
End Class
If you happen to have the Main class as a Singleton you can skip the whole sending 'Me' into the constructor business. Then you can just call Main.Index without storing it as a property on all TestObjects.