how to get the Index of object in collection

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

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.

Answer 3

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