workaround to VB.Net 2003 System.Collections.Generic?

Question

I'm working in an old web application (vb.net 2003) and I'm trying to use a generic list of a custom class.

I realized that System.Collections.Generic was introduced in .Net 2 according to link

Is there any alternative to the list? For instance an array of class?

Let's say I have the following class definition:

Public Class Box
  Public x As Integer
  Public y As Integer
End Class

And an array of Class Box:

Dim BoxList() As Box
BoxList(0).x = 1
BoxList(0).y = 1

BoxList(1).x = 2
BoxList(2).y = 2

But I'm getting an error when BoxList(0).x = 1 error: Object reference not set to an instance of an object

I'm just guessing here.

Answer 1

Use ArrayList, like this:

Dim BoxList As New ArrayList
Dim box = New Box()
box.x = 1
box.y = 2
BoxList.Add(box)

Note: It is recommended that you add a constructor to the Box class that will accept the x and y values, like this:

Public Class Box
    Public x As Integer
    Public y As Integer

    Public Sub New(ByVal _x As Integer, ByVal _y As Integer)
        x = _x
        y = _y
    End Sub
End Class

Now you can shorten your ArrayList code to this:

Dim BoxList As New ArrayList
BoxList.Add(New Box(1, 2))

To use the values in the ArrayList you will need to un-box (pun not intended) the value out of the ArrayList, like this:

For Each box In BoxList
    ' Use x value, like this
    CType(box, Box).x
Next

OR (as Meta-Knight suggested)

For Each box As Box In BoxList
    ' Now box is typed as Box and not object, so just use it
    box.x
Next

Answer 2

You can create your own custom collection class - this is what we had to do back before generics. This article from MSDN gives you the specifics:

''' Code copied directly from article
Public Class WidgetCollection
   Inherits System.Collections.CollectionBase

    Public Sub Add(ByVal awidget As Widget)
       List.Add(aWidget)
    End Sub
    Public Sub Remove(ByVal index as Integer)
       If index > Count - 1 Or index < 0 Then
          System.Windows.Forms.MessageBox.Show("Index not valid!")
       Else
          List.RemoveAt(index)
       End If
    End Sub
    Public ReadOnly Property Item(ByVal index as Integer) As Widget
       Get
          Return CType(List.Item(index), Widget)
       End Get
    End Property

End Class