workaround to VB.Net 2003 System.Collections.Generic? - vb.net

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