how do i initialize my arraylist - vb.net

Question

I have a function that adds items to my arraylist. my problem is that it only holds one item at a time since it is reinitializing the array lit every time I click my button. what is the syntax in VB to only initialize the array if it has not been created yet?
Dim itemSelectAs New ArrayList()
Dim Quantities As New ArrayList()
Dim itemQtyOrdered As Integer
Public Sub ShtickDataList_ItemCommand(ByVal source As Object, ByVal e As System.Web.UI.WebControls.ListViewCommandEventArgs) Handles ShtickDataList.ItemCommand
If e.CommandName = "addToCart" Then
Dim itemQuantity As DropDownList = e.Item.FindControl("QuantityDropDown")
itemQtyOrdered = itemQuantity.SelectedValue
ItemSelect.Add(e.CommandArgument)
Quantities.Add(itemQtyOrdered)
Session("itemInCart") = ItemSelect
Session("quantities") = Quantities
viewInvoice()
End If
End Sub
Protected Sub viewInvoice()
Dim itemSelected As ArrayList = DirectCast(Session("itemInCart"), ArrayList)
Dim QuantityofItem As ArrayList = DirectCast(Session("quantities"), ArrayList)
Dim conn As SqlConnection
Dim comm As SqlCommand
Dim reader As SqlDataReader
Dim purimConnection2 As String = ConfigurationManager.ConnectionStrings("Purim").ConnectionString
conn = New SqlConnection(purimConnection2)
comm = New SqlCommand("SELECT ProductName FROM Products WHERE ProductID = #ProductID", conn)
Dim i As Integer
For i = 0 To ItemSelect.Count - 1
comm.Parameters.Add("#ProductID", Data.SqlDbType.Int)
comm.Parameters("#ProductID").Value = (ItemSelected.Count - 1)
'Next
Try
conn.Open()
reader = comm.ExecuteReader()
ViewCartlink.Text = "View Cart: (" & ItemSelected.Count & ")"
Finally
conn.Close()
End Try
End Sub

Answer 1

First you need to dimension your array list.
Dim array_list as ArrayList()
Then you can instantiate one
array_list = new ArrayList
Or you can combine it into one step:
Dim array_list = new ArrayList()
After that you can add and remove elements from your array list with
array_list.add(obj)
and remove with
array_list.remove(obj)
It looks like your problem is related to accessing the members of an arraylist. New items are always added to the end of an arraylist. To access them directly, you will need their index. If you know the index of the item you want to access use
array_list(i)
If you don't you will need to iterate over the array. To do this you have two options. You can use "for each" or you can use a normal for loop and use array_list.count as your upper bound.
You're recreating your two session values every time you call your button click menu. You need to pull them out of the Session variable and put them in local variables and put them back into the session variable.
Your button method should be:
Public Sub ShtickDataList_ItemCommand(ByVal source As Object, ByVal e As System.Web.UI.WebControls.ListViewCommandEventArgs) Handles ShtickDataList.ItemCommand
if isNothing(itemSelect) Then itemSelect = New ArrayList()
if isNothing(itemQtyOrdered) Then itemQtyOrdered= New ArrayList()
If e.CommandName = "addToCart" Then
Dim itemQuantity As DropDownList = e.Item.FindControl("QuantityDropDown")
itemQtyOrdered = itemQuantity.SelectedValue
ItemSelect.Add(e.CommandArgument)
Quantities.Add(itemQtyOrdered)
Session("itemInCart") = ItemSelect
Session("quantities") = Quantities
viewInvoice()
End If
End Sub
And change your Global calls to:
Dim itemSelect As ArrayList() = Session("itemInCart")
Dim Quantities As New ArrayList() = Session("quantities")

Answer 2

Define your array outside the button click event. (Form level)
Then in the button click event try this:
If myArrayList Is Nothing then
'initializes the array list only if that hasn't happened yet
myArrayList = new ArrayList
End If
'adds the item to the existing list without causing it to reintialize
myArrayList.add(item)
That way it is initialized if it hasn't been, but not if it already has. If it is initialized at the form level ie... its declared as new already then you can just add to it.
Basically make sure that you aren't calling New for the arrayList in the button click event.
Editing for web form:
You should probably check where you initialize your arrayList. Like in the Page_Load:
If Not Page.IsPostBack Then
myArrayList = New ArrayList
End If
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