Inventory Collection in Visual Basic - vb.net

Question

This is what the teacher is asking:
User selects an item from the list
The information about the item displays to the right (description, retail price, units)
The user enters a quantity and clicks Add to Cart
The subtotal, tax and grand total display
User clicks complete purchase button and a confirm order box appears. User clicks OK and the form clears for another transaction.
This is what I done:
Any Suggestions as of why I keep getting this error "Argument Index is not a valid value"
Imports System.IO
Public Class MainForm
Const strFILENAME As String = "Inventory.txt"
Dim dblTaxRate As Double = 8.75
Dim InventoryCollection As New Collection
Public Sub AddRecord(ByVal InvItem As Inventory)
Try
inventoryCollection.Add(InvItem, InvItem.InventoryNumber)
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub
Private Sub ClearMainForm()
txtDesc.Text = String.Empty
txtRetail.Text = ""
txtOnHand.Text = ""
txtInvNumber.Text = String.Empty
End Sub
Private Sub UpdateListBox()
lstInventory.Items.Clear()
Dim InvItem As Inventory
For Each InvItem In inventoryCollection
lstInventory.Items.Add(InvItem.InventoryNumber)
Next
If lstInventory.Items.Count > 0 Then
lstInventory.SelectedIndex = 0
Else
ClearMainForm()
End If
End Sub
Private Sub SaveRecord(ByVal objInventory As Inventory)
Dim Writer As StreamWriter
Try
Writer = File.AppendText("Inventory.txt")
Writer.WriteLine(objInventory.InventoryNumber)
Writer.WriteLine(objInventory.Description)
Writer.WriteLine(objInventory.Retail.ToString())
Writer.WriteLine(objInventory.OnHand.ToString())
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub
Private Sub MainForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim objInventory As New Inventory
Dim inventoryFile As System.IO.StreamReader
Dim blnFound As Boolean = False
Dim inventoryCollection As New Collection
Try
' Open the file.
If System.IO.File.Exists(strFILENAME) Then
End If
inventoryFile = System.IO.File.OpenText(strFILENAME)
'Enter loop and read till end of file.
Do Until inventoryFile.Peek = -1
'Read lines from file, save into Inventory object properties.
objInventory.InventoryNumber = inventoryFile.ReadLine
objInventory.Description = inventoryFile.ReadLine
objInventory.PartCost = inventoryFile.ReadLine
objInventory.Retail = inventoryFile.ReadLine
objInventory.OnHand = inventoryFile.ReadLine
'Display data in text boxes.
lstInventory.Items.Add(objInventory.InventoryNumber)
Loop
'Close the file.
inventoryFile.Close()
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub
Private Sub DisplayInput(ByVal InvItem As Inventory)
'Display from Collection to Label boxes
Try
txtDesc.Text = InvItem.Description
txtOnHand.Text = InvItem.OnHand.ToString()
txtRetail.Text = InvItem.Retail.ToString()
txtInvNumber.Text = InvItem.InventoryNumber
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub
Private Sub lstInventory_SelectedIndexChanged(sender As Object, e As EventArgs) Handles lstInventory.SelectedIndexChanged
Dim objInventory As Inventory
'See if an Item is Selected
If lstInventory.SelectedIndex <> -1 Then
'Retrieve student's data from inventoryCollection. Convert object into Inventory object.
Try
objInventory = CType(inventoryCollection.Item(lstInventory.SelectedItem), Inventory)
Catch ex As Exception
'Display error message.
MessageBox.Show(ex.Message)
Console.WriteLine("")
End Try
End If
End Sub
Private Sub btnClear_Click(sender As Object, e As EventArgs) Handles btnClear.Click
'Clear Form
ClearMainForm()
End Sub
Private Sub btnExits_Click(sender As Object, e As EventArgs) Handles btnExits.Click
'Close the Form
Me.Close()
End Sub
Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
Dim InvID As New Inventory
InventoryCollection.Add(InvID, InvID.InventoryNumber)
End Sub
End Class
Public Class Inventory
Private StrinvNumber As String
Private strdesc As String
Private decCost As Decimal
Private decretailPrice As Decimal
Private IntqtyOnHand As Integer
'Constructor
Public Sub New()
StrinvNumber = String.Empty
strdesc = String.Empty
decCost = 0.0
decretailPrice = 0.0
IntqtyOnHand = 0.0
End Sub
Public Property InventoryNumber() As String
Get
Return StrinvNumber
End Get
Set(ByVal value As String)
StrinvNumber = value
End Set
End Property
Public Property Description() As String
Get
Return strdesc
End Get
Set(ByVal value As String)
strdesc = value
End Set
End Property
Public Property PartCost() As Decimal
Get
Return decCost
End Get
Set(ByVal value As Decimal)
decCost = value
End Set
End Property
Public Property Retail() As Decimal
Get
Return decretailPrice
End Get
Set(ByVal value As Decimal)
decretailPrice = value
End Set
End Property
Public Property OnHand() As Integer
Get
Return IntqtyOnHand
End Get
Set(ByVal value As Integer)
IntqtyOnHand = value
End Set
End Property
End Class

Answer 1

Presumably the issue is here:
objInventory = CType(inventoryCollection.Item(lstInventory.SelectedItem), Inventory)
Does lstInventory contain Integer values and are those values valid indexes into inventoryCollection? I would expect not. Are those two lists supposed to correspond to each other? If so then you should be using SelectedIndex rather than SelectedItem. Probably you should have just bound the list to the ListBox in the first place and then the SelectedItem would be the object you needed already.