Not able to return items in listbox

Question

I am using the code below to try and get a list of the items that are in the listbox. So there could be any number of items/rows and they are a 10 digit number. When I use the code below the "NPIListBox.Items.Count" only returns a count of 1 even when there are 3 items/rows in the list box. Any idea why I can get an accurate count when Next is clicked? My goal is to pass all items in the list box into a session so that I can use the values on another page. Thanks!

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If IsPostBack Then
           PopulateListBox()
    End If

End Sub

Protected Sub cmdNext_Click(sender As Object, e As System.Web.UI.ImageClickEventArgs) Handles cmdNext.Click
    Dim n As Integer = NPIListbox.Items.Count
    Dim arr As String() = New String(n - 1) {}
    For i As Integer = 0 To arr.Length - 1
        arr(i) = NPIListbox.Items(i).ToString.Substring(NPIListbox.Items(i).ToString.Length - 10)
    Next
    Session("arr") = arr

    Response.Redirect("~/frmDescription.aspx")
End Sub

Private Sub PopulateListBox()

    If DoctorNameTextBox.Text = "" Then

    Else
        ' Get value from text box
        Dim textBoxValue As String = Me.DoctorNameTextBox.Text

        ' Create new item to add to list box
        Dim newItem As New ListItem(textBoxValue)

        ' Add item to list box and set selected index
        NPIListbox.Items.Add(newItem)
        NPIListbox.SelectedIndex = NPIListbox.Items.Count - 1


    End If

End Sub

The list box is populated using the following javascript code

 <script language="javascript" type="text/javascript">
  function getSelected(source, eventArgs) {
      var s = $get("<%=DoctorNameTextBox.ClientID %>").value;

      var opt = document.createElement("option");
      opt.text = s.substring(s.length - 10);
      opt.value = s.substring(s.length - 10);

      document.getElementById('<%= NPIListbox.ClientID %>').options.add(opt);

  }

Answer 1

Since you're adding the options with JS, your server doesn't know this. When you post the values back, they didn't exist at run-time so it grabs the original count (1).

You'll have to use another method to get the newly added values, example: add them to a hidden field with runat='server' and grab the values from that.