listbox selected items into textbox not working - asp.net

Question

my vb.net will not use listbox1.selecteditems it always comes up with a blue line underneath even though when i search online everyone is using this.
my goal is to get the selected items and list them in a textbox
Protected Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim li As ListItem
For Each li In ListBox1.Items
If li.Selected Then
TextBox1.Text &= li.Text & vbCrLf
End If
UpdatePanel2.Update()
Next
End Sub
End Class

Answer 1

To determine the selection in a multi-selection list control
Loop through the control's Items collection and test the Selected property of every individual item.
For Each li In ListBox1.Items
If li.Selected Then
TextBox1.Text &= li.Text & vbCrLf
End If
Next
MSDN: To determine the selection in a multi-selection list control
SelectedItems is not available in ASP.Net, this property exists only for Winforms-Listbox Controls.

Answer 2

I think the problem here is that you're binding to the control immediately prior to trying to retrieve a selected value from it. When the control is initially bound to the datasource it won't have any selected items.
You need to split it out so that you bind the listbox when the page is loaded, then the user selects some things in that box, clicks your Button2, and the value of the textbox is updated on postback.
First, on page load:
Protected Sub Page_Load(object sender, EventArgs, e)
listcmd.Connection = conn1
conn1.Open()
listcmd.CommandText = "SELECT distinct B603SalesAsOFMASTER.SDESCR FROM B603SalesAsOFMASTER"
listda.Fill(saolist, "listboxtext")
Dim dt As DataTable = saolist.Tables("listboxtext")
ListBox1.DataSource = dt
ListBox1.DataValueField = "SDESCR"
ListBox1.DataMember = "SDESCR"
ListBox1.DataBind()
conn1.Close()
End Sub
Then, this code is run when the user clicks Button2
Protected Sub Button2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button2.Click
For i As Integer = 0 To ListBox1.SelectedItems.Count - 1
TextBox1.Text &= DirectCast(ListBox1.SelectedItems(i), DataRowView)(1).ToString & vbCrLf
Next
CheckBox1.Visible = True
TextBox1.Visible = True
End Sub

Answer 3

To view (Text Item in textbox)
textbox3.Text = listBox1.GetItemText(listBox1.SelectedItem);