how to get the index of the specific selecteditem in listbox?

I have this code in my search textbox:

listbox.Items.Clear()
        For Each item As String In c
            If item.Contains(textbox.Text) Then
                listbox.Items.Add(item)
            End If
        Next

The problem is thru searching the search item gets the value of the first index of the first item in the listbox which doesn't equal to their original index. I'm using array.