Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        ListBox1.DataSource = Directory.GetFiles("C:\SomeDir", "*.txt")
    End Sub
 
    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, _
      ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        Dim sr As New StreamReader(ListBox1.SelectedValue.ToString())
        TextBox1.Text = sr.ReadToEnd()
        sr.Close()
    End Sub