Dim lines_in As List(Of String) = _
IO.File.ReadAllLines("Textdatei (Zeilen)").ToList
 
Dim lines_out As New SortedList(Of Integer, String)
Dim rndm As New Random()
If lines_in.Count < 20 Then Stop
 
For i As Integer = 0 To 19
    Dim r As Integer = rndm.Next(0, lines_in.Count)
    lines_out.Add(r, lines_in(r))  'Schlüssel = Dateiposition notieren
Next i
 
'Ausgabe-Array erstellen und füllen
Dim lines(lines_out.Count - 1) As String
lines_out.Values.CopyTo(lines, 0)
 
IO.File.WriteAllLines("Ausgabedatei", lines)