Private Sub btnLos_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles btnLos.Click
 
        Dim s1 As String
        Dim s2 As String
        Dim pos As Integer
 
        s1 = txtSuchwort.Text
        s2 = txtString.Text
 
        pos = s2.IndexOf(s1, pos)
 
        If pos >= 0 Then
            txtString.Select(s1, pos) 'Fehler: Ungültige Konvertierung von der 
            ' Zeichenfolge al in Typ Integer.
            txtString.HideSelection = False
        End If       
 
    End Sub