Private Sub TextBox1_KeyPress(ByVal sender As System.Object, ByVal e As _
      System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        Select Case Asc(e.KeyChar)
            Case Keys.A, Keys.S
'A und S zulassen                
 
            Case Else
                ' alle anderen Eingaben unterdrücken
                e.Handled = True
        End Select
    End Sub