Private serialPortReadThread As Thread
 
    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) _
      Handles MyBase.Load
 
        SerialPort1.Open()
 
        'Lesethread starten
        serialPortReadThread = New Thread( _
            Sub()
                ReadSerialPort(SerialPort1)
            End Sub)
        serialPortReadThread.Start()
 
    End Sub
 
    Private Sub ReadSerialPort(sp As SerialPort)
        Try
            Do While True
                Dim s As String = sp.ReadTo(ChrW(13))
                Invoke( _
                    Sub()
                        InListeEintragen(s)
                    End Sub)
 
            Loop
        Catch ex As ThreadInterruptedException
            ' Thread wurde interrupted.
        End Try
    End Sub
 
    Private Sub InListeEintragen(s As String)
 
        ListBox1.Items.Add(s)
        If Not ok Then
            SendZwei(s)
        End If
 
    End Sub
 
 
    Private Sub Form1_FormClosed(sender As System.Object, e As _
      System.Windows.Forms.FormClosedEventArgs) Handles MyBase.FormClosed
        ' Lesethread unterbrechen, sodass er beendet wird
        serialPortReadThread.Interrupt()
    End Sub