my program :

  if (!serialPort1.IsOpen)
                {
                    serialPort1.Open();
                }
                else
                {
                    serialPort1.Close();
                    serialPort1.Open();
                }

 serialPort1.WriteLine("///");

when serialPort1.Open ,  catch the wrong message 'IoException' and the 'COM8' port does't exist

 But I am sure that 'COM8' is the bluetooth port ,  what's wrong ? who can help me out ?

thank you very much 

As I noted in the other thread.  I recommend not using serial ports unless you *really* need too!  From the user's guide:

By far the easiest and best way to use a serial port connection is to use BluetoothClient with service class BluetoothService.SerialPort, i.e. using code very much like shown above.  Both BluetoothClient and virtual serial ports use the RFCOMM protocol so the two are equivalent.  This method is much easier to set-up that a virtual serial port, there is no global state to configure etc; and is also more robust, for instance if the peer device is taken out of range or turned-off one learns this directly soon after, whereas with a serial port one has to use timeouts and retries to detect it.

However there are cases where a virtual serial port is required, for instance where another program needs to access the connection — a common case is where your program is configuring a connection that will be used by a printer driver utility.  [...]

thank you very much for your help, I am thinking bout your suggestion