Mine

        public static bool SetBluetoothPortName(BluetoothAddress deviceaddress, string portname)
        {
            if (portname == null)
                return false;   

            string strDeviceAddress = deviceaddress.ToString("8");

            using (RegistryKey rkPorts = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Bluetooth\Serial\Ports", true))
            {
                RegistryKey rkNewPort;

                rkNewPort = rkPorts.OpenSubKey(strDeviceAddress, true);
                if (rkNewPort == null)
                {
                    rkNewPort = rkPorts.CreateSubKey(strDeviceAddress);

                    if (rkNewPort == null)
                    {
                        return false;
                    }
                }
                else
                {
                    // deregister for old port
                    string oldPort = ((string)rkNewPort.GetValue("Port", null)).Trim('\0');
                    if (oldPort != null)
                        DeregisterDevice(oldPort);
                }

                rkNewPort.SetValue("KeepDCB", 0);
                rkNewPort.SetValue("RemoteDCB", 0);
                rkNewPort.SetValue("Encryption", 0);
                rkNewPort.SetValue("Authentication", 0);
                rkNewPort.SetValue("Port", portname);
                rkNewPort.SetValue("Server", 0);

                rkNewPort.Close();
                rkNewPort = null;

                PORTEMUPortParams pep = new PORTEMUPortParams();
                pep.uiportflags = RFCOMM_PORT_FLAGS.REMOTE_DCB;
                pep.flocal = false;
                pep.device = deviceaddress.ToInt64();
                pep.uuidService = BluetoothService.SerialPort;

                string portprefix = portname.Substring(0, 3);
                int portindex = int.Parse(portname[3].ToString());

                IntPtr handle = RegisterDevice(portprefix, portindex, "btd.dll", ref pep);
                if (handle == IntPtr.Zero)
                {
                    rkPorts.DeleteSubKeyTree(strDeviceAddress);

                    return false;
                }
            }

            return true;
        }