Hi,I'm develop an application with C# for Windows mobile 5

After I connect to an devices (GUID service is serial port),I use this:

StreamWriter sw = new StreamWriter(bluetoothClient.GetStream());
          sw.Write("AT0011111111;\r");  

          MessageBox.Show("OK!", "Success");
          sw.Close();

to send a command to the devices and there is no error.

But when I close the application and restart it to connect the same device.

 the method sw.write show error and with no detail.

Am I forgot something when I close the application??

Hmm curious.  What type of device is it you're connecting too?  Maybe the device itself doesn't handle this properly.

Also try calling bluetoothClient.Close() too, but it shouldn't be necessary.

Finally try with a different device, for instance set-up a virtual serial port on your Win32 PC through its Bluetooth Control panel and see if it works as expected.