this is my code :

bc = new BluetoothClient();

 bc.Client.Connect(new BluetoothEndPoint(btaddress, service));

 byte[ msg = stringTobyte("12345");

 bc.Client.Send(msg, 0, 5, SocketFlags.None);

It can't send msg, what's wrong?  who can help me out ?

thank you very much

See the user's guide (either in the release or at http://32feet.net/files/folders/documents/entry5846.aspx), there's a short sample there.

You want to remove the .Client bits, its not a good idea to access the internal bits of the classes.  So just call bc.Connect, and access the connection through a stream e.g.

   NetworkStream peer = bc.GetStream();
   bc.Write(msg, 0, msg.Length);