I want to set timeout to the read and write function,

but when I read the CanTimeout value ,it is false? (BluetoothClient.GetStream() = false)

How can I use this ?

Well that stream is simply a standard System.Net.Sockets.NetworkStream.  What platform are you running on?  CFv1?

I've just realised that timeouts aren't supported on CFv2.  The Stream has the three properties, but they're not implemented, CanTimeout returns false and ReadTimeout and WriteTimeout throw an exception.  See e.g. http://www.codeplex.com/32feet/WorkItem/View.aspx?WorkItemId=6597