How to designate a network interface when join multicast group

Question

Consider the case that a Win8 laptop is connected to both Ethernet and a wifi network. If an app needs to join a mutlicast group, how can it designate which network interface to join the multicast group? The DatagramSocket.JoinMulticastGroup API seems to do not allow me to do that. http://msdn.microsoft.com/en-us/library/windows/apps/windows.networking.sockets.datagramsocket.joinmulticastgroup