/// <summary>
        /// Returns or creats a chat room for these two users or create a new one if nothing exists
        /// </summary>
        /// <param name="user1ID"></param>
        /// <param name="user1Name"></param>
        /// <param name="user2ID"></param>
        /// <param name="user2Name"></param>
        /// <returns></returns>
        private static ChatRoom GetRoom(string user1ID,string user1Name,string user2ID,string user2Name)
        {
            ChatRoom room=null;

            string rid1=CreateRoomID(user1ID,user2ID);

            string rid2=CreateRoomID(user2ID,user1ID);

            Monitor.Enter(Rooms);

            if (Rooms.Contains(rid1))

                room=(ChatRoom)Rooms[rid1];

            else

            {

                if (Rooms.Contains(rid2))

                    room=(ChatRoom)Rooms[rid2];

                else

                {

                    //if (user1Name=="" && user2Name=="")

                    // return null;

                    //else

                    room=new ChatRoom(user1ID,user1Name,user2ID,user2Name);

                    Rooms.Add(rid1,room);

                }

            }

            Monitor.Exit(Rooms);

            return room;
        }