public void Delete(int id, Action <Exception> callback)
        {
            try
            {
                using (ParticipantsEntities _entities = new ParticipantsEntities())
                {
                    var toBeRemoved = _entities.Participants.Single(item => item.Id == id);
                    _entities.Participants.Remove(toBeRemoved);

                    _entities.SaveChanges();

                    callback(null);
                }
            }
            catch (Exception e)
            {
                callback(e);
            }
        }
        public void Update(Participant participant, Action <Exception> callback)
        {
            try
            {
                using (ParticipantsEntities _entities = new ParticipantsEntities())
                {
                    var old = _entities.Participants.Single(item => item.Id == participant.Id);
                    old.FirstName = participant.FirstName;    // Not nice, but demo purpose only... ;-)
                    old.LastName  = participant.LastName;
                    old.Company   = participant.Company;

                    _entities.SaveChanges();

                    callback(null);
                }
            }
            catch (Exception e)
            {
                callback(e);
            }
        }