// Add a token-user line in TokenUser (remove and put the new token if a token is already recorded)
 private void addTokenUser(String token, String username)
 {
     removePossibleToken(username);
     if (!isTokenExisting(token))
     {
         TokenUser.TryAdd(token, username);
     }
     else
     {
         throw new TokenExistingException("This token already exist!");
     }
 }
        //Return the new client token in order to return it to the client
        public String addNewUser(User user, Client client)
        {
            if (isUsernameExisting(user.Username))
            {
                throw new UsernameTakenException("This user is already taken!");
            }

            String token = generateToken();

            RegisteredMembers.TryAdd(user.Username, user);
            TokenUser.TryAdd(token, user.Username);
            OnlineUsers.TryAdd(token, client);

            saveUser(user.Username);

            return(token);
        }