public string writePost(string forumName, string subForumName, int parentPostNo, string username, string title,
                                string content)
        {
            List <string> input = new List <string>()
            {
                forumName, subForumName, username, title, content
            };

            if (Constants.isValidInput(input) && Constants.isValidPost(title, content))
            {
                lock (o)
                {
                    User user = _um.getUserFromForum(forumName, username);
                    if (user == null)
                    {
                        return("Wrong forum name or username.");
                    }
                    return(_fm.writePost(forumName, subForumName, parentPostNo, user, title, content));
                }
            }
            return(Constants.INVALID_INPUT);
        }