C#的NHibernate初始化ISessionFactory

private static ISessionFactory _sessionFactory;

    public static ISessionFactory InitSessionFactory()
    {
        _sessionFactory = Fluently.Configure()
            .Database(MySQLConfiguration.Standard.ConnectionString(db => db
            .Server("localhost")
            .Username("root")
            .Password("root")
            .Database("bailaodadb")))
            .Mappings(x => x.FluentMappings.AddFromAssemblyOf<NHibernateHelper>())
            .BuildSessionFactory();
        return _sessionFactory;
    }

    public static ISession OpenSession()
    {
        if (_sessionFactory==null)
        {
            _sessionFactory=InitSessionFactory();
        }
        return _sessionFactory.OpenSession();
    }

            /*异常提示：
                    _sessionFactory=null
         * 引发的异常:“FluentNHibernate.Cfg.FluentConfigurationException”(位于 FluentNHibernate.dll 中)
         * “FluentNHibernate.Cfg.FluentConfigurationException”类型的未经处理的异常在 FluentNHibernate.dll 中发生. 
           其他信息: An invalid or incomplete configuration was used while creating a SessionFactory. 
           Check PotentialReasons collection, and InnerException for more detail.
         */
                    我想问一下我的代码到底哪里错了，获取不到sessionfactory.

http://blog.csdn.net/jilongliang/article/details/50328973