i cannot understand what reson will cause this problem? code is below:

public class ConfigReader
{
private static WeakReference sfcache = new WeakReference(null);

private static WeakReference configCache = new WeakReference(null);


public static ISessionFactory SessionFactory
{
get
{
ISessionFactory _sessions = sfcache.Target as ISessionFactory;
if ( _sessions == null )
{
_sessions = ;
sfcache.Target = _sessions;
}

return _sessions;
}
}

public static Configuration Configuration
{
get
{
Configuration _config = configCache.Target as Configuration;
if ( _config == null )
{
_config = new Configuration().Configure();
configCache.Target = _config;
}

return _config;
}
}
}

public class XXX
{
public static IList query( string sql )
{
IList _list = null;
using (ISession _session = ConfigReader.SessionFactory.OpenSession())
{
_list = _session.Find( sql );
_session.Close();
}

return _list;
}
}