ルールが変わったんだよ

・Types should not extend certain base types

For .NET Framework version 1, it was recommended to derive new exceptions from ApplicationException. The recommendation has changed and new exceptions should derive from System.Exception or one of its subclasses in the System namespace.

例外クラスはApplicationExceptionから派生するなとのこと。FxCop先生の指導が厳しい・・・