回答編集履歴

  [XmlSerializer.UnknownNode イベント (System.Xml.Serialization) | Microsoft Docs](https://docs.microsoft.com/ja-jp/dotnet/api/system.xml.serialization.xmlserializer.unknownnode)

  これはどうでしょう？

  提示の例ではエラーにできました。

 

  public class Team

  {

  public List<Person> list;

 

  public static Team Load(string filepath)

  {

  try

  {

  var serializer = new XmlSerializer(typeof(Team));

  serializer.UnknownNode += (s, e) => throw new Exception();

  //serializer.UnknownAttribute += (s, e) => throw new Exception();

  //serializer.UnknownElement += (s, e) => throw new Exception();

 

  using(var sr = new StreamReader(filepath, new UTF8Encoding(false)))

  {

  return (Team)serializer.Deserialize(sr);

  }

  }

  catch

  {

  return null;

  }

  }

  }

  ```