I am learning XML Serialization and meet an issue, I have two claess [System.Xml.Serialization.XmlInclude(typeof(SubClass))] public class BaseClass { } public class SubClass : BaseClass { } I am trying to serialize a SubClass object into XML file, I use blow code XmlSerializer xs = new XmlSerializer(typeof(Base)); xs.Serialize(fs, SubClassObject); I noticed Serialization succeed, but the XML file is kind of like ... If I use XmlSerializer xs = new XmlSerializer(typeof(Base)); SubClassObject = xs.Deserialize(fs) as SubClass; I noticed it will complain xsi:type is unknown attribute(I registered an event), although all information embedded in the XML was parsed successfully and members in SubClassObject was restored correctly. Anyone has any idea why there is error in parsing xsi:type and anything I did wrong? Thanks