I left one thing unsaid in the serialization rules and Aaron's sharp eyes caught it promptly. As he mentioned in his blog, mixing interface programming model (such as IXmlSerializable or ISerializable) with DataContract programming model is disallowed in V1. Here is an example of such a class.
public class XmlDataContractType : IXmlSerializable
public int MyDataMember;
As Aaron mentioned we could have chosen IXmlSerializable in this case. I agree that it is natural for the interface to trump attribute since it impacts the derived types as well. However, this meant that the DataContract does not impact anyone.
On the other hand, picking DataContract enables scenarios where a type is serialized with multiple serializers. Both IXmlSerializable and ISerializable are programming models that are also supported by other serializers (XmlSerializer and BinaryFormatter). A user might want to make a type IXmlSerializable or ISerializable to be used by ‘legacy’ serializers and make it a DataContract type for WCF. But this approach fails as soon as we consider inherited types. Consider two derived types of XmlDataContractType as defined below.
public class DerivedXmlType : XmlDataContractType
public class DerivedDataContractType : XmlDataContractType
Class DerivedXmlType does not define DataContract but is an IXmlSerializable by virtue of inheritance. In this case IXmlSerializable projection of XmlDataContractType should be used. Since DerivedDataContractType is a DataContract it will end up using the DataContract projection of XmlDataContractType. We definitely did not want to encourage this dual projection of XmlDataContractType.