After the DataContract type implements the IExtensibleDataObject interface, an ExtensionDataObject property is added; this property plays an important role in forward-compatible serialization. WCF will use DataContractSerializer for DataContract type serialization/deserialization. When DataContractSerializer finds that a certain type (used for operation parameters or return value) has implemented the IExtensibleDataObject interface, it will store any data (this is obtained from the message stream during deserialization) that doesn't have corresponding property/fields in the type definition into the ExtensionDataObject property so that these data will not get lost. And if the deserialized instance (with some unknown data stored in ExtensionDataObject) is serialized into the message later, DataContractSerializer will write out ExtensionDataObject into the message stream again. This ensures that the data in the new version of DataContract can be consumed by the service/client with the old type definition correctly, instead of raising unexpected type, mismatching, or serialization exceptions.

The following modified data type can be consumed by the service/client that has the old definition, as explained earlier, without synchronizing the DataContract type definition:

[DataContract]
    public class FCQuestion : IExtensibleDataObject
    {
        [DataMember]
        public string Subject { get; set; }
        [DataMember]
        public string Answer { get; set; }
 [DataMember]
 public string Comment { get; set; }

        public ExtensionDataObject ExtensionData
        {            get; set;       }
    }

Currently, using the IExtensibleDataObject interface can make the DataContractSerializer preserve unknown data properties/fields when deserializing/serializing custom data types. However, the ExtensionDataObject property is an opaque object to developers and we do not have means to manually read the data stored in it. In case we want to manually extract the additional unknown property/fields, we can consider directly accessing the underlying SOAP message via MessageInspector or other extension points.