Assembly: System.Runtime.Serialization (in System.Runtime.Serialization.dll)
The interface provides a single property that sets or returns a structure used to store data that is external to a data contract. The extra data is stored in an instance of the ExtensionDataObject class and accessed through the ExtensionData property. In a roundtrip operation where data is received, processed, and sent back, the extra data is sent back to the original sender intact. This is useful to store data received from future versions of the contract. If you do not implement the interface, any extra data is ignored and discarded during a roundtrip operation.
To use this versioning feature
Implement the interface in a class.
Add the ExtensionData property to your type.
Add a private member of type ExtensionDataObject to the class.
Implement get and set methods for the property using the new private member.
For more information about versioning of types, see Data Contract Versioning. For information about creating forward-compatible data contracts, see Forward Compatible Data Contracts. For more information about data contracts, see Using Data Contracts.
The following code shows an instance of a type (PersonVersion2) that is the second version of a serializable type (Person). The second version contains extra data (ID field) that is not present in the first version.