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Programming WCF Services, 2nd Edition by Juval Lowy

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Data Contract Hierarchy

Your data contract class may be a subclass of another data contract class. WCF requires that every level in the class hierarchy explicitly opt in for a given data contract, because the DataContract attribute is not inheritable:

[DataContract]
class Contact
{
   [DataMember]
   public string FirstName;

   [DataMember]
   public string LastName;
}
[DataContract]
class Customer : Contact
{
   [DataMember]
   public int OrderNumber;
}

Failing to designate every level in the class hierarchy as serializable or as a data contract will result in an InvalidDataContractException at the service load time. WCF lets you mix the Serializable and DataContract attributes in the class hierarchy:

[Serializable]
class Contact
{...}

[DataContract]
class Customer : Contact
{...}

However, the Serializable attribute will typically be at the root of the class hierarchy, if it appears at all, because new classes should use the DataContract attribute. When you export a data contract hierarchy, the metadata maintains the hierarchy, and all levels of the class hierarchy are exported when you make use of the subclass in a service contract:

[ServiceContract]
interface IContactManager
{
   [OperationContract]
   void AddCustomer(Customer customer); //Contact is exported as well
   ...
}

Known Types

In traditional object-oriented programming, a reference to a subclass is also a reference to its base class, so the subclass maintains an Is-A relationship with its base class. Any method that expects a reference to a base class ...

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