<SerializedListObjects>
   <A i:type="B">
   <A i:type="C">
</SerializedListObjects>

<SerializedListObjects>
   <B>
   <C>
</SerializedListObjects>

[Serializable]
  [DataContract(Name = "A")]
  public abstract class A
  {
  }

[Serializable]
  [DataContract(Name = "B")]
  public class B : A
  {
  }