How to Serialize list<objects> using DataContractSerializer

Library shared between server and client contains interface :

[ServiceContract(SessionMode=SessionMode.Required, CallbackContract=typeof(IClient))]
public interface IServer
{

     // Node interface methods
    [OperationContract(Name = "UploadResults", IsOneWay = true)]
    void UploadResults(string result);
}

Server code implementing the interface :

public void UploadResults(string result)
{
  try
  {
    // Get portfolio from stream
    DataContractSerializer serializer = new DataContractSerializer(typeof(MyClass));
    using (MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(result)))
    {
      MyClass class = (MyClass)serializer.ReadObject(ms);

      .....   
    }
  }
  catch (Exception ex)
  {
  }
}

Client code calling Server:

 // Send myclass object to server
  try
  {
    // Write portfolio to stream
    DataContractSerializer serializer = new DataContractSerializer(typeof(MyClass));
    using (var stream = new MemoryStream())
    {
      serializer.WriteObject(stream, myclassObj开发者_如何学运维);

      // Send result back to server
      string result = Encoding.UTF8.GetString(stream.GetBuffer(), 0, (int)stream.Position);
      Server.UploadResults(result);
    }
  }
  catch (Exception ex)
  {
  }

MyClass :

  [DataContract]
  [KnownType(typeof(AClass))]
  [KnownType(typeof(List<AClass>))]
  public class MyClass
  {
    [DataMember]
   public List<AClass> History { get; set; }

    [DataMember]
   public int Count { get; set; }
  }

AClass :

[DataContract]

public class AClass
{
  [DataMember]
 public int x;
 [DataMember]
  public int y;
}