public class WCallUpdate : NotificationData
{
    private string m_from = "";
    [DataMember]
    public string From
    {
        get { return m_from; }
        set { m_from = value; }
    }
    private WCall m_wCall = new WCall();
    [DataMember]
    public WCall Call
    {
        get { return m_wCall; }
        set { m_wCall = value; }
    }
}

/// <summary>
/// Basic class used in the notification service
/// </summary>
[DataContract]
public class NotificationData
{
}

/// <summary>
/// Enum containing all the events used in the application
/// </summary>
[DataContract]
public enum NotificationTypeKey
{
    [EnumMember]
    Default = 0,
    [EnumMember]
    IWorkQueueServiceAttributionAddedEvent = 1,
    [EnumMember]
    IWorkQueueServiceAttributionUpdatedEvent = 2,
    [EnumMember]
    IWorkQueueServiceAttributionRemovedEvent = 3,
}

#region Create Message
    /// <summary>
    /// Creates a memoryStream from a notificationData
    /// note: we insert also the notificationTypeKey at the beginning of the
    /// stream in order to treat the memoryStream correctly on the client side
    /// </summary>
    /// <param name="notificationTypeKey"></param>
    /// <param name="notificationData"></param>
    /// <returns></returns>
    public MemoryStream CreateMessage(NotificationTypeKey notificationTypeKey, NotificationData notificationData)
    {
        MemoryStream stream = new MemoryStream();
        BinaryFormatter formatter = new BinaryFormatter();
        try
        {
            formatter.Serialize(stream, notificationTypeKey);
            formatter.Serialize(stream, notificationData);
        }
        catch (Exception ex)
        {
            Logger.Exception(ex);
        }
        return stream;
    }
    #endregion

WCallUpdate  m_wCallUpdate = new WCallUpdate();
NotificationTypeKey  m_notificationTypeKey = new NotificationTypeKey.Default;
CreateMessage(notificationTypeKey , wCallUpdate );

System.Runtime.Serialization.SerializationException: Type 'Xxx.DataContracts.WCall' in Assembly 'Xxx.DataContract, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.
   at System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type)
   at System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context)
   at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo()
   at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter)
   at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter)
   at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Write(WriteObjectInfo objectInfo, NameInfo memberNameInfo, NameInfo typeNameInfo)
   at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck)
   at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck)
   at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph)
   at Xxx.Notification.NotificationMessageFactory.CreateMessage(NotificationTypeKey notificationTypeKey, NotificationData notificationData) in Xxx.NotificationNotificationCenter.cs:line 36

#region NotificationData
/// <summary>
/// Basic class used in the notification service
/// </summary>
[Serializable]
[DataContract]
public class NotificationData
{
}
#endregion

[Serializable]
public class WCallUpdate : NotificationData
{
    private string m_from = "";
    [DataMember]
    public string From
    {
        get { return m_from; }
        set { m_from = value; }
    }
    private WCall m_wCall = new WCall();
    [DataMember]
    public WCall Call
    {
        get { return m_wCall; }
        set { m_wCall = value; }
    }
}

#region DataContractSerializer
        /// <summary>
        /// Creates a Data Contract Serializer for the provided type. The type must be marked with
        /// the data contract attribute to be serialized successfully.
        /// </summary>
        /// <typeparam name="T">The type to be serialized</typeparam>
        /// <returns>A data contract serializer</returns>
        public static DataContractSerializer CreateDataContractSerializer<T>() where T : class
        {
            DataContractSerializer serializer = new DataContractSerializer(typeof(T));
            return serializer;
        }
        #endregion

AA D1=new AA(); //Derived type
BB D2=new BB(); //Derived type
CC D3=new CC(); //Derived type
X D4=new X(); //Base Class

XList<X> AllData=new XList<X>();
AllData.Add(D1);
AllData.Add(D2);
AllData.Add(D3);
AllData.Add(D4);
// -----------------------------------
AllData.Save(@"C:TempDemo.xml");
// -----------------------------------
// Retrieve data from XML file
// -----------------------------------
XList<X> AllData=new XList<X>();
AllData.Open(@"C:TempDemo.xml");
// -----------------------------------