DataContractSerialization works for one object, fails for a similar one - datacontractserializer

Question

I serialize successfully the following class
[DataContract]
public class Event
{
[DataMember] public virtual string EventId { get; set; }
[DataMember] public virtual string Name { get; set; }
[DataMember] public virtual string Editable { get; set; }
[DataMember] public virtual DateTime? CreatedOn { get; set; }
[DataMember] public virtual Function Function { get; set; }
[DataMember] public virtual User CreatedBy { get; set; }
[DataMember] public virtual IList<EventParameter> Parameters { get; set; }
[DataMember] public virtual IList<EventParameterOption> Options { get; set; }
public Event()
{
Parameters = new List<EventParameter>();
Options = new List<EventParameterOption>();
}
}
but the same code produces an exception when serializing the following
[DataContract]
public class Attribute
{
[DataMember] public virtual string AttributeId { get; set; }
[DataMember] public virtual string Name { get; set; }
[DataMember] public virtual string Description { get; set; }
[DataMember] public virtual string Status { get; set; }
[DataMember] public virtual Function Function { get; set; }
[DataMember] public virtual Category Category { get; set; }
[DataMember] public virtual string Editable { get; set; }
[DataMember] public virtual string Column { get; set; }
[DataMember] public virtual DateTime? LastExecution { get; set; }
[DataMember] public virtual DateTime? LastUpdate { get; set; }
[DataMember] public virtual DateTime? CreatedOn { get; set; }
[DataMember] public virtual User CreatedBy { get; set; }
[DataMember] public virtual IList<AttributeParameter> Parameters { get; set; }
public Attribute()
{
Parameters = new List<AttributeParameter>();
}
}
and the exception points to the User property of the Attribute class.
Type 'UserProxy' with data contract name 'UserProxy:http://schemas.datacontract.org/2004/07/' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
The code I use for serializing is
public static byte[] SerializeObjectDataContract(object obj)
{
using (var stream = new MemoryStream())
{
var serializer = new DataContractSerializer(obj.GetType());
serializer.WriteObject(stream, obj);
stream.Position = 0;
return stream.ToArray();
}
}
I really don't understand what differs between the two serializations. Removing the DataMember attribute from the User property allows the serialization to complete, however I do need to serialize the User too.