Why am I getting the exception "Consider using a DataContractResolver or add any types not known statically to the list of known types"

Question

I'm trying to serialise a object to Xml using the DataContractSerializer. I have the following classes;

[ActiveRecord(Lazy = true)]
[KnownType(typeof(RoomType))]
[DataContract]
public class Room : ActiveRecordBase<Room>
{
    [PrimaryKey]
    [DataMember]
    public virtual Int64 RoomId { get; protected set; }

    [BelongsTo("RoomTypeId")]
    [DataMember]
    public virtual RoomType RoomType { get; set; }

    [Property]
    [DataMember]
    public virtual Int64 HotelId { get; set; }

    [Property]
    [DataMember]
    public virtual string Name { get; set; }

    [Property]
    [DataMember]
    public virtual string Description { get; set; }

    public static Room[] FindByHotelId(Int64 HotelId)
    {
        return (Room[])FindAllByProperty(typeof(Room), "HotelId", HotelId);
    }
}

The RoomType class is

[ActiveRecord(Lazy = true)]
[DataContract]
public class RoomType : ActiveRecordBase<RoomType>
{
    [PrimaryKey]
    [DataMember]
    public virtual int RoomTypeId { get; protected set; }

    [Property]
    [DataMember]
    public virtual string Name { get; set; }

}

I use the following method to serialise the object

    internal static XElement ObjectToXElement<T>(T source)
    {
        XDocument oXDocument = new XDocument();

        try
        {
            using (var writer = oXDocument.CreateWriter())
            {
                // write xml into the writer
                var serializer = new DataContractSerializer(source.GetType());
                serializer.WriteObject(writer, source);
            }
        }
        catch(Exception e)
        {
            using (var writer = oXDocument.CreateWriter())
            {
                // write xml into the writer
                var serializer = new DataContractSerializer(oError.GetType());
                serializer.WriteObject(writer, oError);
            }
        }

        return oXDocument.Root;
    }

The actual object I'm serialising is;

[KnownType(typeof(List<Room>))]
[KnownType(typeof(RoomType))]
[DataContract]
public class RoomTypeResponse
{
    [DataMember]
    public int Code { get; set; }

    [DataMember]
    public string Message { get; set; }

    [DataMember]
    public List<Room> Rooms { get; set; }

    public RoomTypeResponse()
    {
        this.Rooms = new List<Room>();
    }
}

But for some reason when I call the method to serialise the object I get the following exception;

Type 'Castle.Proxies.RoomTypeProxy' with data contract name 'RoomTypeProxy:http://schemas.datacontract.org/2004/07/Castle.Proxies' 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.

If I comment out the property in Room class, it works fine

[BelongsTo("RoomTypeId")]
[DataMember]
public virtual RoomType RoomType { get; set; }

I'm not sure why I am getting the exception, because I've added the knowtype attribute for RoomType ? What am I missing, that is causing this problem.

Answer 1

The problem is that one type (Castle.Proxies.RoomTypeProxy) is generated at runtime, so .NET knows nothing about it. This is not an NHibernate-specific problem. If you disable lazy loading and proxy generation, the problem will go away, but I understand it might be difficult. Other option would be to use another serializer, like BinaryFormatter, but I don't know if that will work for you.