C# – Why can’t I use WCF DataContract and ISerializable on the same class

I have a class that I need to be able to serialize to a SQLServer session variable and be available over a WCF Service. I have declared it as follows

namespace MyNM
{
[Serializable] 
[DataContract(Name = "Foo", Namespace = "http://www.mydomain.co.uk")]

public class Foo : IEntity, ISafeCopy<Foo>
{
    [DataMember(Order = 0)] 
    public virtual Guid Id { get; set; }

    [DataMember(Order = 1)] 
    public virtual string a { get; set; }

    DataMember(Order = 2)]
    public virtual Bar c { get; set; }

    /* ISafeCopy implementation */
}


[Serializable]
[DataContract(Name = "Bar ", Namespace = "http://www.mydomain.co.uk")]
public class Bar : IEntity, ISafeCopy<Bar>
{
    #region Implementation of IEntity

    DataMember(Order = 0)]
    public virtual Guid Id { get; set; }
    [DataMember(Order = 1)]
    public virtual Baz y { get; set; }

    #endregion

    /* ISafeCopy implementation*/
}


[Serializable]
[DataContract]
public enum Baz
{
    [EnumMember(Value = "one")]
    one,
    [EnumMember(Value = "two")]
    two,
    [EnumMember(Value = "three")]
    three
}

But when I try and call this service, I get the following error in the trace log.

"System.Runtime.Serialization.InvalidDataContractException:
Type
'BarProxybcb100e8617f40ceaa832fe4bb94533c'
cannot be ISerializable and have
DataContractAttribute attribute."

If I take out the Serializable attribute, the WCF service works, but when the object can't be serialized to session. If I remove the DataContract attribute from class Bar, the WCF service fails saying

Type
'BarProxy3bb05a31167f4ba492909ec941a54533'
with data contract name
'BarProxy3bb05a31167f4ba492909ec941a54533:http://schemas.datacontract.org/2004/07/'
is not expected. 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

I've tried adding a KnownType attribute to the foo class

[KnownType(typeof(Bar))]

But I still get the same error.

Can anyone help me out with this?

Many thanks

Dave