How to pass a Derived Class's fields over WCF?

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C#

.NET

WCF

I have derived class "bar" that has additional fields to the base class. The WCF service must accept the base class type (because there are several derived classes that have to use the same service, each derived class will have additional fields). I need to transfer the derived class's fields over WCF, below is an example.

[KnownType(typeof(bar))]
[DataContract]
public class fooBase
{
[DataMember]
    public string a;
}

[DataContract]
public class bar : fooBase
{
    [DataMember]
    public string b;
}

My WCF service accepts fooBase. When the client passes an instance of bar to the service, a is populated with the correct data, but b is always null.

Is it possible for WCF to transfer the fields of derived classes? If so, how? If not, is this possible with Remoting?

Thanks!