[ServiceContract]
public interface IFooBarService
{
    [OperationContract]
    bool SetFooBar(FooBar fb);
}

[DataContract]
public class FooBar
{
    [DataMember]
    public string ExternalId;
    [DataMember]
    public string Something;
    [DataMember]
    public List<Foo> Foos;
    [DataMember]
    public List<Bar> Bars;
}