protobuf-net Serializing as base class
[ProtoContract]
class Base
{
    [ProtoMember(1)]
    public string PublicInfo { get; set; }
}

class Derived : Base
{
    public string SecretInfo { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        Derived d = new Derived()
        {
            PublicInfo = "public info",
            SecretInfo = "secret info"
        };
        using (var ms = new MemoryStream())
        {
            Serializer.NonGeneric.Serialize(ms, d as Base);
            ms.Seek(0, SeekOrigin.Begin);
            Base deserialized = Serializer.Deserialize<Base>(ms);
            Console.WriteLine("Deserialized type: " + deserialized.GetType());
            Console.WriteLine("Deserialized value: " + deserialized.PublicInfo);
        }
        Console.ReadLine();
    }
}

Deserialized type: Base
 Deserialized value: public info

[ProtoContract] class Generic {
  [ProtoMember(1)] public string PublicInfo { get; set; }
}

class Specialized {
   public Generic Generic { get; set; }
   public string SecretInfo { get; set; }
}

namespace NHibernate.Proxy {
    internal interface INHibernateProxy {} // pretty spectacularly evil
}
...
class Derived : Base, INHibernateProxy {}