I am trying to serialize a ItemTransaction and protobuf-net (r282) is having a problem. ItemTransaction : IEnumerable<KeyValuePair<Type, IItemCollection>></code> and ItemCollection is like this: FooCollection : ItemCollection<Foo> ItemCollection<T> : BindingList<T>, IItemCollection IItemCollection : IList<Item> where T is a derived type of Item. ItemCollection also has a property of type IItemCollection. I am serializing like this: IItemCollection itemCol = someService.Blah(...); ... SerializeWithLengthPrefix<IItemCollection>(stream, itemCol, PrefixStyle.Base128); My eventual goal is to serialize ItemTransaction, but am snagged with IItemCollection. Item and it's derived types can be [de]serialized with no issues, see [1], but deserializing an IItemCollection fails (serializing works). ItemCollection has a ItemExpression property and when deserializing protobuf can't create an abstract class. This makes sense to me, but I'm not sure how to get through it. ItemExpression<T> : ItemExpression, IItemExpression ItemExpression : Expression ItemExpression is abstract as is Expression How do I get this to work properly? Also, I am concerned that ItemTransaction will fail since the IItemCollections are going to be differing and unknown at compile time (an ItemTransaction will have FooCollection, BarCollection, FlimCollection, FlamCollection, etc). What am I missing (Marc) ? [1] http://stackoverflow.com/questions/2276104/protobuf-net-deserializing-across-assembly-boundaries