gistfile1.cs

public abstract class Thingoe {
  public Thingoe(string id, int quantity) { ... }
  public string Id { get; private set; }
  public int Quantity { get; private set; }
}

public class WidgetThingoe : Thingoe {
  public WidgetThingoe(string id, int quantity) : base(id,quantity) {}
}
public class OtherThingoe : Thingoe {
  public OtherThingoe(string id, int quantity) : base(id,quantity) {}
}

@mausch,
Closed type is fine. We'd like to be able to work with ids and quantities across all thingoes, but also restrict certain operations to specific types of thingoes.

e.g. (in haskellish)

getIds :: Thingoe -> String
widgets :: [WidgetThingoe]

widgetIds :: [WidgetThingoe] -> [String]
widgetIds = map getIds

I think I'm after structural typing?

The motivation for this is porting some existing code that uses sub-typing, and I'm trying to think of alternative representations that have the same power.