Fluent NHibernate: cannot cast PersistentGenericBag to type 'System.Collections.Generic.ISet`

Question

I have an object Topic which is a self-related hierarchy where the child Topics are defined as

public class Topic : Entity {
   public ISet<Topic> ChildTopics { get; internal set; }
   public Topic ParentTopic { get; set; }
   ...
}

I'm writing a form (MVC3) to produce a drop-down list (Html.DropDownListFor) of the first-level topics (theoretically this will eventually AJAX into a second drop-down for second-level topics), but when it goes to save, it produces the ever-popular "Cannot cast..." exception (see question title).

The usual cause of this is that you used List or Set instead of IList or ISet, but I am using ISet, and it specifically says it can't cast to ISet.

The reason this is a set is because you wouldn't want a Topic being a child of another Topic more than once. The table mapping create by Fluent NH automapping is correct with this override:

mapping.HasMany<Topic>(t => t.ChildTopics).AsSet().Inverse().KeyColumn("TopicId");

I believe this is an issue with using ASP.Net 4.0 System.Collections.Generic.SortedSet instead of Iesi.Collections.Generic.SortedSet. I have a zillion other issues at the moment, so I can't verify that is completely the issue, but if/when I do verify that was the case, I'll be back. — Carl Bussema
IESI.Collections is not really required with newer versions of NHibernate, they can use the .net framework's ISet interface. Dunno if that helps. — tom.dietrich

Carl Bussema Carl Bussema · Accepted Answer · 2012-03-09T19:16:42

In my project, as of NHibernate 3.2.0.400, this error still occurs if I use System.Collections.Generic.ISet<T> instead of Iesi.Collections.Generic.ISet<T>. Simply changing the references around is minimal work and solves the problem.

Fluent NHibernate: cannot cast PersistentGenericBag to type 'System.Collections.Generic.ISet`

1 Answers