NHibernate: use of IEnumerable as collection type results in error

Question

I have a class which uses an ISet as a collection type as below:

  public class Client
  {
    private ISet<Contact> _contacts = new HashedSet<Contact>();
    public virtual ISet<Contact> Contacts { get { return _contacts; } }
  }

I don't want the collection itself to be able to be modified externally. However, if I change the property's type to IEnumerable as below:

  public class Client
  {
    private ISet<Contact> _contacts = new HashedSet<Contact>();
    public virtual IEnumerable<Contact> Contacts { get { return _contacts; } }
  }

Then whenever I try to use this class I get the NHibernate error:

System.InvalidCastException: Unable to cast object of type 'NHibernate.Collection.Generic.PersistentGenericBag1[Kctc.BusinessLayer.ProbateAssist.Entities.Contact]' to type 'Iesi.Collections.Generic.ISet1[Kctc.BusinessLayer.ProbateAssist.Entities.Contact]'.

What gives? How can I make the public version of the collection read only?

NB I've also tried to use ReadOnlyCollection, and get the same error.

You should show what your nhibernate config files look like (or what is your fluent configuration if you use FluentNhibernate) — samy, Oct 26 '10 at 21:22

score 1 · Accepted Answer · answered Oct 27 '10 at 02:16

1

You are probably using Fluent automapping, which is guessing the type as Bag instead of Set based on the interface type. You'll have to override it in that case.

IEnumerable<T> works fine when mapping manually or with XML.

answered Oct 27 '10 at 02:16

Diego Mijelshon

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Brilliant! I would never have guessed it was a mapping problem. Thank you. – David Oct 27 '10 at 07:58
For posterity, I fixed this problem by adding .AsSet() to the end of the HasMany line for the Contacts collection in the Client mapping file. – David Oct 27 '10 at 07:59

NHibernate: use of IEnumerable as collection type results in error

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