Question

Should a lazy loaded collection in NHibernate ever give me a NullReferenceException? I’m getting an exception in a method like the following:

public void Test(ISession session, int id)
{
    var entity = session.Load<MyEntity>(id);
    entity.LazyLoadedCollection.Add(SomeItem);
}

The call to LazyLoadedCollection is throwing. My mapping looks like:

<bag lazy="true" table="MyTable">
  <key>
    <column name="LazyLoadedCollection" />
  </key>
  <many-to-many class="LazyLoadedItem">
    <column name="LazyLoadedItemId" />
  </many-to-many>
</bag>

Is this the expected behavior in NHibernate?

Answer 1

It’s hard to say without seeing your class, but one thing you may not have realized is that you need to populate each collection in your class’s constructor.

NHibernate will replace these collections with its own at certain times, but you still need to make sure they’re initially populated with a HashedSet<T>, List<T>, or something else depending on the interface you’re using.