C# - NHibernate unable to cast NHibernate.Collection.Generic.PersistentGenericSet to System.Collections.Generic.IList

Question

For some reason, NHibernate is telling me it can't convert NHibernate.Collection.Generic.PersistentGenericSet[Ingredient] to System.Collection.Generic.IList[Ingredient], when I try to get the data from the database. This is a simplfied version of my class mapping/implementation:

public class Product {
    protected Product() { };

    public virtual Name { get; set; }
    public virtual IList<Ingredient> {
        get { return new List<Ingredient>(ingredients).AsReadOnly(); }
        protected set { ingredients = value; }
    }

    private IList<Ingredient> ingredients = new List<Ingredient>();
}

--

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
    <class name="LanchesAdmin.Core.Domain.Product, LanchesAdmin.Core" table="product" >
        <id name="ID" generator="identity">
            <column name="id" not-null="true" />
        </id>

        <property name="Name" column="name" length="64" not-null="true" />

        <set name="Ingredients" table="ingredient" fetch="join" lazy="true" inverse="true">
            <key column="product_id" />
            <one-to-many class="LanchesAdmin.Core.Domain.Ingredient, LanchesAdmin.Core" />
        </set>
    </class>
</hibernate-mapping>

I've seen several related questions, but they all tell me to use IList, and that's exactly what I'm doing.

Answer 1

The reason is that you are using a set. And PersistentGenericSet<T> simply doesn't implement the IList<T> interface.

I think you should simply make the property of type IEnumerable<Ingredient>.
This solves two problems:

1. You get rid of the error you mentioned
2. The API of your class directly communicates that consumers of your class aren't allowed to add ingredients via that property.

Answer 2

Either change type of the property to ISet from IList.

Or change mapping from <set> to <list>.

Answer 3

Do you want a list, bag or set? Your domain model contains a IList<T> but you've mapped it as a <set>.

Either change your mapping to a <bag> or <list> or change your domain model to use a ISet<T>.