Invalid Cast (check your mapping for property type mismatches)

Question

Scenario
I have a category class in DB:

CREATE TABLE [dbo].[Category](
    [pk_cat_id] [int] NOT NULL,
    [name] [varchar](50) NOT NULL,
    [parent_cat_id] [int] NULL
 CONSTRAINT [PK_Category] PRIMARY KEY NONCLUSTERED 
(
    [pk_cat_id] ASC
))

Category class has association to itself. It is a recursive, bidirectional association(many-to-one and one-to-many). Both refer to the same foreign key column: parent_cat_id.
A category can have one parent at-most and no or more child categories.

This is Category.hbm.xml:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns ="urn:nhibernate-mapping-2.2"
                   assembly ="NHibernateIntro.Core"
                   namespace ="NHibernateIntro.Core.Domain">

  <class name="Category" table="Category">

    <id name="CategoryId" column="pk_cat_id">
      <generator class="hilo"/>
    </id>

    <property name="Name" column="name" type="string" length="50" not-null="true" />

    <many-to-one name="ParentCategory" class="Category" column="parent_cat_id" />

    <bag name="childCategories" cascade="all-delete-orphan" inverse="true">
      <key column="parent_cat_id"/>
      <one-to-many class="Category"/>      
    </bag>

  </class>
</hibernate-mapping>

This is Category.cs:

using System;
using System.Collections.Generic;
using Iesi.Collections.Generic;

namespace NHibernateIntro.Core.Domain
{
    public class Category
    {
        private Category parent_category;
        private ISet<Category> child_Categories = new HashedSet<Category>();

        public virtual int CategoryId { get; set; }
        public virtual string Name { get; set; }

        public Category() { }

        public Category( string cat_name )
        {
            Name = cat_name;
        }

        public virtual Category ParentCategory 
        {
            get
            {
                if (parent_category == null)
                    parent_category = new Category();

                return parent_category;
            }
            set{ parent_category = value; }
        }

        public virtual ISet<Category> childCategories
        {
            get { return child_Categories; }
            set { child_Categories = value; }
        } 
    }
}

This is the Main method:

public static void Run(ISessionFactory factory)
{
    int computerId = 1;

    using (ISession session = factory.OpenSession())
    using (session.BeginTransaction())
    {
        Category computer = session.Get<Category>(computerId); // **This line causes   Error(stated below)**
// Please see 'CONFUSING' tag below.
            Category laptops = new Category("Laptops");
            computer.childCategories.Add(laptops);
            laptops.ParentCategory = computer;
            session.Save(laptops);
            session.Transaction.Commit();
        }
    }

CONFUSING : When i debugged the code it stuck here at this line: "set{ parent_category = value; }". I am confused because I am assigning to Cateory then why setter of parentCategory is being called here?

Error: Invalid Cast (check your mapping for property type mismatches); setter of NHibernateIntro.Core.Domain.Category

Inner Error: Unable to cast object of type'NHibernate.Collection.Generic.PersistentGenericBag1[NHibernateIntro.Core.Domain.Category]' to type 'Iesi.Collections.Generic.ISet1[NHibernateIntro.Core.Domain.Category]'.

kINDLY hELP!!

Answer 1

Change

private ISet<Category> child_Categories = new HashedSet<Category>();

to

private ICollection<Category> child_Categories = new HashSet<Category>();

and it should work. Note that I'm using the C# HashSet, not the Iesi.Collections HashedSet. More recent versions of NHibernate may support HashSet directly.

Answer 2

Use set in the mapping file instead of bag. Because ISet cannot be casted to IList and bag mapping is compatible with a .NET IList