Create ths DB schema (using SQLite3 and System.Data.SQLite, but the same happens with MS SQL Server 2008)

create table Parent (
id integer primary key autoincrement,
Name varchar(50)
);
create table Child (
id integer primary key autoincrement,
Name varchar(50),
parentid integer,
foreign key (parentid) references Parent (id)
);

Populate is as follows

sqlite> select * from Parent;
1|First parent

sqlite> select * from Child;
1|First child|1
2|Second child|1
3|Third child|1

Define the following class Parent

namespace NhibernateExperiments
{
using System.Collections.Generic;

using Iesi.Collections.Generic;

/// <summary>
/// TODO: Update summary.
/// </summary>
public class Parent
{
private readonly Iesi.Collections.Generic.ISet<Child> children;

private int id;

public Parent(string name)
{
this.Name = name;
this.children = new HashedSet<Child>();
}

private Parent()
{
}

public string Name { get; private set; }

public IEnumerable<Child> Children
{
get
{
return this.children;
}
}
}
}

Define the following class Child

namespace NhibernateExperiments
{
/// <summary>
/// TODO: Update summary.
/// </summary>
public class Child
{
private int id;

public Child(string name)
{
Name = name;
}

private Child()
{
}

public string Name { get; private set; }
}
}

Define the following mapping. Note the <loader> tag used to load the collection using a named query

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="nhibernateExperiments"
namespace="NhibernateExperiments">

<class name="Parent" lazy="false">
<id name="id" access="field">
<generator class="native"/>
</id>
<property name="Name" />
<set name="children" lazy="true" access="field">
<key column="parentid"/>
<one-to-many class="Child"/>
<loader query-ref="GetChildren"/>
</set>
</class>

<class name="Child" lazy="false">
<id name="id" column="id" access="field">
<generator class="native"/>
</id>
<property name="Name" />
</class>

<sql-query name="GetChildren" xml:space="preserve">
<return class="Child"/>
SELECT
children0_.parentid as parentid1_,
children0_.id as id,
children0_.id as id1_0_,
children0_.Name as Name1_0_
FROM Child children0_
WHERE children0_.parentid=?
</sql-query>

</hibernate-mapping>

And this is the main program

namespace NhibernateExperiments
{
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;

using NHibernate;
using NHibernate.Cfg;

using Environment = NHibernate.Cfg.Environment;

public class Program
{
public static void Main(string[] args)
{
Configuration configuration = new Configuration()
.SetProperties(new Dictionary<string, string>
{
{ Environment.ConnectionDriver, "NHibernate.Driver.SQLite20Driver" },
{ Environment.ConnectionString, "Data Source=h:
parentchild;Version=3" },
{ Environment.Dialect, "NHibernate.Dialect.SQLiteDialect" },
{ Environment.QuerySubstitutions, "true=1;false=0" },
{ Environment.ShowSql, "true" },
}).AddAssembly(Assembly.GetExecutingAssembly());
ISessionFactory buildSessionFactory = configuration.BuildSessionFactory();
ISession session = buildSessionFactory.OpenSession();
Parent parent = session.QueryOver<Parent>().Where(p => p.Name == "First parent").SingleOrDefault<Parent>();
Console.Out.WriteLine("Children of " + parent.Name + " are " + string.Join(",", parent.Children.Select(c => c.Name)));
}
}
}

Result is a NullReferenceException {"Object reference not set to an instance of an object."}

> NHibernate.dll!NHibernate.Collection.Generic.PersistentGenericSet<NhibernateExperiments.Child>.System.Collections.Generic.IEnumerable<T>.GetEnumerator() Line 230 C#
[External Code]
nhibernateExperiments.exe!NhibernateExperiments.Program.Main(string[] args) Line 29 + 0x133 bytes C#

Member NHibernate.Collection.Generic.PersistentGenericSet<T>.gset is null because never initialized: since the collection is loaded using a NHibernate.Persister.Collection.NamedQueryCollectionInitializer instead of the normal CollectionLoader, NHibernate.Engine.Loading.CollectionLoadContext.GetLoadingCollection() is not called and gset is not initialized.