<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="Entities" namespace="Entities.Business">
    <class name="ConstructionElement" table="CNSTR_ELEM">
        <id name="Id" column="ID" type="Int64">
            <generator class="sequence">
       <param name="sequence">CNSTR_ELEM_SEQ</param>
    </generator>
        </id>

    <component name="MaterialDescription" class="Entities.Base.MultiNameEntity">
      <bag name="NamesI" table="CNSTR_ELEM_CD" lazy="false" cascade="all">
        <key column="PARENT_ID"/>
        <composite-element class="Entities.Localization.LocalizedString, Entities">
          <property column="CULTURE_ID" name="Language" type="Common.Enums.RequestedLanguage, Common"/>
          <property column="TEXT" name="Value" type="System.String"/>
        </composite-element>
      </bag>
    </component>
    </class>
</hibernate-mapping>

public class MultiNameEntity
{
        private ObservableCollection<LocalizedString> _names;
        private IList<LocalizedString> _namesI;
      
        protected virtual IList<LocalizedString> NamesI
        {
            get
            {
                if (_names != null)
                    return _names;
                return _namesI;
            }
            set { _namesI = value; }
        }

        public virtual ObservableCollection<LocalizedString> Names
        {
            get
            {
                if (_names == null)
                {
                    if (_namesI != null && NHibernateUtils.IsInitialized(_namesI))
                    {
                        _names = new ObservableCollection<LocalizedString>();
                        _names.AddRange(_namesI);
                    }
                }
                return _names;
            }
            set { _names = value; }
        }
}

Unable to cast object of type 'System.Collections.ObjectModel.ObservableCollection`1[Entities.Localization.LocalizedString]' to type 'NHibernate.Collection.IPersistentCollection'.

public class ConstructionElement : Entity
{
   public virtual MultiNameEntity MaterialDescription { get; set; }      
}   

public abstract class Entity : IEntity
{
   public virtual long Id {get; set;}
}

Unable to cast object of type 'System.Collections.Generic.List`1[Entities.Localization.LocalizedString]' to type 'NHibernate.Collection.IPersistentCollection'.

       ITransaction transaction=session.BeginTransaction();
       ConstructionElement ce = new ConstructionElement();
        MultiNameEntity me =new MultiNameEntity();
        LocalizedString ls = new LocalizedString();
        ls.Language = Language.English;
        ls.Text = "test";
        me.Names = new List<LocalizedString>();
        me.Names.Add(ls);
        ce.MaterialDescription = me;
        session.Save(ce);
        transaction.Commit();