public class WrapperClass
    {
        private List<CollectionClass> _collectionClasses;

        public int Count
        {
            get { return _collectionClasses.Count; }
        }

        public CollectionClass this[int index]    // Indexer declaration
        {
            get
            {
                return _collectionClasses[index];
            }
        }
   
        public void Add(CollectionClass collectionClass)
        {
            _collectionClasses.Add(collectionClass);
        }

        protected internal WrapperClass()
        {
            _collectionClasses = new Collection<CollectionClass>();
        }
   }

public class Contact
{
    // Accessed directly by NHibernate;
    // must be named the same as the typed public property
    // except using camelcase-underscore
    private IList _addresses;

    private ContactAddressCollection _addressesTyped;

    public virtual ContactAddressCollection Addresses
    {
        get
        {
            if (this._addressesTyped == null)
            {
                // first access of strongly typed collection;
                // wrap weakly typed collection used by NHibernate
                this._addressesTyped = new ContactAddressCollection(this._addresses, ...);
            }
            else
            {
                // Reassign the weakly typed collection to the typed collection
                // just in case NHibernate recreated it ...
                this._addressestyped.ExternalCollection = this.AddressesUntyped;
            }
            return this._addressesTyped;
        }
    }

    // Used only by public typed collection property
    private IList AddressesUntyped
    {
        get
        {
            if (this._addresses == null)
            {
                // Initialize it in case NHibernate hasn't set it yet
                this._addresses = new ArrayList();
            }
            return this._addresses;
        }
    }
}

<hibernate-mapping
    ...
    <class name="Contact" ... >
        ...
        <bag name="Addresses" inverse="true" lazy="true" cascade="all-delete-orphan" access="field.camelcase-underscore">
            <key column="..." />
            <one-to-many class="ContactAddress"/>
        </bag>
    </class>
</hibernate-mapping>