How to implement a custom type for NHibernate property

http://blog.miraclespain.com/archive/2008/Mar-18.html

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

  <class name="SoftInn.Domain.Archives.Archive" table="archive" lazy="false">   
    <!--Mapped classes must declare the primary key column of the database table. 
    Most classes will also have a property holding the unique identifier of an instance. 
    The <id> element defines the mapping from that property to the primary key column.-->
    <id name="Id" column="archive_id" type="Int64">
      <!--"column" is The name of the primary key column-->
      <generator class="SoftInn.Services.Internal.NHibernate.IdGenerator, SoftInn.Services">
        <param name="table">nextinstanceid</param>
        <param name="column">next</param>
        <param name="max_lo">9</param>
      </generator>
    </id>
    
    <version name="LastUpdateNumber" column="last_update_number" type="Int32" unsaved-value="-1"/>
    <component name="OwnerId" class="SoftInn.Domain.ObjectId">
      <property name="ClassId" column= "node_type" type="Int32"/>
      <property name="InstanceId" column= "node_id" type="Int64"/>
    </component>

    <property name="User" type="SoftInn.Services.Internal.NHibernate.UserType, SoftInn.Services" not-null="false">
      <column name="siuser_id"/>
    </property>    
    <property name="FullName" column= "full_name" type="String"/>    
    <property name="Name" column= "archive_name" type="String"/>
    <property name="NodeNamePath" column= "node_name_path" type="String"/>
    <property name="Size" column="archive_size" type="Int64"/>
    <property name="ZipSize" column="archive_physical_zip_size" type="Int64"/>    
    <property name="ContainsReports" column="contains_reports" type="YesNo"/>
    <property name="ContainsPermissions" column="contains_permissions" type="YesNo"/>
    <property name="FilteredByDateRange" column="is_filteredby_data_range" type="YesNo"/>
    <property name="StartDate" column= "start_datetime" type="SoftInn.Services.Internal.NHibernate.DateTimeType, SoftInn.Services"/>
    <property name="EndDate" column= "end_datetime" type="SoftInn.Services.Internal.NHibernate.DateTimeType, SoftInn.Services"/>
    <component name="PermissionOwnerId" class="SoftInn.Domain.ObjectId">
      <property name="ClassId" column= "permissionowner_type" type="Int32"/>
      <property name="InstanceId" column= "permissionowner_id" type="Int64"/>
    </component>
    <property name="PermissionOwnerName" column= "permissionowner_name" type="String"/>
    <property name="CreateDate" column= "create_datetime" type="SoftInn.Services.Internal.NHibernate.DateTimeType, SoftInn.Services"/>
    <property name="Status" column= "status" type="String"/>    
  </class>

</hibernate-mapping>

namespace SoftInn.Services.Internal.NHibernate {

    /// <summary>
    /// Used to retrieve and store generic User object relationships via an "InstanceId" column
    /// </summary>
    public class UserType : BaseEntityType {

        /// <see cref="SoftInn.Services.Internal.NHibernate.BaseEntityType.ReturnedClass"/>
        public override Type ReturnedClass {
            get { return typeof(User); }
        }
    }
}

　　

using System;
using NHibernate;
using NHibernate.Engine;
using NHibernate.Type;
using NHibernate.UserTypes;
using SoftInn.Domain;
using SoftInn.Platform.Log;

namespace SoftInn.Services.Internal.NHibernate {

    /// <summary>
    /// Abstract base class for classes used to retrieve and store 
    /// generic Persistent object relationships via an "InstanceId" column.
    /// </summary>
    public abstract class BaseEntityType : ICompositeUserType {

        #region ICompositeUserType Members

        /// <summary>See base class documentation</summary>
        public string[] PropertyNames {
            get { return new String[] { "InstanceId" }; }
        }

        /// <summary>See base class documentation</summary>
        public IType[] PropertyTypes {
            get {
                return new IType[] { NHibernateUtil.Int64 };
            }
        }

        /// <summary>See base class documentation</summary>
        public abstract Type ReturnedClass {
            get;
        }

        /// <summary>See base class documentation</summary>
        public object Assemble(object cached, ISessionImplementor session, object owner) {
            return DeepCopy(cached);
        }

        /// <summary>See base class documentation</summary>
        public object DeepCopy(object value) {
            return value;
        }

        /// <summary>See base class documentation</summary>
        public object Disassemble(object value, ISessionImplementor session) {
            return DeepCopy(value);
        }

        /// <summary>See base class documentation</summary>
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1800:DoNotCastUnnecessarily")]
        public object GetPropertyValue(object component, int property) {
            if (component == null) {
                throw new ArgumentNullException("component");
            }

            if (property == 0) {
                return ((Entity)component).ObjectId.InstanceId;
            } else {
                return null;
            }
        }

        /// <summary>See base class documentation</summary>
        public bool IsMutable {
            get { return true; }
        }

        /// <summary>See base class documentation</summary>
        public object NullSafeGet(System.Data.IDataReader dr, string[] names, ISessionImplementor session, object owner) {
            object instanceIdObj = NHibernateUtil.Int64.NullSafeGet(dr, names[0], session, owner);

            if (instanceIdObj == null ) {
                // Null instanceId means a null entity reference.
                return null;
            }

            Int64 instanceId = (Int64)instanceIdObj;

            // Construct an ObjectId for this instanceId.
            ObjectId entityId = new ObjectId(ReturnedClass, instanceId);

            try {
                // Find and return the corresponding Entity object.
                return ServiceDirectory.PersistServices.FindById(entityId, true);

            } catch (SoftInn.Services.Exceptions.ObjectNotFoundException) {
                // The referenced entity no longer exists, but we still have a reference to it by ID.
                LogManager.LogError(ServicesLogMessages.LOG_CONTEXT,
                    ServicesLogMessages.PERSIST_MISSING_OBJECT,
                    null, entityId.ToString(), GetType().FullName + ".NullSafeGet");
                //throw;
                return null;
            }
        }

        /// <summary>See base class documentation</summary>
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2233:OperationsShouldNotOverflow", MessageId = "index+1")]
        public void NullSafeSet(System.Data.IDbCommand cmd, object value, int index, ISessionImplementor session) {

            Entity entity = (value == null) ? null : (Entity)value;

            if (entity != null && entity.ObjectId != null) {
                // Entity with an ObjectId... store the instance id and class id.
                Int64 instanceId = entity.ObjectId.InstanceId;
                NHibernateUtil.Int64.NullSafeSet(cmd, instanceId, index, session);

            } else {
                // Null entity... store null instance id and class id.
                NHibernateUtil.Int64.NullSafeSet(cmd, null, index, session);
            }
        }

        /// <summary>See base class documentation</summary>
        public void SetPropertyValue(object component, int property, object value) {
        }

        #endregion

        /// <summary>See base class documentation</summary>
        public new bool Equals(object obj1, object obj2) {
            if (obj1 == null) {
                return obj2 == null;
            } else {
                return obj1.Equals(obj2);
            }
        }

        /// <summary>See base class documentation</summary>
        public int GetHashCode(object obj1) {
            if (obj1 != null) {
                return obj1.GetHashCode();
            }
            else {
                return this.GetHashCode();
            }
        }

        /// <summary>See base class documentation</summary>
        public object Replace(object original, object target, ISessionImplementor session, object owner) {
            return original;
        }

    }
}