NHibernate DaoFactory Exposer with Castle MikroKernel

Introduction

In a previous CodeProject article, NHibernate Best Practices with ASP .NET, Generics and Unit Tests by Bill McCafferty, Core project contains DAO interfaces and domain objects while Data project contains implementation of DAO interfaces.

For performance reasons, since we do not want to iterate over collections to find a matching object, we use DAO and thus NHibernate to query for objects we are interested in. Data project contains a reference to Core project. We cannot add Data project as reference to Core project, as this will cause a circular reference and .NET does not allow us to use circular references. As a side effect or result of this restriction:

• We explicitly have to pass DAO instances to our domain objects during construction, or
• Set DAO property of our domain object with an instance of DAO object which has to be created explicitly.

These two methods cause presentation layer developers (WinForm or Web) to write some repetitive code like this:

public void DoSomething()
{
  // DaoFactory can be stored as variable in session (Web) 
  // or as a static variable (WinForm)
  IDaoFactory daoFactory = new DaoFactory();
  
  // This can also be stored as variable in session (Web) 
  // or as a static variable (WinForm)
  ISomeDomainObjectDao dao = daoFactory.GetSomeDomainObjectDao();
  
  //Method-1: Pass dao instance as a parameter in constructor
  SomeDomainObject domObj = new SomeDomainObject(dao);

  //Method-2: Set DAO property of the domain object with an instance of the DAO
  SomeDomainObject domObj = new SomeDomainObject();
  domObj.SomeDomainObjectDao = dao;
}

We can do a favour to presentation layer developers by automatically initializing internal DAO objects. Below is a description of how we can achieve this.

Castle MikroKernel/Windsor

Castle MikroKernel is an inversion of control container that provides us with a very rich set of extensibility. You can find out more information about Castle/MikroKernel here.

In order to work with Castle MikroKernel:

1. We need three assemblies from Castle Project:

• Castle.Model

• Castle.Windsor

• Castle.MikroKernel 

2. We have to specify IoC components in our App/Web.config like the example provided below:

   <configSections>
     <section name="nhibernate" type="System.Configuration.NameValueSectionHandler, 
   	System, Version=1.0.1.0,Culture=neutral, 
   	PublicKeyToken=b77a5c561934e089" />
     <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,
   	log4net, Version=1.2.9.0, Culture=neutral, 
   	PublicKeyToken=b32731d11ce58905" />
     <section name="castle" 
   	type="Castle.Windsor.Configuration.AppDomain.CastleSectionHandler, 
   	Castle.Windsor" />
   </configSections>
   
   <castle>
       <components>
         <component
           id="SampleDaoFactory"
           service="Fully qualified namespace where IDaoFactory interface resides"
           type="Fully qualified namespace where IDaoFactory implementation resides">
         </component>
         <component
           id="AnotherSampleDaoFactory"
           service="Fully qualified namespace where IDaoFactory interface resides"
           type="Fully qualified namespace where IDaoFactory implementation resides">
         </component>
       </components>
     </castle>

DaoFactoryExposer Class Hierarchy

As you can see in the simplified model below, we work with three classes and one interface in this sample implementation.

• DomainObject: This is our domain object that needs a Dao instance to query the database
• IDaoFactory: Specifies which DAO objects are supported by the factory
• DaoFactory: Implements IDaoFactory interface. Creates Dao instances on request
• DaoFactoryExposer: Exposes a DaoFactory instance by using Castle/MikroKernel IoC. As soon as we get a DaoFactory instance, we will also be able to get any needed Dao instance as specified by IDaoFactory interface:

DaoFactoryExposer

/******************************************************************************
  File  : DaoFactoryExposer.cs
  Class : DaoFactoryExposer
  Description:

  Created By: Ali Özgür
  Created On: 31/01/2007 14:55:24
  Contact   : ali_ozgur@hotmail.com

  Revisions:
*******************************************************************************/

using System;
using System.Collections.Generic;
using System.Text;

using Castle.Windsor;
using Castle.Windsor.Configuration.Interpreters;

namespace Bilgi.SampleProject.Core
{
  /// <summary>
  /// This class exposes dao factory using Castle.Windsoe IoC 
  /// container.
  /// </summary>
  public class DaoFactoryExposer
  {
    private IDaoFactory _daoFactory = null;

    public DaoFactoryExposer( )
    {
    }

    public IDaoFactory DaoFactory
    {
      get
      {
        if (_daoFactory != null)
        {
          return _daoFactory;
        }
        else
        {
          IWindsorContainer container = new WindsorContainer(new XmlInterpreter());
          _daoFactory = container["SampleDaoFactory"] as IDaoFactory;

          if (_daoFactory == null)
          {
            throw new TypeLoadException("Can not load dao factory from container!");
          }

          return _daoFactory;
        }
      }
    }
  }
}

IDaoFactory Interface

using System;
using System.Collections.Generic;
using System.Text;

namespace Bilgi.SampleProject.Core
{
  public interface IDaoFactory
  {
    ISomeNeededDao GetSomeNeededDao( );    
  }
}

IDaoFactory Implementation (DaoFactory)

using System;
using System.Collections.Generic;
using System.Text;

namespace Bilgi.SampleProject.Data
{
  public class DaoFactory:IDaoFactory
  {
    ISomeNeededDao _someNeededDao = null;
    
    #region IDaoFactory Members

    public ISomeNeededDao GetSomeNeededDao( )
    {
      if (_someNeededDao == null)
      {
        _someNeededDao = new SomeNeededDao();
      }
      return _someNeededDao;
    }
    
    #endregion
  }
}

DomainObject

using System;
using System.Collections.Generic;
using System.Text;

namespace Bilgi.SampleProject.Core
{
  public class DomainObject
  {
    #region Dao related
    
    private DaoFactoryExposer _daoFactoryExposer = new DaoFactoryExposer();

    IDaoFactory _daoFactory = null;
    public IDaoFactory DaoFactory
    {
      get
      {
        if (_daoFactory == null)
        {
          return _daoFactoryExposer.DaoFactory;
        }
        else
        {
          return _daoFactory;
        }
      }
      set
      {
        _daoFactory = value;
      }
    }

    ISomeNeededDao _someNeededDao = null;
    public ISomeNeededDao SomeNeededDao
    {
      get
      {
        if (_someNeededDao == null)
        {
          return DaoFactory.GetSomeNeededDao();
        }
        else
        {
          return _someNeededDao;
        }
      }
      set { _someNeededDao = value; }
    }

    #endregion //Dao related    
    
    //Some method that needs SomeNeededDao instance
    public void AddOrder(Customer customer, Product product)
    {
      // Here SomeNeededDao instance is created automatically by using DaoFactoryExposer
      decimal customerBalance = SomeNeededDao.QueryCustomerBalance(customer);
      if(customerBalance <= 0 )
      {
        throw new Exception("Not enough balance!");
      }
    }    
  }
}

History

• 2^nd February, 2007: Initial post