Nhibernate

Or two of code.NHibernate uses mapping files to guide its translation from the database to business objects and back again. As an alternative, you can use attributes on classes and properties, instead of mapping files. To keep things as simple as possible, we're going to use mapping files in this article, rather than attributes. In addition, mapping files make for a cleaner separation between business logic and persistence code.So, one need only add a few lines of code to an application and create a simple mapping file for each persistent class, and NHibernate takes care of all database operations. It is amazing how much development time is saved by using NHibernate.Note that NHibernate is not the only ORM framework in the .NET universe. There are literally dozens of commercial and open source products that provide the same services. NHibernate is among the most popular, probably because of its heritage as a descendant of Hibernate, a popular ORM Framework in the Java universe. In addition, Microsoft has promised an 'Entity Framework' for ADO.NET, to provide ORM services. However, the product has been delayed, and it may not be released for some time.Installing NHibernateThe first step in using NHibernate is to download NHibernate and Log4Net, an open-source logging application that NHibernate can use to record errors and warnings. NHibernate contains the most recent Log4Net binary, or you can download the entire Log4Net install package. Here are the download locations:NHibernateLog4Net Log4Net is not strictly required to use NHibernate, but its automatic logging can be very useful when debugging. Getting StartedIn this article, I am going to use a very simple demo application (the demo app) that does no real work, other than demonstrating data access with NHibernate. It is a console application, which simplifies things by eliminating UI code. The application creates some

Business objects, uses NHibernate to persist them, and then reads them back from the database. You will need to do several things to run the demo app on your machine:Replace references to NHibernate and Log4NetAttach the NhibernateSimpleDemo databaseModify the connection stringThe demo app contains references to NHibernate and Log4Net. The references should be valid on your PC, so long as NHibernate and Log4Net are installed in their default locations. If the references aren't valid, you will need to replace them with references to NHibernate (NHibernate.dll) and Log4Net (log4net.dll) as they are installed on your PC. The DLLs can be found in the NHibernate installation folder on your development PC. The demo app is configured to use SQL Server Express 2005. The database files (NhibernateSimpleDemo.mdf and NhibernateSimpleDemo.ldf) are packaged with the demo app. You will need to attach the database to SQL Server on your machine.Finally, the connection string in the App.config file assumes that you are running a named instance of SQL Server Express 2005, and that the instance is named 'SQLEXPRESS'. If your PC is running a different configuration of SQL Server 2005, you will need to modify the connection string in the App.config file. Note that the database will not work with older versions of SQL Server.The Business ModelThere are two ways to develop an application with NHibernate. The first is a "data-centric" approach, which starts with a data model and creates business objects from the database. The second is an "object-centric" approach, which starts with a business model and creates a database to persist the model. The demo app uses the object-centric approach.Here is the business model for the demo app:The model represents the skeleton of an order system. The model is not complete—there are just enough classes to demonstrate object persistence with NHibernate. And there is

Log4NetYou may recall that we imported a reference to Log4Net into the demo app project when we set it up. The first step to configuring NHibernate is to configure Log4Net. Note that if you use Log4Net (its use is optional), Log4Net must be configured before NHibernate, since NHibernate will expect to see Log4Net when it is initialized.Log4Net configuration is easy. First, make sure that Log4Net is enabled in the App.config file:logger name="NHibernate">level value="DEBUG" />/logger>Next, add the following attribute to your code. The demo app adds it above the namespace declaration for the PersistenceManager class:[assembly: log4net.Config.XmlConfigurator(Watch=true)]namespace NHibernateSimpleDemo{ public class PersistenceManager : IDisposable { … }The final step to configuring Log4Net is to call its Configure() method. The demo app encapsulates the call in a ConfigureLog4Net() method, which is called from the PersistenceManager constructor:private void ConfigureLog4Net(){ log4net.Config.XmlConfigurator.Configure();}Configuring NHibernate – Configuration CodeThe demo app configures NHibernate when it initializes the PersistenceManager. The PersistenceManager does the configuration in a private method, which is called from the PersistenceManager constructor. The configuration code is straightforward:private void ConfigureNHibernate(){ Configuration cfg = new Configuration(); cfg.Configure(); Assembly thisAssembly = typeof(Customer).Assembly; cfg.AddAssembly(thisAssembly); m_SessionFactory = cfg.BuildSessionFactory();}First, we create an NHibernate Configuration object. Then we pass it the class mappings from the mapping files. Note that the AddAssembly() method requires that all mapping files be embedded in the project assembly. To do this, set the BuildAction property of each mapping file to Embedded Resource.Once we have passed mapping files to the Configuration object, we simply need to tell it to create a SessionFactory for us. NHibernate will find and read the configuration data it needs; we do not need to specify whether the data is found in App.config or in a separate XML file, and we do not need to explicitly load the data.BuildSessionFactory() returns a SessionFactory object, which the demo app