NHibernate

NHibernate is a port of the Hibernate object–relational mapping (ORM) tool for the Microsoft .NET platform. It provides a framework for mapping an object-oriented domain model to a traditional relational database. Its purpose is to relieve the developer from a significant portion of relational data persistence-related programming tasks. NHibernate is free and open-source software that is distributed under the GNU Lesser General Public License.

Feature summary
NHibernate's primary feature is mapping from .NET classes to database tables (and from CLR data types to SQL data types). NHibernate provides data query and retrieval tools. It generates the SQL commands and removes the need of manual data set handling and object conversion, keeping the application portable to most SQL databases, with database portability delivered at very little performance overhead.

NHibernate provides transparent persistence for Plain Old CLR Objects (POCOs). The only strict requirement for a persistent class is a no-argument constructor, which does not have to be public. (Proper behavior in some applications also requires special attention to the Equals and GetHashCode methods.)

History
Tom Barrett started NHibernate, and later picked up by Mike Doerfler and Peter Smulovics. At the end of 2005, JBoss, Inc. (now part of Red Hat) hired Sergey Koshcheyev, the then lead developer of NHibernate, to work full-time on its future versions. At the end of 2006, JBoss stopped the support to this project. NHibernate is now entirely developed and led by the community. The NHibernate logo, designed by Andrew Mayorov, was chosen sometime before 2008. It features a dormouse, which is known for its long periods of hibernation.

Version 1.0 mirrored the feature set of Hibernate 2.1, as well as a number of features from Hibernate 3.

NHibernate 1.2.1, released in November 2007, introduced many more features from Hibernate 3 and support for .NET 2.0, stored procedures, generics, and nullable types.

NHibernate 2.0
NHibernate 2.0 was released on August 23, 2008. It is comparable to Hibernate 3.2 in terms of features. With the version 2.0 release, NHibernate dropped support for .NET 1.1.

NHibernate 2.1 was released July 17, 2009.

NHibernate 3.0
NHibernate 3.0 was released on December 4, 2010, and is the first version to use .NET 3.5, with features such as:
 * Integrated LINQ support
 * Strongly typed criteria-like API called QueryOver
 * New AST-based parser for HQL engine
 * Support for lazy loading columns

NHibernate 3.2
NHibernate 3.2 was released in April, 2011. New features included:
 * Mapping by code: fluent configuration, .hbm.xml files are no longer required;
 * Subselect: ability to map SQL views as entities;
 * HQL paging: TAKE and SKIP on HQL;
 * Integrated bytecode provider: one less DLL to deploy.

NHibernate 4.0
NHibernate 4.0 was released on August 17, 2014. This version requires .NET Framework 4.0 or later.

NHibernate 5.0
NHibernate 5.0 was released on October 10, 2017. It provides support for asynchronous programming. This version requires .NET Framework 4.6.1 or later.

NHibernate 5.1
NHibernate 5.1 was released on March 17, 2018. It supports .NET Standard 2.0 and .NET Core 2.0.

NHibernate 5.2
NHibernate 5.2 was released on December 4, 2018.

NHibernate 5.3
NHibernate 5.3 was released on July 19, 2020.

Contributions
As open source software, NHibernate has received many contributions from its users. Implementation of LINQ has allowed Language Integrated Query use with NHibernate.

NHibernate Profiler
The NHibernate Profiler is an Object–Relational Mapping tool (ORM) that serves as a real-time visual debugger for NHibernate. It identifies inefficient SQL data queries to eliminate unnecessary work by the database to boost overall performance of the application. The NHibernate Profiler also alerts users to data queries that cost too much in time and directs them to the exact line in the C# code.

Sample
Here a code snippet to add an object to the database and shows how to retrieve, modify and update an object in the database using NHibernate.

NHibernate's configuration may affect when NHibernate executes SQL statements.