Introduction

NHibernate and Entity Framework are two of the most popular O/RM frameworks on the .NET world. Although they share some functionality, there are some aspects on which they are quite different. This post will describe this differences and will hopefully help you get started with the one you know less. Mind you, this is a personal selection of features to compare, it is by no way an exhaustive list.

History

First, a bit of history. NHibernate is an open-source project that was first ported from Java’s venerable Hibernate framework, one of the first O/RM frameworks, but nowadays it is not tied to it, for example, it has .NET specific features, and has evolved in different ways from those of its Java counterpart. Current version is 3.3, with 3.4 on the horizon. It currently targets .NET 3.5, but can be used as well in .NET 4, it only makes no use of any of its specific functionality. You can find its home page at NHForge.

Entity Framework 1 came out with .NET 3.5 and is now on its second major version, despite being version 4. Code First sits on top of it and but came separately and will also continue to be released out of line with major .NET distributions. It is currently on version 4.3.1 and version 5 will be released together with .NET Framework 4.5. All versions will target the current version of .NET, at the time of their release. Its home location is located at MSDN.

Architecture

In NHibernate, there is a separation between the Unit of Work and the configuration and model instances. You start off by creating a Configuration object, where you specify all global NHibernate settings such as the database and dialect to use, the batch sizes, the mappings, etc, then you build an ISessionFactory from it. The ISessionFactory holds model and metadata that is tied to a particular database and to the settings that came from the Configuration object, and, there will typically be only one instance of each in a process. Finally, you create instances of ISession from the ISessionFactory, which is the NHibernate representation of the Unit of Work and Identity Map. This is a lightweight object, it basically opens and closes a database connection as required and keeps track of the entities associated with it. ISession objects are cheap to create and dispose, because all of the model complexity is stored in the ISessionFactory and Configuration objects.

As for Entity Framework, the ObjectContext/DbContext holds the configuration, model and acts as the Unit of Work, holding references to all of the known entity instances. This class is therefore not lightweight as its NHibernate counterpart and it is not uncommon to see examples where an instance is cached on a field.

Mappings

Both NHibernate and Entity Framework (Code First) support the use of POCOs to represent entities, no base classes are required (or even possible, in the case of NHibernate).

As for mapping to and from the database, NHibernate supports three types of mappings:

• XML-based, which have the advantage of not tying the entity classes to a particular O/RM; the XML files can be deployed as files on the file system or as embedded resources in an assembly;
• Attribute-based, for keeping both the entities and database details on the same place at the expense of polluting the entity classes with NHibernate-specific attributes;
• Strongly-typed code-based, which allows dynamic creation of the model and strongly typing it, so that if, for example, a property name changes, the mapping will also be updated.

Entity Framework can use:

• Attribute-based (although attributes cannot express all of the available possibilities – for example, cascading);
• Strongly-typed code mappings.

Database Support

With NHibernate you can use mostly any database you want, including:

• SQL Server;
• SQL Server Compact;
• SQL Server Azure;
• Oracle;
• DB2;
• PostgreSQL;
• MySQL;
• Sybase Adaptive Server/SQL Anywhere;
• Firebird;
• SQLLite;
• Informix;
• Any through OLE DB;
• Any through ODBC.

Out of the box, Entity Framework only supports SQL Server, but a number of providers exist, both free and commercial, for some of the most used databases, such as Oracle and MySQL. See a list here.

Inheritance Strategies

Both NHibernate and Entity Framework support the three canonical inheritance strategies: Table Per Type Hierarchy (Single Table Inheritance), Table Per Type (Class Table Inheritance) and Table Per Concrete Type (Concrete Table Inheritance).

Associations

Regarding associations, both support one to one, one to many and many to many. However, NHibernate offers far more collection types:

• Bags of entities or values: unordered, possibly with duplicates;
• Lists of entities or values: ordered, indexed by a number column;
• Maps of entities or values: indexed by either an entity or any value;
• Sets of entities or values: unordered, no duplicates;
• Arrays of entities or values: indexed, immutable.

Querying

NHibernate exposes several querying APIs:

• LINQ is probably the most used nowadays, and really does not need to be introduced;
• Hibernate Query Language (HQL) is a database-agnostic, object-oriented SQL-alike language that exists since NHibernate’s creation and still offers the most advanced querying possibilities; well suited for dynamic queries, even if using string concatenation;
• Criteria API is an implementation of the Query Object pattern where you create a semi-abstract conceptual representation of the query you wish to execute by means of a class model; also a good choice for dynamic querying;
• Query Over offers a similar API to Criteria, but using strongly-typed LINQ expressions instead of strings; for this, although more refactor-friendlier that Criteria, it is also less suited for dynamic queries;
• SQL, including stored procedures, can also be used;
• Integration with Lucene.NET indexer is available.

As for Entity Framework:

• LINQ to Entities is fully supported, and its implementation is considered very complete; it is the API of choice for most developers;
• Entity-SQL, HQL’s counterpart, is also an object-oriented, database-independent querying language that can be used for dynamic queries;
• SQL, of course, is also supported.

Caching

Both NHibernate and Entity Framework, of course, feature first-level cache. NHibernate also supports a second-level cache, that can be used among multiple ISessionFactorys, even in different processes/machines:

• Hashtable (in-memory);
• SysCache (uses ASP.NET as the cache provider);
• SysCache2 (same as above but with support for SQL Server SQL Dependencies);
• Prevalence;
• SharedCache;
• Memcached;
• Redis;
• NCache;
• Appfabric Caching.

Out of the box, Entity Framework does not have any second-level cache mechanism, however, there are some public samples that show how we can add this.

ID Generators

NHibernate supports different ID generation strategies, coming from the database and otherwise:

• Identity (for SQL Server, MySQL, and databases who support identity columns);
• Sequence (for Oracle, PostgreSQL, and others who support sequences);
• Trigger-based;
• HiLo;
• Sequence HiLo (for databases that support sequences);
• Several GUID flavors, both in GUID as well as in string format;
• Increment (for single-user uses);
• Assigned (must know what you’re doing);
• Sequence-style (either uses an actual sequence or a single-column table);
• Table of ids;
• Pooled (similar to HiLo but stores high values in a table);
• Native (uses whatever mechanism the current database supports, identity or sequence).

Entity Framework only supports:

• Identity generation;
• GUIDs;
• Assigned values.

Properties

NHibernate supports properties of entity types (one to one or many to one), collections (one to many or many to many) as well as scalars and enumerations. It offers a mechanism for having complex property types generated from the database, which even include support for querying. It also supports properties originated from SQL formulas.

Entity Framework only supports scalars, entity types and collections. Enumerations support will come in the next version.

Events and Interception

NHibernate has a very rich event model, that exposes more than 20 events, either for synchronous pre-execution or asynchronous post-execution, including:

• Pre/Post-Load;
• Pre/Post-Delete;
• Pre/Post-Insert;
• Pre/Post-Update;
• Pre/Post-Flush.

It also features interception of class instancing and SQL generation.

As for Entity Framework, only two events exist:

• ObjectMaterialized (after loading an entity from the database);
• SavingChanges (before saving changes, which include deleting, inserting and updating).

Tracking Changes

For NHibernate as well as Entity Framework, all changes are tracked by their respective Unit of Work implementation. Entities can be attached and detached to it, Entity Framework does, however, also support self-tracking entities.

Optimistic Concurrency Control

NHibernate supports all of the imaginable scenarios:

• SQL Server’s ROWVERSION;
• Oracle’s ORA_ROWSCN;
• A column containing date and time;
• A column containing a version number;
• All/dirty columns comparison.

Entity Framework is more focused on Entity Framework, so it only supports:

• SQL Server’s ROWVERSION;
• Comparing all/some columns.

Batching

NHibernate has full support for insertion batching, but only if the ID generator in use is not database-based (for example, it cannot be used with Identity), whereas Entity Framework has no batching at all.

Cascading

Both support cascading for collections and associations: when an entity is deleted, their conceptual children are also deleted. NHibernate also offers the possibility to set the foreign key column on children to NULL instead of removing them.

Flushing Changes

NHibernate’s ISession has a FlushMode property that can have the following values:

• Auto: changes are sent to the database when necessary, for example, if there are dirty instances of an entity type, and a query is performed against this entity type, or if the ISession is being disposed;
• Commit: changes are sent when committing the current transaction;
• Never: changes are only sent when explicitly calling Flush().

As for Entity Framework, changes have to be explicitly sent through a call to AcceptAllChanges()/SaveChanges().

Lazy Loading

NHibernate supports lazy loading for

• Associated entities (one to one, many to one);
• Collections (one to many, many to many);
• Scalar properties (thing of BLOBs or CLOBs).

Entity Framework only supports lazy loading for:

• Associated entities;
• Collections.

Generating and Updating the Database

Both NHibernate and Entity Framework Code First (with the Migrations API) allow creating the database model from the mapping and updating it if the mapping changes.

Extensibility

As you can guess, NHibernate is far more extensible than Entity Framework. Basically, everything can be extended, from ID generation, to LINQ to SQL transformation, HQL native SQL support, custom column types, custom association collections, SQL generation, supported databases, etc. With Entity Framework your options are more limited, at least, because practically no information exists as to what can be extended/changed. It features a provider model that can be extended to support any database.

Integration With Other Microsoft APIs and Tools

When it comes to integration with Microsoft technologies, it will come as no surprise that Entity Framework offers the best support. For example, the following technologies are fully supported:

• ASP.NET (through the EntityDataSource);
• ASP.NET Dynamic Data;
• WCF Data Services;
• WCF RIA Services;
• Visual Studio (through the integrated designer).

Documentation

This is another point where Entity Framework is superior: NHibernate lacks, for starters, an up to date API reference synchronized with its current version. It does have a community mailing list, blogs and wikis, although not much used. Entity Framework has a number of resources on MSDN and, of course, several forums and discussion groups exist.

Conclusion

Like I said, this is a personal list. I may come as a surprise to some that Entity Framework is so behind NHibernate in so many aspects, but it is true that NHibernate is much older and, due to its open-source nature, is not tied to product-specific timeframes and can thus evolve much more rapidly. I do like both, and I chose whichever is best for the job I have at hands. I am looking forward to the changes in EF5 which will add significant value to an already interesting product.

So, what do you think? Did I forget anything important or is there anything else worth talking about? Looking forward for your comments!