Tag Archives: NHibernate

The Case for NHibernate

Recently, my employer has instituted a program called “Innovator of the Quarter”.  The idea is for employees to submit ideas, such as new product features, process improvements, or tools to increase productivity, to a committee.  The committee will then pick the idea it thinks has the most merit, and the person who submits the winning idea will get $500 and the chance to implement their idea, or at least build a proof-of-concept.

When this program was first announced, several ideas occurred to me, but the one I thought would result in the greatest benefit for the company (and, admittedly, the greatest reduction of development pain for me) was to use NHibernate for our data access instead of the hand-rolled DAL we currently use.  I put together a proof-of-concept project based on one of our smaller systems (which I unfortunately can’t share since it’s proprietary), and wrote up a short explanation of the benefits I thought adopting NHibernate would provide.

I wanted to post the contents of that entire document here (slightly sanitized for product and team names) so that readers could point out any glaring mistakes or make suggestions for additions.  So here it is:

The Case for NHibernate

One of the most fundamental parts of a system is its data-access strategy.  IT organizations building on the .NET platform have a multitude of options for addressing this part of their systems.  Many of these options are provided by Microsoft itself, while others have grown up out of the open-source community.  They range from thin wrappers over the ADO.NET API, such as the Enterprise Library Data Application Block, which provide similar functionality to plain ADO with an easier-to-use programming model, to full-fledged Object-Relational mapping (ORM) tools, such as the Entity Framework or LINQ to SQL.

On our team, data access has traditionally been implemented using tools that operate very close to the ADO “metal”, using stored procedures for much of the application logic.  The reasoning for adopting these tools and practices at the time our first system was created included both developer skillsets and performance benefits.  Not long ago, there was a substantial performance benefit to using stored procedures as opposed to “inline” SQL.  However, with the improvements made to SQL Server, these advantages have been eroded to the point that stored procedures no longer carry much performance advantage over parameterized SQL queries.

Since the performance differences between the two are now far less significant, other advantages of the various data-access strategies, and specifically those of ORMs, should be examined.  I will outline here the multiple advantages I believe using NHibernate would bring to our projects.  Examples will be taken from the proof-of-concept project stored in source control.

Development Time


Chief among the advantages of using any ORM tool is the reduction in development time that is possible if the tool is used appropriately.  In particular, the standard Create, Read, Update, and Delete (CRUD) operations that commonly need to be performed on any business entity are easier to implement using an ORM tool.  Take, for example, the Contact entity from the proof of concept system, which looks something like this:


In order to implement persistence for an entity like this using stored procedures, we would need a minimum of four procedures, one each for INSERT, UPDATE, SELECT and DELETE.  Also, application code must also be written to put the data into the appropriate properties of a .NET object when retrieved from the database, and from the .NET object properties to stored procedure parameters when saving back to the database.  This sort of “right-hand, left-hand” code is tedious and time-consuming to write, and is really just specifying the same idea four or five times over.  The CRUD procedures for the Contact object in the project the proof-of-concept was based on are about 350 lines of T-SQL put together, and the .NET code to move the properties and parameters back and forth is about 180 lines.

On the other hand, using NHibernate mappings, we specify the relationships between our object properties and database table columns in only one place.  Through the use of conventions provided by Fluent NHibernate, these mappings can be specified concisely and clearly.  In the case of the Contact entity, the mapping can be specified in about 30 lines of C# (you can see this in the file ContactMap.cs in the proof of concept project).

Once these mappings have been specified, CRUD operations can be executed in the application.  For example, to retrieve a particular Contact from the database, we can simply write:

Contact con = session.Get<Contact>(contactID);

Where “contactID”  is the primary key value for the contact we want to retrieve.  The “session” object here is a key part of the NHibernate infrastructure; all interactions with the database are executed within a session.  The other 3 CRUD operations are similarly simple.  Examples of each can be seen in the ContactsUC.ascx.cs control in the proof of concept project.





Besides simple retrieval and modification by primary key, the main way that we interact with persistent data is retrieval by a set of more complex criteria.  This is exemplified by the numerous search pages in our systems.  Currently, these searches are accomplished using dynamic SQL generated within a stored procedure.  As anyone who’s ever worked on a search page can attest, these SPs (along with the pages themselves) can get very large very quickly.  This approach necessitates the use of significant conditional logic within the T-SQL code, which can make the procedure quite confusing due to the limited methods of encapsulation provided by the language and environment.

There are a couple of options for doing these complex searches using NHibernate:  the Criteria API and the new LINQ provider.  Both of these have their strong points, but it’s what they have in common that’s most important.  They provide a way to execute dynamic queries using only application code without resorting to inline SQL.

There are a couple of advantages to using C# to specify searches rather than in T-SQL stored procedures.   The first is in the kinds of tools we have available for achieving logical separation.  Using both native language constructs and refactoring tools, we can break search logic down into much more easily understandable pieces, rather than having to navigate through a sea of IF blocks in T-SQL.  The second advantage is a reduction of duplication.  In our current search implementations, checking search values for nulls, empty strings, etc. occurs in both application and database code.  Using NHibernate, we cut down the number of these checks by half, since we only need to interrogate those values in the application.

An example of how a search page might be implemented can be seen in the proof of concept project in Search.aspx.cs.  Note that this page would certainly benefit from additional refactoring, but it should give the reader a general idea of how better separation of concerns and elimination of duplication can be achieved using this method.

Performance Concerns

As was stated earlier, one of the primary motivations for the data access strategy used by our systems currently was performance, particularly in the area of search.  In order to consider adoption of a new data access strategy, it must be shown that it performs comparably to the current strategy.  Fortunately, SQL Profiler can help us determine the performance impact by showing us the resulting queries produced both by the stored procedures and by NHibernate and the time they take to execute.

Though nowhere near exhaustive, the tests I performed showed that the queries produced by NHibernate, when not nearly textually identical to the ones produced by the stored procedures, executed with time differences that were essentially statistically insignificant.  (E.g., 1885 ms vs. 1886 ms)  In fact, in several instances, the NHibernate queries actually performed better than their stored-procedure-produced counterparts.

It would be prudent to note, however, that this is only one search page.  Different entity relationships may give rise to situations where SQL hints provided by a developer would have a significant impact on query performance, but this would have to be approached on a case-by-case basis to determine if such optimization is worthwhile.  If such a case does present itself, nothing would prevent us from using a stored procedure to perform that particular operation.  While it is best to be consistent with the data-access methods used in a project, using NHibernate is certainly not an all-or-nothing proposition.

When it comes right down to it, NHibernate is an abstraction on top of ADO.NET.  Abstractions are created to increase productivity, not performance.  It may be that in certain cases, the delegation of responsibility to the abstraction layer may result in greater performance due to the elimination of human error or ignorance.  However, a determined developer with intimate knowledge of the underlying technology will often be able to write code that outperforms the abstraction.  In other words, there is no question that an abstraction comes at a price.  The challenge is not to eliminate all abstractions that can be outperformed by hand-tuned code in order to eke out the absolute best performance, but to weigh the benefits and costs of each abstraction to determine if it is, overall, beneficial to the project.

The software development industry as a whole is becoming more and more willing to adopt the ORM abstraction.  This can be seen on a number of different platforms, from Ruby on Rails’ ActiveRecord to the Java Hibernate project (upon which NHibernate was originally based).  Microsoft itself has acknowledged the benefits this kind of technology provides, evidenced by their offering of not one, but two ORM solutions:  LINQ to SQL and the Entity Framework.


NHibernate is hardly the only player in the .NET Object-Relational Mapping space.  Microsoft has two different offerings, LINQ to SQL and the Entity Framework, and there are multiple other commercial and open-source frameworks that offer similar functionality.  So why use NHibernate over these other technologies?


As Microsoft’s first foray into ORM, LINQ to SQL is a fairly lightweight framework for turning your database tables into entities usable by your application.  For example, if you had a table named “Contact”, you could simply drag that table from the Server Explorer in Visual Studio onto the LINQ to SQL design surface to create a persistent “Contact” class.  This works well for simple scenarios, but LINQ to SQL has several significant limitations.  Among these are only supporting one-to-one mapping between classes and tables, limited support for inherited classes, and a less than stellar workflow for modification of mappings.  In short, LINQ to SQL is likely not a good fit for our existing systems.

Entity Framework

The ADO.NET Entity Framework is Microsoft’s full-fledged ORM solution.  While it boasts a larger set of features than LINQ to SQL, it also has a number of shortcomings.    Rather than allowing the user to map tables to an existing set of entity classes, EF generates its own new classes that must be used in order to persist data.  Also, lazy-loading of associated entities (e.g. waiting to load a set of Contacts belonging to a Location until and only if they are needed) is poorly supported.  In addition to these and other problems, the XML mapping files themselves are tremendously complex, and consequently are very difficult to modify when needed.  While there is hope that some of the problems with EF may be addressed in the upcoming version, its release is tied to Visual Studio 2010, which it still a while off.

Other Third-Party ORMs

There are a multitude of ORM options for the .NET platform other than the ones already discussed, including SubSonic, LLBLGen, and Telerik’s OpenAccess among others.  Their feature sets vary widely, and while each has its merits, they all share a disadvantage against NHibernate:  the size of their user-base.  Due to its widespread adoption, there are simply more resources for learning about and troubleshooting NHibernate than any other .NET ORM.  When faced with a technical challenge, the availability of online resources can be the difference between solving the problem in a matter of minutes or a matter of days.


Bottom line, adding the capabilities of NHibernate to our projects will mean increased productivity.  Less time will be spent on repetitive tasks, leaving more time to focus on the problem domain and the needs of our clients.

Any feedback you’d like to provide in the comments is welcome!

UPDATE: Chris Benard suggested that posting the CRUD operations and the mapping would be helpful to people without access to the codebase, and wouldn’t expose any IP.  I agree, so now it’s there, in the form of GitHub gists.  Hopefully that’s an improvement!

Fluent NHibernate Auto Persistence Model

I wrote the other day about how conventions in Fluent NHibernate can make your life easier by decreasing the amount of code you have to write to map your objects to their relational representations.  Well, if you’re willing to go along with even more convention, you can even do away with some of your mapping files entirely.

When setting up your NHibernate configuration via FNH, normally you specify where to find your mappings by using something similar to the following:

.Mappings(m => m.FluentMappings.AddFromAssemblyOf<Entity>())

Notice the “FluentMappings” property being used here.  This will look for any mappings you have explicitly specified in classes that derive from ClassMap, a base class provided by FNH.  However, if we change that to use the “AutoMappings” property instead, like so:

.Mappings(m => m.AutoMappings.Add(AutoPersistenceModel.MapEntitiesFromAssemblyOf<Entity>()))

FNH will determine the mappings all by itself based on conventions, and you don’t have to write a single ClassMap class.

Used in conjunction with NHibernate’s SchemaUpdate feature, this becomes really powerful.  You can simply add a property to an entity class contained in an assembly for which you are using auto mapping.  When you run SchemaUpdate, either at the start of your application, or maybe in a separate console app, your database schema will be updated to include a new column to store that property value.  To illustrate, I’ll show you a simple example.  

I’ve got a small entity named “Course” that represents a college course.

    public class Course
        public virtual int Id { get; private set; }
        public virtual string Name { get; set; }
        public virtual string CatalogNumber { get; set; }

And the database table it maps to looks like this:


Say that I wanted to add a property to my Course entity to tell how many credits the course was worth.  I just add the property to the class:

    public class Course
        public virtual int Id { get; private set; }
        public virtual string Name { get; set; }
        public virtual string CatalogNumber { get; set; }
        public virtual int NumberOfCredits { get; set; }

And run SchemaUpdate through a simple console app that looks like this:

    class Program
        static void Main(string[] args)
                .ConnectionString(c =>
            .Mappings(m =>

        private static void BuildSchema(Configuration config)
            new SchemaUpdate(config).Execute(true, true);

I’ll see the script being used to update the schema in the console window:

And my database table will be updated with a new column for the new property:


Having not used this feature of Fluent NHibernate in anything close to a production capacity, I can’t speak to how well it scales.  However, I think this has a lot of potential for rapid prototyping, especially when you pair it with ASP.NET MVC and the templated controllers and views that you can generate.  A little alteration of the T4 templates used for the controllers could do some default persistence via NHibernate, and then we’d be quite a bit closer to the ease of protoyping provided by Rails.

Again, hats off to the Fluent NHibernate team!

Fluent NHibernate Rocks

At work, we are preparing to institute an “Innovation Idea of the Quarter” program.  I say “Innovation Idea” and not “Innovator” because the idea doesn’t necessarily need to end up being implemented for the person who came up with it to win.  I think this is great, because it really encourages people to think outside of the box from the way we normally develop, and not to just propose things they know for sure would be “safe” enough to actually go into the product.

As you can probably tell from the title of this post, I’ve already decided what my initial submission is going to be.  A few months back, when we were first floating the idea of the innovation program, I started trying to prototype using NHibernate in one of our smaller systems as a proof-of-concept.  I eventually got one of search pages working, but it took a lot of trial and error with the “hbm.xml” mapping files, since I didn’t know until I ran the program when I had misspelled something.  Since the innovation program still in incubation when I finished, I set my proof-of-concept aside.

Fast forward a few months, and I had noticed that the Fluent NHibernate project had come a long way.  They’ve now got  a significant amount of documentation on their wiki site (http://fluentnhibernate.org/), pre-built binary packages, and a pretty active mailing list (http://groups.google.com/group/fluent-nhibernate).  After perusing the site a bit, and determining that, based on the amount of activity I saw, Fluent NHibernate probably did have some legs and wouldn’t just be a flash in the pan, I decided to convert proof-of-concept over to using Fluent NHibernate.

I was not disappointed.  One of the key benefits of using FNH is that you get compile-time checking of your object property names, since you use lambda expressions to specify your mappings.  You also don’t have to use the verbose type name syntax (e.g. “My.Really.Long.Chain.Of.Namespaces, ClassName”) when creating relationships between entities.  But the thing I liked the most was the ability to define conventions for just about everything.

The team I work on has an… ahem… interesting pattern for column naming:  basically, Hungarian notation for columns.  If a column is an integer column in the database, it will have an “int” prefix on the column name.  If it’s a char or varchar column, it will have a “str” prefix, and so on.  While I may not agree with this “coding standard”, the team has at least been consistent about it, so I was able to take advantage of conventions.  

In FNH, you define a convention by creating a class that implements one of the convention interfaces provided by FNH, such as IPropertyConvention (you can see the entire list of available interfaces here).  That interface requires you to implement two methods, Accept and Apply.  

The Accept method lets you define what kinds of properties or classes will be affected by the convention.  You get full access to all of the information about the property, such as the name, the data type, and so on, to let you determine if the convention ought to apply to the property.  For example, the Accept method for the string column convention I mentioned earlier looks like this:

public bool Accept(IProperty target)

    return target.PropertyType == typeof (string);

The Apply method lets you perform mapping actions on the property or class just as you would in a ClassMap<T> class, which is where mappings are defined in FNH (each one of those classes would correspond to a “hbm.xml” file used by NHibernate when not using FNH).  This is easier shown than explained, so here’s the Apply method for that string column convention:

public void Apply(IProperty target)
    target.ColumnNames.Add("str" + target.Property.Name);

So, with that convention in place, all I have to do to define a mapping between an entity property named Address and a column named strAddress is:

Map(x => x.Address);

You can, of course, override the conventions you define at any point that you system deviates, so you’re never painting youself into a corner by defining conventions. You’re just taking care of the majority case, and eliminating a lot of code (and magic strings) in the process.  

Since I had conventions and Intellisense working for me, I was able to convert all the hbm.xml mappings I had defined the first time to FNH mappings in a single evening.  If you’re using NHibernate in your application, you should take a serious look at Fluent NHibernate.  Not just for the reasons I’ve covered here, but also for the help it provides with refactoring (property names in mappings are affected by refactoring tools) and the ability to test your mappings.  Kudos to James Gregory,  Paul Batum, Andrew Stewart, Chad Myers, Jeremy Miller, et al. for providing a way for the rest of us to improve our NHibernate experience.