NHibernate Type Safety using Lambda Expressions

I can't remember if this has been around before, I do vaguely remember seeing something like it.

However, I just wanted to apply a snippet of code I found on Paul's blog the other day to NHibernate. Of course we will definitely have type safety in queries when Linq-to-NHibernate is completed. But surely linq-to-nhibernate is not going to be the _only_ way of writing queries.

Using the original code snippet 'as is' would look something like this:

ICriteria c = session.CreateCriteria(typeof(Person));
c.Add(Restrictions.Eq(Property.GetFor(() => new Person().FirstName), "John"));

This is ok, but a little long winded, so I implemented another class called RestrictBy which can break down a simple lambda expression. So now I can use:

ICriteria c = session.CreateCriteria(typeof(Person));
c.Add(RestrictBy.Eq(() => new Person().FirstName == "John"));
c.UniqueResult();

This also means you'll get compile time errors for incompatible types such as:

Using the lambda expression we get to evaluate the _actual_ property and that property's type. If you wanted to clean this up some more, its not hard to then go and add some extension methods to ICriteria to produce:

ICriteria c = session.CreateCriteria(typeof(Person));
c.AddEq(() => new Person().FirstName == "John");
c.UniqueResult();

So that's it, a simple example at the moment, but it might be an option to look into a little more because the ICriteria interface is still NHibernate's native querying interface and still provides a lot of power, flexibility and programmatic building of queries.

Sourcecode:

• RestrictBy.cs also a couple of unit tests here
  
• Paul's Property.GetFor()