NHibernate LINQ provider extension

In NHibernate almost all OO query systems (HQL, Criteria, QueryOver) are extensible. LINQ is, in its nature, a big extensible extension, but can you extend our LINQ-provider to translate your LINQ-extension in SQL ?
As usual in NHibernate, the answer is : yes, you can!!

The LINQ extension

As example I’m going to use an extension of string to mimic the SQL LIKE clause.

public static class MyLinqExtensions
{
    public static bool IsLike(this string source, string pattern)
    {
        pattern = Regex.Escape(pattern);
        pattern = pattern.Replace("%", ".*?").Replace("_", ".");
        pattern = pattern.Replace(@"\[", "[").Replace(@"\]","]").Replace(@"\^", "^");

        return Regex.IsMatch(source, pattern);
    }
}

I can use my extension in memory but what I need is instruct NHibernate about its translation for my persistence-queries.

The integration

The integration point is a class implementing : ILinqToHqlGeneratorsRegistry
The LinqToHqlGeneratorsRegistry provides the way to recognize and translate methods and properties of varios classes. Our DefaultLinqToHqlGeneratorsRegistry can manage LINQ extensions as Any, All, Min, Max etc., methods as StratsWith, EndsWith, Contains, IndexOf etc. or properties as Year, Month, Day, Length etc.
First of all I have to implements a IHqlGeneratorForMethod. NHibernate provides a cooked base class and my implementation will look as:

public class IsLikeGenerator : BaseHqlGeneratorForMethod
{
    public IsLikeGenerator()
    {
        SupportedMethods = new[] {ReflectionHelper.GetMethod(() => MyLinqExtensions.IsLike(null, null))};
    }

    public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject,
        ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
    {
        return treeBuilder.Like(visitor.Visit(arguments[0]).AsExpression(),
                                visitor.Visit(arguments[1]).AsExpression());
    }
}

Having the HQL generator for my extension I can implements my ILinqToHqlGeneratorsRegistry extending the default implementation.

public class MyLinqToHqlGeneratorsRegistry: DefaultLinqToHqlGeneratorsRegistry
{
    public MyLinqToHqlGeneratorsRegistry():base()
    {
        RegisterGenerator(ReflectionHelper.GetMethod(() => MyLinqExtensions.IsLike(null, null)),
                          new IsLikeGenerator());
    }
}

The configuration

The last step to use my IsLike extension to query the DB is the configuration of my LinqToHQLGeneratorsRegistry.
The name of the property for the XML configuration is : linqtohql.generatorsregistry
In NHibernate 3 you can use the Loquacious-configuration :

configuration.LinqToHqlGeneratorsRegistry<MyLinqToHqlGeneratorsRegistry>();

 And now...

var contacts = (from c in db.Customers where c.ContactName.IsLike("%Thomas%") select c).ToList();

Welcome to the world of options,
Welcome to NHibernate3!!!