NHibernate (3.1.0.4000) NullReferenceException using Query<> and NHibernate Facility

Question

I have a problem with NHibernate, I can't seem to find any solution for. In my project I have a simple entity (Batch), but whenever I try and run the following test, I get an exception. I've triede a couple of different ways to perform a similar query, but almost identical exception for all (it differs in which LINQ method being executed).

The first test:

[Test]
public void QueryLatestBatch()
{
    using (var session = SessionManager.OpenSession())
    {
        var batch = session.Query<Batch>()
            .FirstOrDefault();

        Assert.That(batch, Is.Not.Null);
    }
}

The exception:

System.NullReferenceException : Object reference not set to an instance of an object.
at NHibernate.Linq.NhQueryProvider.PrepareQuery(Expression expression, ref IQuery query, ref NhLinqExpression nhQuery)
at NHibernate.Linq.NhQueryProvider.Execute(Expression expression)
at System.Linq.Queryable.FirstOrDefault(IQueryable`1 source)

The second test:

[Test]
public void QueryLatestBatch2()
{
    using (var session = SessionManager.OpenSession())
    {
        var batch = session.Query<Batch>()
            .OrderBy(x => x.Executed)
            .Take(1)
            .SingleOrDefault();

        Assert.That(batch, Is.Not.Null);
    }
}

The exception:

System.NullReferenceException : Object reference not set to an instance of an object.
at NHibernate.Linq.NhQueryProvider.PrepareQuery(Expression expression, ref IQuery query, ref NhLinqExpression nhQuery)
at NHibernate.Linq.NhQueryProvider.Execute(Expression expression)
at System.Linq.Queryable.SingleOrDefault(IQueryable`1 source)

However, this one is passing (using QueryOver<>):

[Test]
public void QueryOverLatestBatch()
{
    using (var session = SessionManager.OpenSession())
    {
        var batch = session.QueryOver<Batch>()
            .OrderBy(x => x.Executed).Asc
            .Take(1)
            .SingleOrDefault();

        Assert.That(batch, Is.Not.Null);
        Assert.That(batch.Executed, Is.LessThan(DateTime.Now));
    }
}

Using the QueryOver<> API is not bad at all, but I'm just kind of baffled that the Query<> API isn't working, which is kind of sad, since the First() operation is very concise, and our developers really enjoy LINQ.

I really hope there is a solution to this, as it seems strange if these methods are failing such a simple test.

EDIT

I'm using Oracle 11g, my mappings are done with FluentNHibernate registered through Castle Windsor with the NHibernate Facility. As I wrote, the odd thing is that the query works perfectly with the QueryOver<> API, but not through LINQ.

There's something wrong with your mappings, configuration, or some other part of your code that you aren't showing, because all those methods work fine for me (and in NH tests) — Diego Mijelshon
Something is seriously wrong with my setup. I can't get any method on Query<> to work now. I guess there's something wrong with my references. — Siewers
I've found what the problem was/is. Apparently I'm doing something wrong in the NHibernateFacility, as that's the one causing all the problems. If I create a session factory outside the facility, I can query using Query<>, but with it, it doesn't work! — Siewers

Siewers Siewers · Accepted Answer · 2011-04-06T08:44:25

There is an issue with the current implementation of the LINQ extensionmethods for NHibernate 3.1.0.4000 used together with NHibernate Facility 2.0RC (and previous versions) (see: https://nhibernate.jira.com/browse/NH-2626 and discussion here: http://groups.google.com/group/castle-project-devel/browse_thread/thread/ac90148a8d4c8477)

The fix I am using at the moment is to simply ignore the LINQ extensionmethods provided by NHibernate and create it myself. They're really just one-liners:

public static class NHibernateLinqExtensions
{
    /// <summary>
    /// Performs a LINQ query on the specified type.
    /// </summary>
    /// <typeparam name="T">The type to perform the query on.</typeparam>
    /// <param name="session"></param>
    /// <returns>A new <see cref="IQueryable{T}"/>.</returns>
    /// <remarks>This method is provided as a workaround for the current bug in the NHibernate LINQ extension methods.</remarks>
    public static IQueryable<T> Linq<T>(this ISession session)
    {
        return new NhQueryable<T>(session.GetSessionImplementation());
    }

    /// <summary>
    /// Performs a LINQ query on the specified type.
    /// </summary>
    /// <typeparam name="T">The type to perform the query on.</typeparam>
    /// <param name="session"></param>
    /// <returns>A new <see cref="IQueryable{T}"/>.</returns>
    /// <remarks>This method is provided as a workaround for the current bug in the NHibernate LINQ extension methods.</remarks>
    public static IQueryable<T> Linq<T>(this IStatelessSession session)
    {
        return new NhQueryable<T>(session.GetSessionImplementation());
    }
}

Then, when I need to do a LINQ query, I just use session.Linq<EntityType>() instead of session.Query<EntityType>.

Hope it helps someone in the same situation that I was.

NHibernate (3.1.0.4000) NullReferenceException using Query<> and NHibernate Facility

2 Answers