NHibernate 3.0: No FirstOrDefault() with QueryOver?

Try

Result precedingOrMatchingResult = Session.QueryOver<Result>().
        Where(r => r.TimeStamp < timeStamp).
        OrderBy(r => r.TimeStamp).Desc.
        SetFetchSize(1).
        UniqueResult();

UniqueResult will return a single value, or null if no value is found, which is kinda what First or Default does.

Setting the Fetch Size to 1 may or may not be required, I'd test that with a profiler.

Result precedingOrMatchingResult = Session.QueryOver<Result>()
                                          .Where(r => r.TimeStamp < timeStamp)
                                          .OrderBy(r => r.TimeStamp).Desc
                                          .SingleOrDefault();

I have now found out that I could use the Take() extension method on the IQueryOver instance, and only the enumerate to a list, like so:

Result precedingOrMatchingResult = Session.QueryOver<Result>().
        Where(r => r.TimeStamp < timeStamp).
        OrderBy(r => r.TimeStamp).Desc.   
        Take(1).List(). //enumerate only on element of the sequence!
        FirstOrDefault(); //get the preceding or matching result, if there is any

NH 3 has an integrated LINQ provider (queries are translated internally to HQL/SQL). You have to add the NHibernate.Linq namespace and then:

Result precedingOrMatchingResult = Session.Query<Result>().
    Where(r => r.TimeStamp < timeStamp).
    OrderByDescending(r => r.TimeStamp).
    FirstOrDefault();