C# in Depth

346 CHAPTER 12 LINQ beyond collectionsfrom anything else, this means that the feature of RDBMS independence comes forfree. The same LINQ queries can be run against SQL Server, Oracle, or Postgres, forexample: any system that NHibernate knows about, with SQL tailored for that particularimplementation.Before writing this book, I hadn’t used NHibernate (although I am reasonablyexperienced with its cousin in the Java world), and it’s a testament to the project thatwithin about an hour I was up and running with the SkeetySoft defect database, usingnothing but the online tutorial. Listing 12.20 shows the same query we used againstLINQ to SQL in listing 12.3 to list all of Tim’s open defects.Listing 12.20LINQ to NHibernate query to list defects assigned to Tim TrotterISessionFactory sessionFactory =new Configuration().Configure().BuildSessionFactory();using (ISession session = sessionFactory.OpenSession()){using (ITransaction tx = session.BeginTransaction()){User tim = (from user in session.Linq()where user.Name == "Tim Trotter"select user).Single();}}var query = from defect in session.Linq()where defect.Status != Status.Closedwhere defect.AssignedTo == timselect defect.Summary;foreach (var summary in query){Console.WriteLine(summary);}tx.Commit();As you can see, once the session and transaction have been set up, the code is similarto that used in LINQ to SQL. The generated SQL is different, although it executes thesame sort of queries. In other cases, identical query expressions can generate differentSQL, mostly due to decisions regarding the lazy or eager loading of entities. This is anexample of a leaky abstraction 7 —where in theory the abstraction layer of LINQ mightbe considered to isolate the developer from the implementation performing theactual query, but in practice the implementation details leak through. Don’t fall forthe abstraction: it takes nothing away from the value of LINQ, but you do need to beaware of what you’re coding against, and keep an eye on what queries are being executedfor you.So, we’ve seen LINQ working against both web services and multiple databases.There’s another piece of infrastructure that is commonly queried, though: an enterprisedirectory.7www.joelonsoftware.com/articles/LeakyAbstractions.htmlLicensed to Rhona Hadida