C# in Depth

LINQ to SQL325using (var context = new DefectModelDataContext()){Defect d = (from defect in context.Defectswhere defect.ID==1select defect).Single();EFinds defect withquery expression}Console.WriteLine (d);Listing 12.5 is easy enough to follow—we open up a context and fetch the firstdefect B. After changing the defect and the entity representing Tim Trotter C, weask LINQ to SQL to save the changes to the database D. Finally, we fetch thedefect E again in a new context and write the details to the console. Just for a bit ofvariety, I’ve shown two different ways of fetching the defect—they’re absolutelyequivalent, because the compiler translates the query expression form into the“method call” form anyway.That’s all the LINQ to SQL we’re going to see—hopefully it’s shown you enough ofthe capabilities to understand how it’s a normal ORM system, but one that has goodsupport for query expressions and the LINQ standard query operators.12.1.5 LINQ to SQL summaryThere are lots of ORMs out there, and many of them allow you to build up queries programmaticallyin a way that can look like LINQ to SQL—if you ignore compile-timechecking. It’s the combination of lambda expressions, expression trees, extensionmethods, and query expressions that make LINQ special, giving these advantages:■ We’ve been able to use familiar syntax to write the query (at least, familiar whenyou know LINQ to Objects!).■ The compiler has been able to do a lot of validation for us.■ Visual Studio 2008 is able to help us build the query with IntelliSense.■ If we need a mixture of client-side and server-side processing, we can do both ina consistent manner.■ We’re still using the database to do the hard work.Of course, this comes at a cost. As with any ORM system, you want to keep an eye onwhat SQL queries are being executed for a particular query expression. That’s wherethe logging is invaluable—but don’t forget to turn it off for production! In particular,you will need to be careful of the infamous “N+1 selects” issue, where an initial querypulls back results from a single table, but using each result transparently executesanother query to lazily load associated entities. Sometimes you’ll be able to find anelegant query expression that results in exactly the SQL you want to use; other timesyou’ll need to bend the query expression out of shape somewhat. Occasionally you’llneed to write the SQL manually or use a stored procedure instead—as is often the casewith ORMs.I find it interesting just to take query expressions that you already know work inLINQ to Objects and see what SQL is generated when you run them against a database.Licensed to Rhona Hadida