Beginning ASP.NET 4.5 in CSharp and VB Opsylum

Mapping Your Data Model to an Object Model x 505
the myEntities variable holds a (scarce) connection to the SQL Server database, it’s a good idea to
wrap the code that uses it in a Using block, so the object is destroyed at the end of the block and the
connection is released. This myEntities object then exposes your data (such as reviews and genres)
that you can use in a query:
VB.NET
Using myEntities As New PlanetWroxEntities()
Dim authorizedReviews = From review In myEntities.Reviews
Where review.Authorized = True
Order By review.CreateDateTime Descending
Select review
...
End Using
C#
using (PlanetWroxEntities myEntities = new PlanetWroxEntities())
{
var authorizedReviews = from review in myEntities.Reviews
where review.Authorized == true
orderby review.CreateDateTime descending
select review;
...
}
Note that this query looks similar to the SQL that you learned in the previous chapters. Under the
hood, the run time converts this LINQ query into its SQL counterpart and executes it against the
underlying database. Within this query, the variable review in the From clause is used to refer to the
review in the other parts of the query (Where, Order By, and Select), enabling you to specify the
select, filter, and ordering criteria.
What’s important to realize is that EF uses a mechanism called lazy loading, which means sub objects
are not loaded until you explicitly tell them to. What this means is that in the previous example the
Genre properties of the Review objects you’ve queried are null in C# and Nothing in VB.NET and
don’t contain any data. As soon as your code tries to access them, they are loaded by executing another
query to the database. This can greatly improve performance if you don’t need these sub objects.
However, if you’re sure you need them in your code beforehand, executing a separate SQL statement for
each item results in a lot of overhead. In that case, you can preload the objects with the initial query. To
express that you want to include these objects as well, you use the Include method for the types you
want to query:
VB.NET
Dim authorizedReviews = From review In myEntities.Reviews.Include("Genre")
Where review.Authorized = True
...
C#
var authorizedReviews = from review in myEntities.Reviews.Include("Genre")
where review.Authorized == true
...
With this addition to the query, the Review objects now have their Genre property correctly filled with
data. Though this may seem a little counterintuitive and counterproductive at first, it’s actually quite