Practical .NET

Use Lambda Expressions in LINQ Includes

You couldn't do it in earlier versions of Entity Framework but, in more recent versions, you can use lambda expressions to save yourself from some runtime errors.

As I discuss in my column on building a query layer in an ASP.NET MVC application, the LINQ Include method is essential to controlling getting all the data you want. When I first started working with Entity Framework, you had to pass to the Include method (in quotes) the name of the navigation property you were using to retrieve related items. Typical code would look like this (this retrieves the SalesOrderItem entities associated each SalesOrderHeader through the SalesOrderItems property):

Dim soDTO = From so In db.SalesOrderHeaders _
               .Include("SalesOrderItems")
            Select so

The problem is that, because the property name is in quotes, the compiler can't check it. If you've mistyped the property name (or picked the wrong one), you won't find out until you run your code and it blows up.

These days, however, you can pass a lambda expression to the Include method. Rewriting my previous example to use a lambda expression would look like this (and you'll get IntelliSense support all the way through):

Dim soDTO = From so In db.SalesOrderHeaders _
                .Include(Function(s) s.SalesOrderItems)
            Select so 

You might still pick the wrong property but you can't misspell it. In C#, the query would look like this:

var soDTO = from so in db.SalesOrderHeaders _
                .Include(s => s.SalesOrderItems)
            select so;

This isn't an unalloyed blessing though. If I wanted to include the Product entity that's available from the SalesOrderItem, I would have to do it this way with a lambda expression:

Dim soDTO = From so In sor.SalesOrderHeaders _
                 .Include(Function(s) s.SalesOrderItems.Select(Function(si) si.Product))
            Where so.SalesOrderId = SalesOrderId
            Select so

I have to admit that string-based method:

Dim soDTO = From so In sor.SalesOrderHeaders _
                 .Include("SalesOrderItems.Product")
            Where so.SalesOrderId = SalesOrderId
            Select so

is a little easier to read.

About the Author

Peter Vogel is a system architect and principal in PH&V Information Services. PH&V provides full-stack consulting from UX design through object modeling to database design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His blog posts on user experience design can be found at http://blog.learningtree.com/tag/ui/.

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