c# - Entity Framework Include() is not working

Question

I have the following EF query:

TestEntities db = new TestEntities();
var questions = from q in db.Questions.Include("QuestionType")
                from sq in db.SurveyQuestions
                where sq.Survey == surveyTypeID
                orderby sq.Order
                select q;

foreach( var question in questions ) {
    // ERROR: Null Reference Exception
    Console.WriteLine("Question Type: " + question.QuestionType.Description);
}

I am getting a null reference exception when I access the QuestionType property. I am using Include("QuestionType") but it doesn't appear to be working. What am I doing wrong?

Edit: It does not throw a null reference exception when I have Lazy Loading turned on.

Edit: Include() seems to be working when i do the following:

var questions = db.Questions.Include("QuestionType").Select(q => q);

When I predicate on a separate entity Include seems to fail. Is that not allowed when using Include? What about my query is causing this thing to not work?

Answer 1

The problem might be related to the subquery in your Linq expression. Subselects, grouping und projections can cause eager loading with Include to fail silently, as mentioned here and explained in more detail here (see answers of Diego Vega somewhere in the middle of the thread).

Although I cannot really see that you violate any of the rules to follow when using Include as described in those posts, you could try to change the query according to the recommendation:

var questions = from q in db.Questions
                from sq in db.SurveyQuestions
                where sq.Survey == surveyTypeID
                orderby sq.Order
                select q;

var questionsWithInclude = ((ObjectQuery)questions).Include("QuestionType");

foreach( var question in questionsWithInclude ) {
    Console.WriteLine("Question Type: " + question.QuestionType.Description);
}

(Or use the extension method mentioned in the posts.)

If I understand the linked posts correctly, this does not necessarily mean that it will work now (probably not), but you will get an exception giving you more details about the problem.