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C# Entity Framework Basics LINQ Queries Sorting Queries

Ruth O'Kelly
Ruth O'Kelly
4,944 Points
Posted by Ruth O'Kelly
Ruth O'Kelly
4,944 Points

Deffered Execution?

At 1:52, James says "let's look at an example of deferred execution". Where was the example? I felt it wasn't clear exactly what was the example of this in the code he then wrote.

Simon Coates
Simon Coates
28,694 Points
Simon Coates
Simon Coates
28,694 Points

I'm struggling with that detail myself. C# is torturing me with magical or only partially explained advanced features, resulting in a perpetual nagging fear that something could go wrong at any second and i'll be left surveying smoking wreckage (and i had such a sunny disposition). Currently, i'm thinking of a query as just the idea of a query with execution of it happening invisibly at moments when you need a value (iteration, toList, Count, etc). But that seems too simple, so I'd assume there's some nuance I'm missing. FYI, getting responses on C# courses (especially advanced ones) can be a little tricky. You can always try tagging James Churchill to see if you get a response.

1 Answer

slaguerta
slaguerta
9,811 Points

If you go to the 4 minute mark of the List Queries video, you'll find a good definition and example of deferred execution for a review. With deferred execution, a query won't be executed against the database until it's enumerated, either by a loop or calling a method like ToList() or Count.

So here's the code from this video, and I've put in some comments on where I think the query is finally being executed (before the execution, the LINQ query is defining the data you'll want back, but doesn't actually query the database yet until the data is requested). 

//deferred execution example
                var comicBooksQuery = context.ComicBooks
                    .Include(cb => cb.Series)
                    .OrderByDescending(cb => cb.IssueNumber);   //not executed, sort of defining the data set in a query here

                var comicBooks = comicBooksQuery.ToList();    //query is executed because of ToList method

                var comicBooks2 = comicBooksQuery        //query begins to be executed here because of the ToList Method
                    .Where(cb => cb.AverageRating < 7)
                    .ToList();                 

                foreach(var comicBook in comicBooks)     //query is executed because enumerating over a loop
                {
                    Console.WriteLine(comicBook.DisplayText);
                }

                Console.WriteLine();
                Console.WriteLine("# of comic books {0}", comicBooks.Count); //query sent here
                Console.WriteLine();

                foreach(var comicBook in comicBooks2) //query sent here
                {
                    Console.WriteLine(comicBook.DisplayText);
                }

                Console.WriteLine();
                Console.WriteLine("# of comic books {0}", comicBooks2.Count); //query sent here

I also found this article to be extremely helpful in kind of detailing deferred execution. (https://blogs.msdn.microsoft.com/charlie/2007/12/10/linq-and-deferred-execution/)

I would also step through the code with the debugger and breakpoints to kind of see how it is working like we do at the end of the "List Queries" video.