Question

Say I have a list of all Projects, and that I group them by Category like this:

var projectsByCat = from p in Projects                     group p by p.Category into g                     orderby g.Count() descending                     select new { Category = g.Key, Projects = g }; 

Now I want to display this as a list in a web page, where first I create the left side div, secondly the right side div. I am ordering by number of Projects in each Category to show the Categories with the most Projects on top – thus I would like to split projectsByCat in two – if I put all the "odd numbered" Categories on the left and the "even numbered" categories on the right, I think I will get a reasonably sane view.

So I thought I could do this to get the odd and even members of projectsByCat:

var oddCategories = projectsByCat.Where((cat, index) => index % 2 != 0);  var evenCategories = projectsByCat.Where((cat, index) => index % 2 == 0); 

And it compiles – however, when I run it, I get an exception such as this:

Unsupported overload used for query operator ‘Where’.

And I thought I was safe since it compiled in the first place.. 😉

Is there an elegant way to do this? And also, is there an elegant explanation for why my creative use of Where() won’t work?

Answer 1

If you’re using LINQ to SQL or LINQ to Entities you should first fully materialize the results into memory:

var oddCategories  = projectsByCat.ToList().Where((c,i) => i % 2 != 0); var evenCategories = projectsByCat.ToList().Where((c,i) => i % 2 == 0); 

It isn’t possible to iterate through results on the database with an indexer without the use of a cursor, which either ORM framework does not do.