Does a Linq expression treat a group specially when iterating it with the 'from' clause?

Question

Consider this query to group a list of students by the 1st letter of their last name:

var query = from s in students
            group s by s.Last[0] into group1
            orderby group1.Key
            from g in group1
            select new { g.First, g.Last }

Now, the part that says from g in group1 is not iterating through the keys of group1, it's iterating through the values of each key in group1.

This is how I'd like it to be handled (to produce a flat list), but it seems counter intuitive to me.

To answer my question, I just need someone to point me to the part of MSDN that explains this or explain why I'm the counter intuitive one ;-)

Answer 1

When you introduce a secondary from clause in a query expression, that translates into a call to SelectMany. As you say, that's basically a flattening operation.

It may make it easier to convert it into two queries:

var groups = from s in students
             group s by s.Last[0];

var query = from grp in groups
            orderby grp.Key
            from item in grp
            select new { item.First, item.Last };

This is exactly equivalent to the original query you've got, but you may find it easier to understand.

Other resources which may help... where Edulinq is a series of blog posts I wrote, reimplementing LINQ to Objects from scratch for educational purposes.

• LINQ Query Expressions (MSDN)
• Edulinq part 9 - SelectMany
• Edulinq part 41 - how query expressions work