Linq and group by clause

Question

I am curious what the x is for in a a linq group by clause: group x by ...

The x can be replaced by an 1:

       var query = from box in c.Boxes
                    join item in c.Items on box equals item.Box
                    group 1 by new { BoxId = box.BoxId, item.ItemType.ItemTypeId } into g
                    select new {  g.Key.BoxId, g.Key.ItemTypeId, Count = g.Count() };

Does anybody have a sample where the x (or wathever local variable you chose in the group by ) is really of some value?

I mean

var query2 =  from box in c.Boxes
                              group box by box.BoxId into q
                              select q.Key;

can be replaced by

var query2 =  from box in c.Boxes
                              group 1 by box.BoxId into q
                              select q.Key;

Answer 1

In group x by... the x is what gets aggregated.

So you could write something like

var childrenByAge = from child in class
                    group getName(child) by child.Age;

This gives you a grouping containing names of children for each age.

Here is a simple example where you can test the difference easily:

    static void Main(string[] args)
    {
        var grouping = from c in "Hello World!"
                       group c by  c;  // Replace 'group c by' with 'group 1 by'
                                       // and compare results


        foreach (var group in grouping)
        {
            Console.Write("KEY:  {0} VALUES :", group.Key);
            foreach (var value in group)
                Console.Write(" {0}", value);

            Console.WriteLine();
        }

        Console.ReadKey();

    }

Answer 2

That's the expression that determines what will go in the output of the group by clause.

You are actually not inspecting the whole output, but only the Keys, which are the same in the examples above, because the grouping is done by the same thing (box.BoxId).

However, replace the last line:

select q.Key;

with

select q;

You will notice that:

• the group 1 by ... query will return an IGrouping<int, int> which will have all values set to 1
• the group box by ... query will return an IGrouping<int, Box>, which will have all the boxId keys and, for each of them, the corresponding Box objects with respect to the grouping criteria.