Doubt about JOIN of LINQ

Question

Is there a right order to use Join? For example, I have two lists: Categories and products, should I first use the from clause with Categories or Products? And then in Join? Example:

    var stockQuery = from category in categories
                     join product in products on category equals product.Category into prodGroup
                     select new { Key = category.Name, Products = prodGroup }; 

The code above works normally, but the code below does not, why?

    var stockQuery = from product in products
                     join category in categories on product.Category equals category into prodGroup
                     select new { Key = category.Name, Products = prodGroup };

Shows the error that category it’s not in the same context, but I still don’t understand why he’s not...

Answer 1

After performing a Join, two variables are in context: the item selected in the clause from and the group established under join. That is, in the following query:

from product in products
join category in categories on product.Category equals category into prodGroup
select /** TODO **/

The available variables are product and prodGroup - the product and the corresponding categories.

The variable category is not available - but does not need to be, as it would equal product.Category. This is how to solve the select

select new { Key = product.Category.Name, Products = prodGroup };