What is KEY in anonymous-types {select new} ?

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First of all i am asking so gladly here because i am getting very helpfull "Explanation"
the question is about Key in anonymous-types :

in my code if i debug on 'query' i will see :

[0] = { GetKey = { Size = "18 ZOLL", Colour = "ACTION" }, GetCount = 1 }

then i will get GetCount = 1
and on GetKey i will see

GetKey = { Size = "18 ZOLL", Colour = "ACTION" }

I need to understand what a Key because with another code i got like puzzled about key ?
a good explanation based on this Example it will help me a lot .
thanks always in advanced

What I have tried:

var query = (from ph in _webshopDbContext.FXXLPurchaseHeaders
                         join pl in _webshopDbContext.FXXLPurchaseLines on ph.No_ equals pl.Document_No_
                         where ph.Ship_to_Name_2 == "Zentrallager" && pl.Brand == "CUBE"
                          && ph.Order_Date >= minDateInclusive
                          && ph.Order_Date < maxDateExclusive
                         group pl by new
                         {
                             pl.Size,
                             pl.Colour
                         }
                         into NewGroupResult
                         select new 
                         {
                            GetKey = NewGroupResult.Key,
                            GetCount = NewGroupResult.Count()

                         });

Posted 23-Sep-19 2:04am

Abdalla Ben Omran

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dnxit · Accepted Answer · 2019-10-01T03:28:00

When you call GroupBy, usually you get a series of groups

C#

IEnumerable<IGrouping<key, IEnumerable<T>>>

where each Grouping has a Key used to create the group
Also has an IEnumerable<t> of whatever items are there in your original data set.

e.g.
we have a customer class with Name, number of order and number of products

CustomerInfo(name, ordersCount, productsCount)

sample data

joe 1 5
jane 4 9
john 2 8
jim 3 1
jean 1 3
jill 2 5
jeb 3 3
jenn 4 7

C#

foreach(var item in data.GroupBy(i => i.OrdersCount)
                        .Select(group => new { 
                             Key = group.Key, 
                             Count = group.Count() 
                        }).OrderBy(x => x.OrdersCount)
{
     Console.WriteLine($"Key : {item.Key} Count : {item.Count}");
}

(Group 1): [joe 1 5, jean 1 3]
(Group 2): [john 2 8, jill 2 5]
(Group 3): [jim 3 1, jeb 3 3]
(Group 4): [jane 4 9, jenn 4 7]

In your case you're setting two keys to identify the record like a composite key

C#

group pl by new
{
    pl.Size,
    pl.Colour
}

that's the reason your key look like this
Key = { Size = "18 ZOLL", Colour = "ACTION" }

So it depends on the structure & data that you're trying to group by, accordingly you'll have a key and subset

C#

IEnumerable<T>

of super set of that data.

I hope it'll clear your doubts :)