C# object sorting by group, using linq

Question

I have a list of objects which are grouped by a particular property. I need to sort this list based on other properties, but it always needs to retain the grouping. So for example, if the list is something like:

{ id=1, partNumber = 100 }
{ id=2, partNumber = 500 }
{ id=2, partNumber = 300 }
{ id=2, partNumber = 600 }
{ id=3, partNumber = 550 }
{ id=3, partNumber = 990 }
{ id=4, partNumber = 200 }
{ id=5, partNumber = 300 }

then the result after sorting by part number ascending would be:

{ id=1, partNumber = 100 }
{ id=4, partNumber = 200 }
{ id=5, partNumber = 300 }
{ id=2, partNumber = 400 }
{ id=2, partNumber = 500 }
{ id=2, partNumber = 600 }
{ id=3, partNumber = 550 }
{ id=3, partNumber = 990 }

It sorts by the minimum PartNumber in each group (or maximum if the sort is descending), but must remain grouped by ID. I have tried various combinations of .OrderBy() and .GroupBy(), but I can't seem to get what I need.

At what point are you grouping the results? Are you grouping at the database level, like through a stored procedure, or is the grouping also done through LINQ? — James Johnson
– James Johnson, Commented Aug 29, 2011 at 17:20

Justin Niessner · Accepted Answer · 2011-08-29 17:38:15Z

5

var orderedList = 
    list.GroupBy(i => i.id)
        .OrderBy(g => g.Min(i => i.partNumber))
        // and if you want to flatten the groupings back out
        .SelectMany(g => g);

edited Aug 29, 2011 at 17:38

answered Aug 29, 2011 at 17:19

Justin Niessner

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3 Comments

Jimmy Over a year ago

and probably a SelectMany to ungroup afterwards

drowned Over a year ago

SelectMany seems to be what I was missing. Thanks a lot.

Aducci Over a year ago

Add a .OrderBy(i => i.partNumber) first and that will give the desired output

Aducci · Accepted Answer · 2011-08-29 17:32:33Z

0

Your before/after test sample doesn't match up, but this should solve your problem.

 var query = from a in list
             orderby a.partNumber
             group a by a.id into ag
             orderby ag.Min(x => x.partNumber)
             from a in ag
             select a;

answered Aug 29, 2011 at 17:32

Aducci

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3 Comments

drowned Over a year ago

My before test sample was an arbitrary list. I'm pretty sure the after sample matches up, in terms of being grouped by ID, ordered by part number. What do you mean?

Aducci Over a year ago

@drowned - In your first set you have id=3, partNumber = 900, that is not in your ordered set, but you have id=3, partNumber = 550

drowned Over a year ago

ah, a typo, ok. I thought you were saying that the sort was wrong. Got it, thanks.

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