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.

Answer 1

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);