Get Distinct using LINQ to Objects

Question

My data looks like the following image

So i loaded it into a collection, using anon objects to bypass creating a class

List<dynamic> myList = new List<dynamic>()
        {
            new {
                WorkOrderID = 40001,
                LastUpdatedAt = DateTime.Parse("2018-10-02"),
                Col3 = "abc",
                Col4 = "xyz"
            },
            new {
                WorkOrderID = 40001,
                LastUpdatedAt = DateTime.Parse("2017-06-01"),
                Col3 = "abc",
                Col4 = "xyz"
            },
            new {
                WorkOrderID = 40002,
                LastUpdatedAt = DateTime.Parse("2018-07-01"),
                Col3 = "abc",
                Col4 = "xyz"
            },
            new {
                WorkOrderID = 40003,
                LastUpdatedAt = DateTime.Parse("2018-09-01"),
                Col3 = "abc",
                Col4 = "xyz"
            },
            new {
                WorkOrderID = 40001,
                LastUpdatedAt = DateTime.Parse("2016-01-01"),
                Col3 = "abc",
                Col4 = "xyz"
            },
            new {
                WorkOrderID = 40002,
                LastUpdatedAt = DateTime.Parse("2016-12-01"),
                Col3 = "abc",
                Col4 = "xyz"
            },
        };

How do I use Linq-to-Objects to extract only DISTINCT WorkOrders that were updated most recently.

In other words, i should end up with the 3 rows highlighted in Orange.

var myFilteredList = myList.GroupBy( //?? Totally lost... help...

It would be awesome if you could be explicit about the results you are expecting. — mjwills
– mjwills, Commented Oct 16, 2018 at 2:44

ProgrammingLlama · Accepted Answer · 2018-10-16 02:52:46Z

2

If I'm not misunderstanding, you want the newest item per work order.

var latestWorkOrders = myList
    .GroupBy(w => w.WorkOrderID)
    .Select(g => g.OrderByDescending(w => w.LastUpdatedAt).First());

It will group by the WorkOrderID, and for each grouping select the newest item.

edited Oct 16, 2018 at 2:52

answered Oct 16, 2018 at 2:51

ProgrammingLlama

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2 Comments

joedotnot Over a year ago

is your left hand side latestWorkOrders a LINQ query, rather than final result? i.e. i still need to do var finalList = latestWorkOrders.ToList()

ProgrammingLlama Over a year ago

Sorry, yes. It's a LINQ query, so you will need .ToList().

Nkosi · Accepted Answer · 2018-10-16 03:35:45Z

2

The following uses the GroupBy overload so the element selection can be done in the same call and convert them to strongly typed anonymous object.

var myFilteredList = myList
    .GroupBy(_ => (int)_.WorkOrderID, (k, g) =>
    {
        var recent = g.OrderByDescending(_ => _.LastUpdatedAt).First();
        return new
        {
            WorkOrderID = k,
            LastUpdatedAt = (DateTime)recent.LastUpdatedAt,
            Col3 = (string)recent.Col3,
            Col4 = (string)recent.Col4
        };
    });

Fiddle

edited Oct 16, 2018 at 3:35

answered Oct 16, 2018 at 3:08

Nkosi

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1 Comment

joedotnot Over a year ago

Thanks for your answer, It does work but i went with the first correct reply; Your left hand side is a linq query, rather than the final list, missing a .ToList() cast at the end ?

Gauravsa · Accepted Answer · 2018-10-16 03:08:41Z

0

List<dynamic> list = myList
               .GroupBy(a => a.WorkOrderID )
               .OrderByDescending(b => b.LastUpdatedAt)
               .Select(g => g.FirstOrDefault())
               .ToList();

foreach (var p in list)
{
    Console.WriteLine(p.WorkOrderID + " : " + p.Col3);
}

Personally, dynamic isnt a good choice. If you have an object, then you can implement Equals and GetHashCode on the Object. This way you can do:

var list = myList.Select(o => new { o.WorkOrderID, ... })
                                       .Distinct();

which is much cleaner.

answered Oct 16, 2018 at 3:08

Gauravsa

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1 Comment

joedotnot Over a year ago

IGrouping<dynamic, dynamic> does not contain a definition for LastUpdatedAt.

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