LINQ - Distinct by value?

Question

Code :

news = (from New myNew in new News()
       select myNew).Distinct().ToList();

but this Distinct is for "object" with same values. I need, into my list, a myNew for each month. (so one for january, one for februaru, and so on). Than, news will get 12 record.

Is it possible a sort of Distinct(myNew.Month)?

If you have multiple records for a month, how do you want decide which one to take? The first one perhaps? — Jamiec, May 09 '12 at 10:14
Well, the answer to the question I added gives myCustomerList.GroupBy(cust => cust.CustomerId).Select(grp => grp.First()); — Danny Varod, May 09 '12 at 10:25
@Perception: This answer is not a duplicate at all from ["Distinct() with lambda?"](http://stackoverflow.com/questions/1300088/distinct-with-lambda) as you can see easily from [my answer](http://stackoverflow.com/a/10514121/284240). — Tim Schmelter, May 10 '12 at 10:41
Does this answer your question? [LINQ's Distinct() on a particular property](https://stackoverflow.com/questions/489258/linqs-distinct-on-a-particular-property) — Magnetron, Jun 01 '21 at 12:09

Tim Schmelter · Accepted Answer · 2012-05-10T11:11:15.220

15

You could group by month and take the first or last or whatever(you haven't told us):

var news = News()
           .GroupBy(n => n.Month)
           .Select(grp => grp.Last());

Edit: From the comment on Habib's answer i see that you want 12 months even if there are no news. Then you need to do a "Linq Outer-Join":

var monthlyNews = from m in Enumerable.Range(1, 12) // left outer join every month
                  join n in News() on m equals n.Month into m_n
                  from n in m_n.DefaultIfEmpty()
                  group n by m into MonthGroups
                  select new {
                      Month = MonthGroups.Key, 
                      LastNews = MonthGroups.Last() 
                  };
foreach (var m in monthlyNews)
{
    int month = m.Month;
    var lastNewsInMonth = m.LastNews;
    if (lastNewsInMonth != null) ; // do something...
}

Edit: Since you have problems to implement the query in your code, you don't need to select the anonymous type which contains also the month. You can also select only the news itself:

var monthlyNews = from m in Enumerable.Range(1, 12) // every motnh
                  join n in news on m equals n.Month into m_n
                  from n in m_n.DefaultIfEmpty()
                  group n by m into MonthGroups
                  select MonthGroups.Last();

Note that you now get 12 news but some of them might be null when there are no news in that month.

edited May 10 '12 at 11:11

answered May 09 '12 at 10:18

Tim Schmelter

450,073
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Why post the answer after this has been marked as a duplicate with the exact same answer? – Danny Varod May 09 '12 at 10:26
@DannyVarod: Because 1. it's not exact the same answer(btw,i haven't seen that answer before) 2. both question might differ more from each other if OP would clarify his requirement. – Tim Schmelter May 09 '12 at 10:36
@DannyVarod: Is it better now? ;-) – Tim Schmelter May 09 '12 at 11:09
The problem here is that News() is a List... seems I can't write your code... – markzzz May 10 '12 at 10:42
@markzzz: So what's the problem? When `News` is a `List` write `...join n in News on m equals n.Month into m_n...`. – Tim Schmelter May 10 '12 at 10:48
But this create a list of pair value (Month, LastNews). I need to create a list of (just) New into "monthlyNews" (which is a List) – markzzz May 10 '12 at 10:57
You can easily change the query to select not the anonymous type with the month but only the news itself. I'll edit my answer. – Tim Schmelter May 10 '12 at 11:09
You rock as well! Thank you :) – markzzz May 10 '12 at 12:26

score 6 · Answer 2 · answered May 09 '12 at 15:20

6

Solution 1. Get MoreLinq (also available as NuGet package and use

  News().DistinctBy(n => n.Property)

Solution 2. Implement an IEqualityComparer and use this Distinct() overload.

answered May 09 '12 at 15:20

georgiosd

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score 5 · Answer 3 · answered May 09 '12 at 10:18

5

var result  =  News()
  .GroupBy(p => p.Month)
  .Select(g => g.First())
  .ToList();

answered May 09 '12 at 10:18

Habib

219,104
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the problem is : if the april doesnt exist in any new...how can I insert "null" instead? – markzzz May 09 '12 at 10:28

score 2 · Answer 4 · answered Aug 20 '14 at 13:10

2

Short hand solution

var vNews  =  News()   
              .GroupBy(p => p.Month, (key, p) => p.FirstOrDefault()) 
              .ToList();

answered Aug 20 '14 at 13:10

Arasu RRK

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score 0 · Answer 5 · answered May 09 '12 at 10:29

0

var vNews  =  News()   
              .GroupBy(p => p.Month)   
              .Select(g => g.First())   
              .ToList();

answered May 09 '12 at 10:29

Kishore Kumar

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LINQ - Distinct by value?

5 Answers5

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