Linq, join and null how to check a value

Question

I try to join two collection into one. If my second is empty I just need a null value, here is my code (it's correct if com is not empty)

    var tmp = List{ Elem {long UserID; string tmpContent} };
    var com = List{ Comment{long UserID; string Content} } ;

    var res = from t in tmp
              group t by t.UserID into g
              join c in com on g.Key equals c.UserID
              select new AnswerSet(new List<Answer>(g), c.Content);

I would like to get AnswerSet(g, Content) ou AnswerSet(g, null) the problem, I guess, is with g.Key equals c.UserID when com is empty

http://stackoverflow.com/a/4739738/397817 DefaultIfEmpty()? – Stephen Kennedy Jun 03 '15 at 11:12 — Stephen Kennedy, Jun 03 '15 at 11:12

score 6 · Accepted Answer · answered Jun 03 '15 at 11:15

6

Basically what you want is a left outer join. you can do that by using join into instead of just join.

var res = from t in tmp
          group t by t.UserID into g
          join c in com on g.Key equals c.UserID into j
          from subc in j.DefaultIfEmpty()
          select new AnswerSet(new List<Answer>(g), subc != null ?subc.Content : null);

answered Jun 03 '15 at 11:15

Kryptos

875
8
19

Thanks, this is perfect. – Adrien Blaizot Jun 03 '15 at 11:46

Linq, join and null how to check a value

1 Answers1