Linq，join和null如何检查值

Question

我尝试将两个集合合二为一。如果我的第二个是空的，我只需要一个空值，这是我的代码（如果com 不为空，则正确）

    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);

我想得到AnswerSet(g, Content) 或AnswerSet(g, null) 的问题，我猜是g.Key equals c.UserID 当com 为空时

Answer 1

基本上你想要的是left outer join。你可以通过使用join into 而不仅仅是join 来做到这一点。

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);