How to rewrite this LINQ using join with lambda expressions?

Question

It seems like most LINQ is written with lambda expressions. How do I go about rewriting this linq using lambda, kinda confusion with the style (especially with joins)?

var responses =
            from c in questionRepository.GetReponses()
            join o in questionRepository.GetQuestions() on
            c.QuestionID equals o.QuestionID
            where c.UserID == 9999
            orderby o.DisplayOrder
       select new { o.QuestionText, c.AnswerValue };

You don't really need to rewrite it. It's fine as it is, to be honest. Sometimes I use one style and sometimes the other, and this is a case where I'd probably go for the query syntax. From MSDN: As a rule when you write LINQ queries, we recommend that you use query syntax whenever possible and method syntax whenever necessary. — Mark Byers
– Mark Byers, Commented Dec 3, 2012 at 21:29
I like this form for JOIN's. With "Lambda Expressions" it requires specifying 4 arguments (plus the receiver) which, while the same as the above, seems more cluttered. — user166390
– user166390, Commented Dec 3, 2012 at 21:30
Though query syntax is easier for joins, lambda syntax is easier to debug. For details see simple-talk.com/dotnet/.net-framework/… — user8128167
– user8128167, Commented Sep 23, 2015 at 17:53

Sprintstar · Accepted Answer · 2018-08-28 09:49:43Z

18

I prefer the "LINQ syntax" for Joins as I think it looks cleaner.

In any case, here is how to translate the LINQ-join to the "Lambda Expression"-join.

The translation for:

from a in AA
join b in BB on
a.Y equals b.Y
select new {a, b}

Is:

AA.Join(                 // L
  BB,                    // R
  a => a.Y, b => b.Y,    // L -> join value, R -> join value
  (a, b) => new {a, b})  // L+R result

The other LINQ keywords are much simpler to convert (e.g. OrderBy(u => u.DisplayOrder) and are just "chained together" with .. - give it a go!

edited Aug 28, 2018 at 9:49

Sprintstar

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answered Dec 3, 2012 at 21:36

user166390

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1 Comment

Luke Hammer Over a year ago

I think you are missing a comma between b => b.Y and (a, b) => new {a, b} I new at this I could be wrong.

Sprintstar · Accepted Answer · 2018-08-28 09:51:29Z

9

var responses = questionRepository.GetReponses()
                   .Join(questionRepository.GetQuestions(), 
                         c => c.QuestionID,
                         o => o.QuestionID,
                         (c, o) => new {c, o})
                   .Where(x => x.c.UserID == 99999)
                   .OrderBy(x => x.o.DisplayOrder)
                   .Select(x => new {x.o.QuestionText, x.c.AnswerValue});

edited Aug 28, 2018 at 9:51

Sprintstar

8,1896 gold badges41 silver badges51 bronze badges

answered Dec 3, 2012 at 21:42

Sergey Berezovskiy

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