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Editorial Team
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Asked: 2026-05-29T07:09:12+00:00

I’m new to Linq to object and have a question about the from clause

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I’m new to Linq to object and have a question about the from clause when used in two contexts:

1) to perform a cross join like the query bellow

  var q1 = from person in people
      from job in jobs
      select new {person, job}

2) to perform an outer join

  var q2 = from person in people
     join pet in pets on person equals pet.Owner into gj
     from subpet in gj
     select new { OwnerName = person.FirstName, PetName = subpet.Name };

does the second from clause acts as a cross join or is it evaluated depending on the context? because
q1 will produce people.Count * jobs.Count element
but q2 will produce only people.Count

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1 Answer

  1. Editorial Team

    According to MS documentation, to make the second query the left outer join you should use the DefaultIfEmpty method as seen here. The from clause always evaluates the same way: it returns each element in the sequence, will it be either predefined source, or a context variable.

    EDIT: I’ll try to explain from the start. At first you inner join (group join, to be specific) the people and the pets. Then you select from the resulting collection (actually the person-pets set) to the new anonymous object, getting the person name (from the gj set element) and the subpet name (for each of the pets in the gj set element). I think the second from does not do crossjoin because it selects from gj, and each person is already a part of gj set element. If you call the gj.DefaultIfEmpty() method in the second from clause, then the person without any pets (with empty pets collection inside the gj set element) will be added to the resulting set. You can read the following articles to understand it better:

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