Accepted answer

I think if you want to use extension methods you need to use the GroupJoin

var query =
                     person => person,
                     pet => pet.Owner,
                     (person, petCollection) =>
                        new { OwnerName = person.Name,
                              Pet = PetCollection.Select( p => p.Name )
                                                 .DefaultIfEmpty() }

You may have to play around with the selection expression. I'm not sure it would give you want you want in the case where you have a 1-to-many relationship.

I think it's a little easier with the LINQ Query syntax

var query = (from person in context.People
             join pet in context.Pets on person equals pet.Owner
             into tempPets
             from pets in tempPets.DefaultIfEmpty()
             select new { OwnerName = person.Name, Pet = pets.Name })


If you actually have a database, this is the most-simple way:

var lsPetOwners = ( from person in context.People
                    from pets in context.Pets
                        .Where(mypet => mypet.Owner == person.ID) 
                     select new { OwnerName = person.Name, Pet = pets.Name }


Left joins in LINQ are possible with the DefaultIfEmpty() method. I don't have the exact syntax for your case though...

Actually I think if you just change pets to pets.DefaultIfEmpty() in the query it might work...

EDIT: I really shouldn't answer things when its late...


Here's a good blog post that's just been posted by Fabrice (author of LINQ in Action) which covers the material in the question that I asked. I'm putting it here for reference as readers of the question will find this useful.

Converting LINQ queries from query syntax to method/operator syntax


I the following error message when faced this same problem:

The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'GroupJoin'.

Solved when I used the same property name, it worked.


join enderecoST in db.PessoaEnderecos on 
         CD_PESSOA          = nf.CD_PESSOA_ST, 
      } equals 
    } into eST



You need to get the joined objects into a set and then apply DefaultIfEmpty as JPunyon said:

Person magnus = new Person { Name = "Hedlund, Magnus" };
Person terry = new Person { Name = "Adams, Terry" };
Person charlotte = new Person { Name = "Weiss, Charlotte" };

Pet barley = new Pet { Name = "Barley", Owner = terry };
List<Person> people = new List<Person> { magnus, terry, charlotte };
List<Pet> pets = new List<Pet>{barley};

var results =
    from person in people
    join pet in pets on person.Name equals pet.Owner.Name into ownedPets
    from ownedPet in ownedPets.DefaultIfEmpty(new Pet())
    orderby person.Name
    select new { OwnerName = person.Name, ownedPet.Name };

foreach (var item in results)
        String.Format("{0,-25} has {1}", item.OwnerName, item.Name ) );


Adams, Terry              has Barley
Hedlund, Magnus           has
Weiss, Charlotte          has

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