Question

Using extension syntax I’m trying to create a left-join using LINQ on two lists that I have. The following is from the Microsoft help but I’ve modified it to show that the pets list has no elements. What I’m ending up with is a list of 0 elements. I assume that this is because an inner-join is taking place. What I want to end up with is a list of 3 elements (the 3 Person objects) with null data filled in for the missing elements. i.e. a Left-Join. Is this possible?

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 }; //Pet boots = new Pet { Name = 'Boots', Owner = terry }; //Pet whiskers = new Pet { Name = 'Whiskers', Owner = charlotte }; //Pet daisy = new Pet { Name = 'Daisy', Owner = magnus };  List<Person> people = new List<Person> { magnus, terry, charlotte }; //List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy }; List<Pet> pets = new List<Pet>();  // Create a list of Person-Pet pairs where  // each element is an anonymous type that contains a // Pet's name and the name of the Person that owns the Pet. var query =     people.Join(pets,                 person => person,                 pet => pet.Owner,                 (person, pet) =>                     new { OwnerName = person.Name, Pet = pet.Name }).ToList(); 

Answer 1

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

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

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 })             .ToList();