Perform left outer joins

A left outer join is a join in which each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection. You can use LINQ to perform a left outer join by calling the DefaultIfEmpty method on the results of a group join.


The example in this topic uses the Pet and Person data classes from Perform inner joins.


The following example demonstrates how to use the DefaultIfEmpty method on the results of a group join to perform a left outer join.

The first step in producing a left outer join of two collections is to perform an inner join by using a group join. (See Perform inner joins for an explanation of this process.) In this example, the list of Person objects is inner-joined to the list of Pet objects based on a Person object that matches Pet.Owner.

The second step is to include each element of the first (left) collection in the result set even if that element has no matches in the right collection. This is accomplished by calling DefaultIfEmpty on each sequence of matching elements from the group join. In this example, DefaultIfEmpty is called on each sequence of matching Pet objects. The method returns a collection that contains a single, default value if the sequence of matching Pet objects is empty for any Person object, thereby ensuring that each Person object is represented in the result collection.


The default value for a reference type is null; therefore, the example checks for a null reference before accessing each element of each Pet collection.

Person magnus = new("Magnus", "Hedlund");
Person terry = new("Terry", "Adams");
Person charlotte = new("Charlotte", "Weiss");
Person arlene = new("Arlene", "Huff");

Pet barley = new("Barley", terry);
Pet boots = new("Boots", terry);
Pet whiskers = new("Whiskers", charlotte);
Pet bluemoon = new("Blue Moon", terry);
Pet daisy = new("Daisy", magnus);

// Create two lists.
List<Person> people = new() { magnus, terry, charlotte, arlene };
List<Pet> pets = new() { barley, boots, whiskers, bluemoon, daisy };

var query =
    from person in people
    join pet in pets on person equals pet.Owner into gj
    from subpet in gj.DefaultIfEmpty()
    select new
        PetName = subpet?.Name ?? string.Empty

foreach (var v in query)
    Console.WriteLine($"{v.FirstName + ":",-15}{v.PetName}");

record class Person(string FirstName, string LastName);
record class Pet(string Name, Person Owner);

// This code produces the following output:
// Magnus:        Daisy
// Terry:         Barley
// Terry:         Boots
// Terry:         Blue Moon
// Charlotte:     Whiskers
// Arlene:

See also