Queryable.GroupJoin<TOuter, TInner, TKey, TResult> Method (IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>)


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Correlates the elements of two sequences based on key equality and groups the results. The default equality comparer is used to compare keys.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IQueryable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
	this IQueryable<TOuter> outer,
	IEnumerable<TInner> inner,
	Expression<Func<TOuter, TKey>> outerKeySelector,
	Expression<Func<TInner, TKey>> innerKeySelector,
	Expression<Func<TOuter, IEnumerable<TInner>, TResult>> resultSelector


Type: System.Linq.IQueryable<TOuter>

The first sequence to join.

Type: System.Collections.Generic.IEnumerable<TInner>

The sequence to join to the first sequence.

Type: System.Linq.Expressions.Expression<Func<TOuter, TKey>>

A function to extract the join key from each element of the first sequence.

Type: System.Linq.Expressions.Expression<Func<TInner, TKey>>

A function to extract the join key from each element of the second sequence.

Type: System.Linq.Expressions.Expression<Func<TOuter, IEnumerable<TInner>, TResult>>

A function to create a result element from an element from the first sequence and a collection of matching elements from the second sequence.

Return Value

Type: System.Linq.IQueryable<TResult>

An IQueryable<T> that contains elements of type TResult obtained by performing a grouped join on two sequences.

Type Parameters


The type of the elements of the first sequence.


The type of the elements of the second sequence.


The type of the keys returned by the key selector functions.


The type of the result elements.

Exception Condition

outer or inner or outerKeySelector or innerKeySelector or resultSelector is null.

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) method generates a MethodCallExpression that represents calling GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the outer parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) depends on the implementation of the type of the outer parameter. The expected behavior is that the outerKeySelector and innerKeySelector functions are used to extract keys from outer and inner, respectively. These keys are compared for equality to match each element in outer with zero or more elements from inner. Then the resultSelector function is invoked to project a result object from each group of correlated elements.

The following code example demonstrates how to use GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) to perform a grouped join on two sequences.

class Person
    public string Name { get; set; }

class Pet
    public string Name { get; set; }
    public Person Owner { get; set; }

public static void GroupJoinEx1()
    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 };

    // Create a list where each element is an anonymous 
    // type that contains a person's name and a collection 
    // of names of the pets that are owned by them.
    var query =
                         person => person,
                         pet => pet.Owner,
                         (person, petCollection) =>
                                 OwnerName = person.Name,
                                 Pets = petCollection.Select(pet => pet.Name)

    foreach (var obj in query)
        // Output the owner's name.
        Console.WriteLine("{0}:", obj.OwnerName);
        // Output each of the owner's pet's names.
        foreach (string pet in obj.Pets)
            Console.WriteLine("  {0}", pet);

    This code produces the following output:

    Hedlund, Magnus:
    Adams, Terry:
    Weiss, Charlotte:

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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