This documentation is archived and is not being maintained.

Enumerable.GroupJoin<TOuter, TInner, TKey, TResult> Method (IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter, TKey>, Func<TInner, TKey>, Func<TOuter, IEnumerable<TInner>, TResult>)

Correlates the elements of two sequences based on equality of keys 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 IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
	this IEnumerable<TOuter> outer,
	IEnumerable<TInner> inner,
	Func<TOuter, TKey> outerKeySelector,
	Func<TInner, TKey> innerKeySelector,
	Func<TOuter, IEnumerable<TInner>, TResult> resultSelector

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.


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

The first sequence to join.

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

The sequence to join to the first sequence.

Type: System.Func<TOuter, TKey>

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

Type: System.Func<TInner, TKey>

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

Type: System.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.Collections.Generic.IEnumerable<TResult>
An IEnumerable<T> that contains elements of type TResult that are obtained by performing a grouped join on two sequences.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TOuter>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).


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

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The default equality comparer, Default, is used to hash and compare keys.

GroupJoin produces hierarchical results, which means that elements from outer are paired with collections of matching elements from inner. GroupJoin enables you to base your results on a whole set of matches for each element of outer.


If there are no correlated elements in inner for a given element of outer, the sequence of matches for that element will be empty but will still appear in the results.

The resultSelector function is called only one time for each outer element together with a collection of all the inner elements that match the outer element. This differs from the Join method, in which the result selector function is invoked on pairs that contain one element from outer and one element from inner.

GroupJoin preserves the order of the elements of outer, and for each element of outer, the order of the matching elements from inner.

GroupJoin has no direct equivalent in traditional relational database terms. However, this method does implement a superset of inner joins and left outer joins. Both of these operations can be written in terms of a grouped join. See Join Operations.

In query expression syntax, a join … into (Visual C#) or Group Join (Visual Basic) clause translates to an invocation of GroupJoin.

The following code example demonstrates how to use GroupJoin<TOuter, TInner, TKey, TResult>(IEnumerable<TOuter>, IEnumerable<TInner>, Func<TOuter, TKey>, Func<TInner, TKey>, 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 they own.
    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:

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0