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EnumerableGroupJoin Method

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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
generic<typename TOuter, typename TInner, typename TKey, typename TResult>
static IEnumerable<TResult>^ GroupJoin(
	IEnumerable<TOuter>^ outer,
	IEnumerable<TInner>^ inner,
	Func<TOuter, TKey>^ outerKeySelector,
	Func<TInner, TKey>^ innerKeySelector,
	Func<TOuter, IEnumerable<TInner>^, TResult>^ resultSelector
static member GroupJoin<'TOuter, 'TInner, 'TKey, 'TResult> : 
        outer:IEnumerable<'TOuter> *
        inner:IEnumerable<'TInner> *
        outerKeySelector:Func<'TOuter, 'TKey> *
        innerKeySelector:Func<'TInner, 'TKey> *
        resultSelector:Func<'TOuter, IEnumerable<'TInner>, 'TResult> -> IEnumerable<'TResult>
Public Shared Function GroupJoin(Of TOuter, TInner, TKey, TResult) (
	outer As IEnumerable(Of TOuter),
	inner As IEnumerable(Of TInner),
	outerKeySelector As Func(Of TOuter, TKey),
	innerKeySelector As Func(Of TInner, TKey),
	resultSelector As Func(Of TOuter, IEnumerable(Of TInner), TResult)
) As IEnumerable(Of TResult)



The first sequence to join.


The sequence to join to the first sequence.


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


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


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


An IEnumerable that contains elements of type TResult that are 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 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, , 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 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:
Structure Person
    Public Name As String
End Structure

Structure Pet
    Public Name As String
    Public Owner As Person
End Structure

Sub GroupJoinEx1()
    Dim magnus As New Person With {.Name = "Hedlund, Magnus"}
    Dim terry As New Person With {.Name = "Adams, Terry"}
    Dim charlotte As New Person With {.Name = "Weiss, Charlotte"}

    Dim barley As New Pet With {.Name = "Barley", .Owner = terry}
    Dim boots As New Pet With {.Name = "Boots", .Owner = terry}
    Dim whiskers As New Pet With {.Name = "Whiskers", .Owner = charlotte}
    Dim daisy As New Pet With {.Name = "Daisy", .Owner = magnus}

    Dim people As New List(Of Person)(New Person() {magnus, terry, charlotte})
    Dim pets As New List(Of Pet)(New Pet() {barley, boots, whiskers, daisy})

    ' Create a collection 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.
    Dim query =
           Function(person) person,
           Function(pet) pet.Owner,
           Function(person, petCollection) _
               New With {.OwnerName = person.Name,
                         .Pets = petCollection.Select(
                                            Function(pet) pet.Name)})

    Dim output As New System.Text.StringBuilder
    For Each obj In query
        ' Output the owner's name.
        output.AppendLine(obj.OwnerName & ":")
        ' Output each of the owner's pet's names.
        For Each pet As String In obj.Pets
            output.AppendLine("  " & pet)

    ' Display the output.
End Sub

' This code produces the following output:
' Hedlund, Magnus
'   Daisy
' Adams, Terry
'   Barley
'   Boots
' Weiss, Charlotte
'   Whiskers
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 7.0
Windows Phone
Available since 8.1
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