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Enumerable.Join(Of TOuter, TInner, TKey, TResult) Method (IEnumerable(Of TOuter), IEnumerable(Of TInner), Func(Of TOuter, TKey), Func(Of TInner, TKey), Func(Of TOuter, TInner, TResult))

Correlates the elements of two sequences based on matching keys. The default equality comparer is used to compare keys.

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

<ExtensionAttribute> _
Public Shared Function Join(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, TInner, TResult) _
) As IEnumerable(Of TResult)

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(Of TOuter)
The first sequence to join.
Type: System.Collections.Generic.IEnumerable(Of TInner)
The sequence to join to the first sequence.
Type: System.Func(Of TOuter, TKey)
A function to extract the join key from each element of the first sequence.
Type: System.Func(Of TInner, TKey)
A function to extract the join key from each element of the second sequence.
Type: System.Func(Of TOuter, TInner, TResult)
A function to create a result element from two matching elements.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TResult)
An IEnumerable(Of T) that has elements of type TResult that are obtained by performing an inner 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(Of 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 Nothing.

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.

A join refers to the operation of correlating the elements of two sources of information based on a common key. Join brings the two information sources and the keys by which they are matched together in one method call. This differs from the use of SelectMany, which requires more than one method call to perform the same operation.

Join preserves the order of the elements of outer, and for each of these elements, the order of the matching elements of inner.

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

In relational database terms, the Join method implements an inner equijoin. 'Inner' means that only elements that have a match in the other sequence are included in the results. An 'equijoin' is a join in which the keys are compared for equality. A left outer join operation has no dedicated standard query operator, but can be performed by using the GroupJoin method. See Join Operations.

The following code example demonstrates how to use Join(Of TOuter, TInner, TKey, TResult)(IEnumerable(Of TOuter), IEnumerable(Of TInner), Func(Of TOuter, TKey), Func(Of TInner, TKey), Func(Of TOuter, TInner, TResult)) to perform an inner join of two sequences based on a common key.

    Structure Person
        Public Name As String
    End Structure

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

    Sub JoinEx1()
        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 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.
        Dim query = _
            people.Join(pets, _
                        Function(person) person, _
                        Function(pet) pet.Owner, _
                        Function(person, pet) _
                            New With {.OwnerName = person.Name, .Pet = pet.Name})

        Dim output As New System.Text.StringBuilder
        For Each obj In query
            output.AppendLine(obj.OwnerName & " - " & obj.Pet)

        ' Display the output.
    End Sub

    ' This code produces the following output:
    ' Hedlund, Magnus - Daisy
    ' Adams, Terry - Barley
    ' Adams, Terry - Boots
    ' Weiss, Charlotte - Whiskers

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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