Queryable.Join(Of TOuter, TInner, TKey, TResult) Method (IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, TInner, TResult)))

.NET Framework (current version)

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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)

Syntax

C++

<ExtensionAttribute>
Public Shared Function Join(Of TOuter, TInner, TKey, TResult) (
	outer As IQueryable(Of TOuter),
	inner As IEnumerable(Of TInner),
	outerKeySelector As Expression(Of Func(Of TOuter, TKey)),
	innerKeySelector As Expression(Of Func(Of TInner, TKey)),
	resultSelector As Expression(Of Func(Of TOuter, TInner, TResult))
) As IQueryable(Of TResult)

Parameters

outer: Type: System.Linq.IQueryable(Of TOuter)
The first sequence to join.

inner: Type: System.Collections.Generic.IEnumerable(Of TInner)
The sequence to join to the first sequence.

outerKeySelector: Type: System.Linq.Expressions.Expression(Of Func(Of TOuter, TKey))
A function to extract the join key from each element of the first sequence.

innerKeySelector: Type: System.Linq.Expressions.Expression(Of Func(Of TInner, TKey))
A function to extract the join key from each element of the second sequence.

resultSelector: Type: System.Linq.Expressions.Expression(Of Func(Of TOuter, TInner, TResult))
A function to create a result element from two matching elements.

Return Value

Type: System.Linq.IQueryable(Of TResult)

An IQueryable(Of T) that has elements of type TResult obtained by performing an inner join on two sequences.

Type Parameters

TOuter: The type of the elements of the first sequence.

TInner: The type of the elements of the second sequence.

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

TResult: The type of the result elements.

Exceptions

Exception	Condition
ArgumentNullException	outer or inner or outerKeySelector or innerKeySelector or resultSelector is null.

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

The Join(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, TInner, TResult))) method generates a MethodCallExpression that represents calling Join(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, TInner, TResult))) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of 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 Join(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, TInner, TResult))) depends on the implementation of the type of the outer parameter. The expected behavior is that of an inner join. The outerKeySelector and innerKeySelector functions are used to extract keys from outer and inner, respectively. These keys are compared for equality to match elements from each sequence. A pair of elements is stored for each element in inner that matches an element in outer. Then the resultSelector function is invoked to project a result object from each pair of matching elements.

Examples

The following code example demonstrates how to use Join(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of 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

Shared 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})

    ' Join the list of Person objects and the list of Pet objects 
    ' to create a list of person-pet pairs where each element is 
    ' an anonymous type that contains pet's name and the name of the
    ' Person object that owns the pet.
    Dim query = people.AsQueryable().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(String.Format( _
            "{0} - {1}", obj.OwnerName, obj.Pet))
    Next

    ' Display the output.
    MsgBox(output.ToString())
End Sub

' This code produces the following output:

' Hedlund, Magnus - Daisy
' Adams, Terry - Barley
' Adams, Terry - Boots
' Weiss, Charlotte - Whiskers

Version Information

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