• Article

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

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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

'Declaration
<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)

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

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.

Parameters

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> that has elements of type TResult 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 IQueryable<TOuter>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

outer or inner or outerKeySelector or innerKeySelector or resultSelector is nulla null reference (Nothing in Visual Basic).

Remarks

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 Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>) method generates a MethodCallExpression that represents calling Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, 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 Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<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<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<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.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:

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


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

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

         public static void JoinEx1()
         {
            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 };

            // 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 the name of pet and the name
            // of the person that owns the pet.
            var query = people.AsQueryable().Join(pets,
                            person => person,
                            pet => pet.Owner,
                            (person, pet) =>
                                new { OwnerName = person.Name, Pet = pet.Name });

            foreach (var obj in query)
            {
               outputBlock.Text += String.Format(
                   "{0} - {1}",
                   obj.OwnerName,
                   obj.Pet) + "\n";
            }
         }

         /*
             This code produces the following output:

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

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.