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

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


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, TInner, TResult>

A function to create a result element from two matching elements.

Return Value

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

An IEnumerable<T> that has elements of type TResult that are obtained by performing an inner 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, 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<TOuter, TInner, TKey, TResult> 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<TOuter, TInner, TKey, TResult> 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<TOuter, TInner, TKey, TResult>.

In relational database terms, the Join<TOuter, TInner, TKey, TResult> 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<TOuter, TInner, TKey, TResult> method. See Join Operations.

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

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 };

    // 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.
    var query =
                    person => person,
                    pet => pet.Owner,
                    (person, pet) =>
                        new { OwnerName = person.Name, Pet = pet.Name });

    foreach (var obj in query)
            "{0} - {1}",

 This code produces the following output:

 Hedlund, Magnus - Daisy
 Adams, Terry - Barley
 Adams, Terry - 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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