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)

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: System.Linq.IQueryable<TOuter>

The first sequence to join.

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

The sequence to join to the first sequence.

Type: System.Linq.Expressions.Expression<Func<TOuter, TKey>>

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

Type: System.Linq.Expressions.Expression<Func<TInner, TKey>>

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

Type: System.Linq.Expressions.Expression<Func<TOuter, TInner, TResult>>

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

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.

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 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.

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.

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)
            "{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 7.1
Windows Phone
Available since 8.1
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