Join(TOuter, TInner, TKey, TResult) Method (IEnumerable(TOuter), IEnumerable(TInner), Func(TOuter, TKey), Func(TInner, TKey), Func(TOuter, TInner, TResult), IEqualityComparer(TKey))

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

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Correlates the elements of two sequences based on matching keys. A specified IEqualityComparer<T> 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,
	IEqualityComparer<TKey> comparer

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<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.
Type: System.Collections.Generic.IEqualityComparer<TKey>
An IEqualityComparer<T> to hash and compare keys.

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.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TOuter>. When you use instance method syntax to call this method, omit the first parameter.


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.

If comparer is null, 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 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.

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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