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

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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)


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

  • 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.


  • outerKeySelector
    Type: System.Func<TOuter, TKey>
    A function to extract the join key from each element of the first sequence.
  • innerKeySelector
    Type: System.Func<TInner, TKey>
    A function to extract the join key from each element of the second sequence.
  • resultSelector
    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.

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.


Exception Condition

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


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 nulla null reference (Nothing 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 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.

Version Information


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0


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