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

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Correlates the elements of two sequences based on key equality and groups the results. A specified IEqualityComparer<T> is used to compare keys.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)


<ExtensionAttribute> _
Public Shared Function GroupJoin(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, IEnumerable(Of TInner), TResult), _
    comparer As IEqualityComparer(Of TKey) _
) As IEnumerable(Of TResult)
public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, IEnumerable<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, IEnumerable<TInner>, TResult>
    A function to create a result element from an element from the first sequence and a collection of matching elements from the second sequence.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>
An IEnumerable<T> that contains elements of type TResult that are obtained by performing a grouped 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.

GroupJoin produces hierarchical results, which means that elements from outer are paired with collections of matching elements from inner. GroupJoin enables you to base your results on a whole set of matches for each element of outer.


If there are no correlated elements in inner for a given element of outer, the sequence of matches for that element will be empty but will still appear in the results.

The resultSelector function is called only one time for each outer element together with a collection of all the inner elements that match the outer element. This differs from the Join method in which the result selector function is invoked on pairs that contain one element from outer and one element from inner.

GroupJoin preserves the order of the elements of outer, and for each element of outer, the order of the matching elements from inner.

GroupJoin has no direct equivalent in traditional relational database terms. However, this method does implement a superset of inner joins and left outer joins. Both of these operations can be written in terms of a grouped join.

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.