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


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)

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: 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, 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.

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 contains elements of type TResult that are obtained by performing a grouped 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.

If comparer is null, the default equality comparer, Default, is used to hash and compare keys.

GroupJoin<TOuter, TInner, TKey, TResult> 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<TOuter, TInner, TKey, TResult> 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<TOuter, TInner, TKey, TResult> 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. See Join Operations.

Universal Windows Platform
Available since 4.5
.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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