Queryable.GroupJoin(Of TOuter, TInner, TKey, TResult) Method (IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, IEnumerable(Of TInner), TResult)), IEqualityComparer(Of TKey))


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

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

Public Shared Function GroupJoin(Of TOuter, TInner, TKey, TResult) (
	outer As IQueryable(Of TOuter),
	inner As IEnumerable(Of TInner),
	outerKeySelector As Expression(Of Func(Of TOuter, TKey)),
	innerKeySelector As Expression(Of Func(Of TInner, TKey)),
	resultSelector As Expression(Of Func(Of TOuter, IEnumerable(Of TInner), TResult)),
	comparer As IEqualityComparer(Of TKey)
) As IQueryable(Of TResult)


Type: System.Linq.IQueryable(Of TOuter)

The first sequence to join.

Type: System.Collections.Generic.IEnumerable(Of TInner)

The sequence to join to the first sequence.

Type: System.Linq.Expressions.Expression(Of Func(Of TOuter, TKey))

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

Type: System.Linq.Expressions.Expression(Of Func(Of TInner, TKey))

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

Type: System.Linq.Expressions.Expression(Of Func(Of TOuter, IEnumerable(Of 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(Of TKey)

A comparer to hash and compare keys.

Return Value

Type: System.Linq.IQueryable(Of TResult)

An IQueryable(Of T) that contains elements of type TResult 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 has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The GroupJoin(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, IEnumerable(Of TInner), TResult)), IEqualityComparer(Of TKey)) method generates a MethodCallExpression that represents calling GroupJoin(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, IEnumerable(Of TInner), TResult)), IEqualityComparer(Of TKey)) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of 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 GroupJoin(Of TOuter, TInner, TKey, TResult)(IQueryable(Of TOuter), IEnumerable(Of TInner), Expression(Of Func(Of TOuter, TKey)), Expression(Of Func(Of TInner, TKey)), Expression(Of Func(Of TOuter, IEnumerable(Of TInner), TResult)), IEqualityComparer(Of TKey)) depends on the implementation of the type of the outer parameter. The expected behavior is that the outerKeySelector and innerKeySelector functions are used to extract keys from outer and inner, respectively. These keys are compared for equality by using comparer. The outcome of the comparisons is used to match each element in outer with zero or more elements from inner. Then the resultSelector function is invoked to project a result object from each group of correlated elements.

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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