Queryable.Join(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, TInner, TResult)), IEqualityComparer(Of TKey))


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Correlates the elements of two sequences based on matching keys. A specified IEqualityComparer(Of T) is used to compare keys.

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

Public Shared Function Join(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, 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, TInner, TResult))

A function to create a result element from two matching elements.

Type: System.Collections.Generic.IEqualityComparer(Of TKey)

An IEqualityComparer(Of T) to hash and compare keys.

Return Value

Type: System.Linq.IQueryable(Of TResult)

An IQueryable(Of T) that has elements of type TResult obtained by performing an inner 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 Join(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, TInner, TResult)), IEqualityComparer(Of TKey)) method generates a MethodCallExpression that represents calling Join(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, 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 Join(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, TInner, TResult)), IEqualityComparer(Of TKey)) depends on the implementation of the type of the outer parameter. The expected behavior is that of an inner join. 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 create a matching pair for each element in inner that matches an element in outer. Then the resultSelector function is invoked to project a result object from each pair of matching 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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