Article
11/18/2011

Queryable.Join<TOuter, TInner, TKey, TResult> Method (IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<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)

Syntax

'Declaration
<ExtensionAttribute> _
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)

public static IQueryable<TResult> Join<TOuter, TInner, TKey, TResult>(
    this IQueryable<TOuter> outer,
    IEnumerable<TInner> inner,
    Expression<Func<TOuter, TKey>> outerKeySelector,
    Expression<Func<TInner, TKey>> innerKeySelector,
    Expression<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.

Parameters

outer
Type: System.Linq.IQueryable<TOuter>
The first sequence to join.

inner
Type: System.Collections.Generic.IEnumerable<TInner>
The sequence to join to the first sequence.

outerKeySelector
Type: System.Linq.Expressions.Expression<Func<TOuter, TKey>>
A function to extract the join key from each element of the first sequence.

innerKeySelector
Type: System.Linq.Expressions.Expression<Func<TInner, TKey>>
A function to extract the join key from each element of the second sequence.

resultSelector
Type: System.Linq.Expressions.Expression<Func<TOuter, TInner, TResult>>
A function to create a result element from two matching elements.

comparer
Type: System.Collections.Generic.IEqualityComparer<TKey>
An IEqualityComparer<T> to hash and compare keys.

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> that has elements of type TResult 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 IQueryable<TOuter>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

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

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>, IEqualityComparer<TKey>) method generates a MethodCallExpression that represents calling Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>, IEqualityComparer<TKey>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<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<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>, IEqualityComparer<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.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

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

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