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

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)

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

The first sequence to join.

inner
Type: System.Collections.Generic.IEnumerable<TInner>

The sequence to join to the first sequence.

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, 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.Collections.Generic.IEnumerable<TResult>
An IEnumerable<T> that has elements of type TResult that are 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 IEnumerable<TOuter>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

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.

A join refers to the operation of correlating the elements of two sources of information based on a common key. Join brings the two information sources and the keys by which they are matched together in one method call. This differs from the use of SelectMany, which requires more than one method call to perform the same operation.

Join preserves the order of the elements of outer, and for each of these elements, the order of the matching elements of inner.

In relational database terms, the Join method implements an inner equijoin. 'Inner' means that only elements that have a match in the other sequence are included in the results. An 'equijoin' is a join in which the keys are compared for equality. A left outer join operation has no dedicated standard query operator, but can be performed by using the GroupJoin method. See Join Operations.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

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