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ParallelEnumerable.ToLookup<TSource, TKey, TElement> Method (ParallelQuery<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, IEqualityComparer<TKey>)

Creates an ILookup<TKey, TElement> from a ParallelQuery<TSource> according to a specified key selector function, a comparer and an element selector function.

Namespace:  System.Linq
Assemblies:   System.Linq.Parallel (in System.Linq.Parallel.dll)
  System.Core (in System.Core.dll)

public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(
	this ParallelQuery<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<TSource, TElement> elementSelector,
	IEqualityComparer<TKey> comparer
)

Type Parameters

TSource

The type of elements of source.

TKey

The type of the key returned by keySelector.

TElement

The type of the value returned by elementSelector.

Parameters

source
Type: System.Linq.ParallelQuery<TSource>

The sequence to create a ILookup<TKey, TElement> from.

keySelector
Type: System.Func<TSource, TKey>

A function to extract a key from each element.

elementSelector
Type: System.Func<TSource, TElement>

A transform function to produce a result element value from each element.

comparer
Type: System.Collections.Generic.IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys.

Return Value

Type: System.Linq.ILookup<TKey, TElement>
A Lookup<(Of <(TKey, TElement>)>) that contains values of type TElement selected from the input sequence.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type ParallelQuery<TSource>. 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
OperationCanceledException

The query was canceled with the token passed in through WithCancellation.

ArgumentNullException

source or keySelector or elementSelector is a null reference (Nothing in Visual Basic).

AggregateException

One or more exceptions occurred during the evaluation of the query.

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1
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