This documentation is archived and is not being maintained.

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

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

Namespace:  System.Linq
Assembly:  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

Type Parameters


The type of elements of source.


The type of the key returned by keySelector.


The type of the value returned by elementSelector.


Type: System.Linq.ParallelQuery<TSource>
The sequence to create a ILookup<TKey, TElement> from.
Type: System.Func<TSource, TKey>
A function to extract a key from each element.
Type: System.Func<TSource, TElement>
A transform function to produce a result element value from each element.

Return Value

Type: System.Linq.ILookup<TKey, TElement>
A ILookup<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).


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


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


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

.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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