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MSDN Library

ParallelEnumerable.GroupBy(TSource, TKey) Method (ParallelQuery(TSource), Func(TSource, TKey))

Groups in parallel the elements of a sequence according to a specified key selector function.

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

static member GroupBy : 
        source:ParallelQuery<'TSource> * 
        keySelector:Func<'TSource, 'TKey> -> ParallelQuery<IGrouping<'TKey, 'TSource>> 

Type Parameters


The type of elements of source.


The type of the key returned by keySelector.


Type: System.Linq.ParallelQuery(TSource)

An OrderedParallelQuery{TSource}that contains elements to sort.

Type: System.Func(TSource, TKey)

A function to extract a key from an element.

Return Value

Type: System.Linq.ParallelQuery(IGrouping(TKey, TSource))
A sequence of groups that are sorted descending according to TKey.

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 action is a null reference (Nothing in Visual Basic).


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

.NET for Windows Phone apps

Supported in: Windows Phone 8.1

Portable Class Library

Supported in: Portable Class Library
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