ParallelEnumerable.GroupBy<TSource, TKey, TResult> Method (ParallelQuery<TSource>, Func<TSource, TKey>, Func<TKey, IEnumerable<TSource>, TResult>)

Groups in parallel the elements of a sequence according to a specified key selector function and creates a result value from each group and its key.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

public static ParallelQuery<TResult> GroupBy<TSource, TKey, TResult>(
	this ParallelQuery<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<TKey, IEnumerable<TSource>, TResult> resultSelector

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the result value returned by resultSelector.


Type: System.Linq.ParallelQuery<TSource>
A sequence whose elements to group.
Type: System.Func<TSource, TKey>
A function to extract the key for each element.
Type: System.Func<TKey, IEnumerable<TSource>, TResult>
A function to create a result value from each group.

Return Value

Type: System.Linq.ParallelQuery<TResult>
A sequence of elements of type TResult where each element represents a projection over a group and its key.

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

.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.
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