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.NET Framework Class Library

Enumerable.GroupBy<TSource, TKey> Method (IEnumerable<TSource>, Func<TSource, TKey>)

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

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey) ( _
	source As IEnumerable(Of TSource), _
	keySelector As Func(Of TSource, TKey) _
) As IEnumerable(Of IGrouping(Of TKey, TSource))

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Type: System.Func<TSource, TKey>

A function to extract the key for each element.

Return Value

Type: System.Collections.Generic.IEnumerable<IGrouping<TKey, TSource>>
An IEnumerable<IGrouping<TKey, TSource>> in C# or IEnumerable(Of IGrouping(Of TKey, TSource)) in Visual Basic where each IGrouping<TKey, TElement> object contains a sequence of objects and a key.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<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).
Exception Condition
ArgumentNullException

source or keySelector 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.

The GroupBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>) method returns a collection of IGrouping<TKey, TElement> objects, one for each distinct key that was encountered. An IGrouping<TKey, TElement> is an IEnumerable<T> that also has a key associated with its elements.

The IGrouping<TKey, TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey, TElement>. Elements in a grouping are yielded in the order they appear in source.

The default equality comparer Default is used to compare keys.

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy. For more information and usage examples, see group clause (C# Reference) and Group By Clause (Visual Basic).

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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