Article
11/18/2011

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

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The keys are compared by using a specified comparer.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey, TResult) ( _
    source As IEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey), _
    resultSelector As Func(Of TKey, IEnumerable(Of TSource), TResult), _
    comparer As IEqualityComparer(Of TKey) _
) As IEnumerable(Of TResult)

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

Type Parameters

TSource
The type of the elements of source.

TKey
The type of the key returned by keySelector.

TResult
The type of the result value returned by resultSelector.

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.

resultSelector
Type: System.Func<TKey, IEnumerable<TSource>, TResult>
A function to create a result value from each group.

comparer
Type: System.Collections.Generic.IEqualityComparer<TKey>
An IEqualityComparer<T> to compare keys with.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>
A collection 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 IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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