Article
11/18/2011

Enumerable.GroupBy<TSource, TKey, TElement, TResult> Method (IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, Func<TKey, IEnumerable<TElement>, 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. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.

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

Syntax

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

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

Type Parameters

TSource
The type of the elements of source.

TKey
The type of the key returned by keySelector.

TElement
The type of the elements in each IGrouping<TKey, TElement>.

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.

elementSelector
Type: System.Func<TSource, TElement>
A function to map each source element to an element in an IGrouping<TKey, TElement>.

resultSelector
Type: System.Func<TKey, IEnumerable<TElement>, 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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