This documentation is archived and is not being maintained.

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

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)

generic<typename TSource, typename TKey, typename TElement, typename TResult>
static IEnumerable<TResult>^ GroupBy(
	IEnumerable<TSource>^ source, 
	Func<TSource, TKey>^ keySelector, 
	Func<TSource, TElement>^ elementSelector, 
	Func<TKey, IEnumerable<TElement>^, TResult>^ resultSelector, 
	IEqualityComparer<TKey>^ comparer

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


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


The type of the result value returned by resultSelector.


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

An IEnumerable<T> whose elements to group.

Type: System::Func<TSource, TKey>

A function to extract the key for each element.

Type: System::Func<TSource, TElement>

A function to map each source element to an element in an IGrouping<TKey, TElement>.

Type: System::Func<TKey, IEnumerable<TElement>, TResult>

A function to create a result value from each group.

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0