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

Groups the elements of a sequence according to a key selector function. The keys are compared by using a comparer and each group's elements are projected by using a specified function.

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

[ExtensionAttribute]
public:
generic<typename TSource, typename TKey, typename TElement>
static IEnumerable<IGrouping<TKey, TElement>^>^ GroupBy(
	IEnumerable<TSource>^ source, 
	Func<TSource, TKey>^ keySelector, 
	Func<TSource, TElement>^ elementSelector, 
	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 the IGrouping<TKey, TElement>.

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>.

comparer
Type: System.Collections.Generic::IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys.

Return Value

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

ExceptionCondition
ArgumentNullException

source or keySelector or elementSelector is nullptr.

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, TElement>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, IEqualityComparer<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 that the elements that produced them appear in source.

If comparer is nullptr, the default equality comparer Default is used to compare keys.

If two keys are considered equal according to comparer, the first key is chosen as the key for that grouping.

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).

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

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