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

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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 Shared Function GroupBy(Of TSource, TKey, TElement) ( _
    source As IEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey), _
    elementSelector As Func(Of TSource, TElement), _
    comparer As IEqualityComparer(Of TKey) _
) As IEnumerable(Of IGrouping(Of TKey, TElement))
public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
    this 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.


  • keySelector
    Type: System..::.Func<(Of <(TSource, TKey>)>)
    A function to extract the key for each element.

Return Value

Type: System.Collections.Generic..::.IEnumerable<(Of <(IGrouping<(Of <(TKey, TElement>)>)>)>)
An IEnumerable<IGrouping<TKey, TElement>> in C# or IEnumerable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping<(Of <(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<(Of <(TSource>)>). When you use instance method syntax to call this method, omit the first parameter.


Exception Condition

source or keySelector or elementSelector is nullNothingnullptra null reference (Nothing in Visual Basic).


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

The IGrouping<(Of <(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<(Of <(TKey, TElement>)>). Elements in a grouping are yielded in the order that the elements that produced them appear in source.

If comparer is nullNothingnullptra null reference (Nothing in Visual Basic), 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 Enumerable.GroupBy.

Version Information

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0


Windows Phone

See Also


Enumerable Class

GroupBy Overload

System.Linq Namespace