Enumerable.GroupBy<TSource, TKey> Method (IEnumerable<TSource>, Func<TSource, 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.
Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)
Syntax
'Declaration
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey) ( _
source As IEnumerable(Of TSource), _
keySelector As Func(Of TSource, TKey) _
) As IEnumerable(Of IGrouping(Of TKey, TSource))
public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by keySelector.
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.
Return Value
Type: System.Collections.Generic.IEnumerable<IGrouping<TKey, TSource>>
An IEnumerable<IGrouping<TKey, TSource>> in C# or IEnumerable(Of IGrouping(Of TKey, TSource)) in Visual Basic where each IGrouping<TKey, TElement> object contains a sequence of objects 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.
Exceptions
| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector is nulla null reference (Nothing in Visual Basic). |
Remarks
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>(IEnumerable<TSource>, Func<TSource, 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 they appear in source.
The default equality comparer Default is used to compare keys.
In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy.
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.