Enumerable.GroupBy(TSource, TKey) Method (IEnumerable(TSource), Func(TSource, TKey), IEqualityComparer(TKey))
Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer.
Assemblies: System.Linq (in System.Linq.dll)
System.Core (in System.Core.dll)
static member GroupBy : source:IEnumerable<'TSource> * keySelector:Func<'TSource, 'TKey> * comparer:IEqualityComparer<'TKey> -> IEnumerable<IGrouping<'TKey, 'TSource>>
The type of the elements of source.
The type of the key returned by keySelector.
- Type: System.Func(, )
A function to extract the key for each element.
Return ValueType: System.Collections.Generic.IEnumerable(IGrouping(, ))
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 collection of objects and a key.
Usage NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(). 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).
source or keySelector is a 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.
For examples of GroupBy, see the following topics.
The 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.
If comparer is a 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 GroupBy. For more information and usage examples, see group clause (C# Reference) and Group By Clause (Visual Basic).
.NET FrameworkSupported in: 4.6, 4.5, 4, 3.5
.NET Framework Client ProfileSupported in: 4, 3.5 SP1
Portable Class LibrarySupported in: Portable Class Library
.NET for Windows Store appsSupported in: Windows 8
Supported in: Windows Phone 8.1
Supported in: Windows Phone Silverlight 8.1
Supported in: Windows Phone Silverlight 8
Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.