Enumerable.GroupBy(TSource, TKey, TElement) Method (IEnumerable(TSource), Func(TSource, TKey), Func(TSource, TElement))
Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function.
Assembly: System.Core (in System.Core.dll)
static member GroupBy : source:IEnumerable<'TSource> * keySelector:Func<'TSource, 'TKey> * elementSelector:Func<'TSource, 'TElement> -> IEnumerable<IGrouping<'TKey, 'TElement>>
The type of the elements of source.
The type of the key returned by keySelector.
The type of the elements in the IGrouping(TKey, TElement).
- Type: System.Func(, )
A function to extract the key for each element.
Return ValueType: System.Collections.Generic.IEnumerable(IGrouping(, ))
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 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 or elementSelector 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.
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 that the elements that produced them appear in source.
The default equality comparer Default is used to compare keys.
The following code example demonstrates how to use to group the elements of a sequence.
In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy. The translation of the query expression in the following example is equivalent to the query in the example above.
In a Visual C# or Visual Basic query expression, the element and key selection expressions occur in the reverse order from their argument positions in a call to the method.