GroupBy<TSource,TKey,TElement>(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.

public: generic <typename TSource, typename TKey, typename TElement> [System::Runtime::CompilerServices::Extension] static System::Collections::Generic::IEnumerable<System::Linq::IGrouping<TKey, TElement> ^> ^ GroupBy(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, TKey> ^ keySelector, Func<TSource, TElement> ^ elementSelector);

public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey,TElement>> GroupBy<TSource,TKey,TElement> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector);

static member GroupBy : seq<'Source> * Func<'Source, 'Key> * Func<'Source, 'Element> -> seq<System.Linq.IGrouping<'Key, 'Element>>

<Extension()> Public Function GroupBy(Of TSource, TKey, TElement) (source As IEnumerable(Of TSource), keySelector As Func(Of TSource, TKey), elementSelector As Func(Of TSource, TElement)) As IEnumerable(Of IGrouping(Of TKey, TElement))

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 IEnumerable<TSource> An IEnumerable<T> whose elements to group.

keySelector Func<TSource,TKey> A function to extract the key for each element.

elementSelector Func<TSource,TElement> A function to map each source element to an element in the IGrouping<TKey,TElement>.

Returns

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.

Exceptions

ArgumentNullException source or keySelector or elementSelector is null .

Examples

The following code example demonstrates how to use GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>) to group the elements of a sequence.

class Pet { public string Name { get; set; } public int Age { get; set; } } // Uses method-based query syntax. public static void GroupByEx1() { // Create a list of pets. List<Pet> pets = new List<Pet>{ new Pet { Name="Barley", Age=8 }, new Pet { Name="Boots", Age=4 }, new Pet { Name="Whiskers", Age=1 }, new Pet { Name="Daisy", Age=4 } }; // Group the pets using Age as the key value // and selecting only the pet's Name for each value. IEnumerable<IGrouping<int, string>> query = pets.GroupBy(pet => pet.Age, pet => pet.Name); // Iterate over each IGrouping in the collection. foreach (IGrouping<int, string> petGroup in query) { // Print the key value of the IGrouping. Console.WriteLine(petGroup.Key); // Iterate over each value in the // IGrouping and print the value. foreach (string name in petGroup) Console.WriteLine(" {0}", name); } } /* This code produces the following output: 8 Barley 4 Boots Daisy 1 Whiskers */

Structure Pet Public Name As String Public Age As Integer End Structure Sub GroupByEx1() 'Create a list of Pet objects. Dim pets As New List(Of Pet)(New Pet() _ {New Pet With {.Name = "Barley", .Age = 8}, New Pet With {.Name = "Boots", .Age = 4}, New Pet With {.Name = "Whiskers", .Age = 1}, New Pet With {.Name = "Daisy", .Age = 4}}) ' Group the pets using Age as the key ' and selecting only the pet's Name for each value. Dim query As IEnumerable(Of IGrouping(Of Integer, String)) = pets.GroupBy(Function(pet) pet.Age, Function(pet) pet.Name) Dim output As New System.Text.StringBuilder ' Iterate over each IGrouping in the collection. For Each petGroup As IGrouping(Of Integer, String) In query ' Print the key value of the IGrouping. output.AppendLine(petGroup.Key) ' Iterate over each value in the IGrouping and print the value. For Each name As String In petGroup output.AppendLine(" " & name) Next Next ' Display the output. Console.WriteLine(output.ToString) End Sub ' This code produces the following output: ' ' 8 ' Barley ' 4 ' Boots ' Daisy ' 1 ' Whiskers

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.

IEnumerable<IGrouping<int, string>> query = from pet in pets group pet.Name by pet.Age;

Dim query = From pet In pets Group pet.Name By Age = pet.Age Into ageGroup = Group

Note 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 GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>) method.

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,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>) 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.

See also