Article
11/17/2011

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

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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.

Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)

Syntax

'Declaration
<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) _
) 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
)

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
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.

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

Return Value

Type: System.Collections.Generic.IEnumerable<IGrouping<TKey, TElement>>
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 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 or elementSelector 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, 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.

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.

   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.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' 8
   '   Barley
   ' 4
   '   Boots
   '   Daisy
   ' 1
   '   Whiskers

      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.
            outputBlock.Text += petGroup.Key + "\n";
            // Iterate over each value in the 
            // IGrouping and print the value.
            foreach (string name in petGroup)
               outputBlock.Text += String.Format("  {0}", name) + "\n";
         }
      }

      /*
       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.

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

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

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.

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.

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