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

   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
      */

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;