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

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey, TResult) ( _
    source As IEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey), _
    resultSelector As Func(Of TKey, IEnumerable(Of TSource), TResult) _
) As IEnumerable(Of TResult)
public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    Func<TKey, IEnumerable<TSource>, TResult> resultSelector
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TKey
    The type of the key returned by keySelector.
  • TResult
    The type of the result value returned by resultSelector.

Parameters

  • keySelector
    Type: System.Func<TSource, TKey>
    A function to extract the key for each element.
  • resultSelector
    Type: System.Func<TKey, IEnumerable<TSource>, TResult>
    A function to create a result value from each group.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>
A collection of elements of type TResult where each element represents a projection over a group and its 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.

Remarks

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy.

Examples

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

   Structure Pet
      Public Name As String
      Public Age As Double
   End Structure

   Public Sub GroupByEx3()
      ' Create a list of pets.
      Dim petsList As New List(Of Pet)(New Pet() _
                                   {New Pet With {.Name = "Barley", .Age = 8.3}, _
                                    New Pet With {.Name = "Boots", .Age = 4.9}, _
                                    New Pet With {.Name = "Whiskers", .Age = 1.5}, _
                                    New Pet With {.Name = "Daisy", .Age = 4.3}})

      ' Group Pet objects by the Math.Floor of their age.
      ' Then project an anonymous type from each group
      ' that consists of the key, the count of the group's
      ' elements, and the minimum and maximum age in the group.
      Dim query = petsList.GroupBy( _
          Function(pet) Math.Floor(pet.Age), _
          Function(age, pets) New With _
              {.Key = age, _
              .Count = pets.Count(), _
              .Min = pets.Min(Function(pet) pet.Age), _
              .Max = pets.Max(Function(Pet) Pet.Age)} _
          )

      Dim output As New System.Text.StringBuilder
      ' Iterate over each anonymous type.
      For Each result In query
         output.AppendLine(vbCrLf & "Age group: " & result.Key)
         output.AppendLine("Number of pets in this age group: " & result.Count)
         output.AppendLine("Minimum age: " & result.Min)
         output.AppendLine("Maximum age: " & result.Max)
      Next

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

   ' This code produces the following output:

   ' Age group: 8
   ' Number of pets in this age group: 1
   ' Minimum age: 8.3
   ' Maximum age: 8.3
   '
   ' Age group: 4
   ' Number of pets in this age group: 2
   ' Minimum age: 4.3
   ' Maximum age: 4.9
   '
   ' Age group: 1
   ' Number of pets in this age group: 1
   ' Minimum age: 1.5
   ' Maximum age: 1.5

      class Pet
      {
         public string Name { get; set; }
         public double Age { get; set; }
      }

      public static void GroupByEx3()
      {
         // Create a list of pets.
         List<Pet> petsList =
             new List<Pet>{ new Pet { Name="Barley", Age=8.3 },
                                new Pet { Name="Boots", Age=4.9 },
                                new Pet { Name="Whiskers", Age=1.5 },
                                new Pet { Name="Daisy", Age=4.3 } };

         // Group Pet objects by the Math.Floor of their age.
         // Then project an anonymous type from each group
         // that consists of the key, the count of the group's
         // elements, and the minimum and maximum age in the group.
         var query = petsList.GroupBy(
             pet => Math.Floor(pet.Age),
             (age, pets) => new
             {
                Key = age,
                Count = pets.Count(),
                Min = pets.Min(pet => pet.Age),
                Max = pets.Max(pet => pet.Age)
             });

         // Iterate over each anonymous type.
         foreach (var result in query)
         {
            outputBlock.Text += "\nAge group: " + result.Key + "\n";
            outputBlock.Text += "Number of pets in this age group: " + result.Count + "\n";
            outputBlock.Text += "Minimum age: " + result.Min + "\n";
            outputBlock.Text += "Maximum age: " + result.Max + "\n";
         }

         /*  This code produces the following output:

             Age group: 8
             Number of pets in this age group: 1
             Minimum age: 8.3
             Maximum age: 8.3

             Age group: 4
             Number of pets in this age group: 2
             Minimum age: 4.3
             Maximum age: 4.9

             Age group: 1
             Number of pets in this age group: 1
             Minimum age: 1.5
             Maximum age: 1.5
         */
      }

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.