This documentation is archived and is not being maintained.

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

<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)

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the result value returned by resultSelector.


Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of T) whose elements to group.
Type: System.Func(Of TSource, TKey)
A function to extract the key for each element.
Type: System.Func(Of TKey, IEnumerable(Of TSource), TResult)
A function to create a result value from each group.

Return Value

Type: System.Collections.Generic.IEnumerable(Of 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(Of TSource). 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).

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

The following code example demonstrates how to use GroupBy(Of TSource, TKey, TResult)(IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TKey, IEnumerable(Of 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)

        ' Display the output.
    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

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.