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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)

Parameters

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

An IEnumerable(Of T) whose elements to group.

keySelector
Type: System.Func(Of TSource, TKey)

A function to extract the key for each element.

resultSelector
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.

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.

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)
    Next

    ' Display the output.
    MsgBox(output.ToString)
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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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