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Enumerable.GroupBy(Of TSource, TKey, TElement, TResult) Method (IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement), Func(Of TKey, IEnumerable(Of TElement), 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. The elements of each group are projected by using a specified function.

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

<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey, TElement, TResult) ( _
	source As IEnumerable(Of TSource), _
	keySelector As Func(Of TSource, TKey), _
	elementSelector As Func(Of TSource, TElement), _
	resultSelector As Func(Of TKey, IEnumerable(Of TElement), TResult) _
) As IEnumerable(Of TResult)
Dim source As IEnumerable(Of TSource)
Dim keySelector As Func(Of TSource, TKey)
Dim elementSelector As Func(Of TSource, TElement)
Dim resultSelector As Func(Of TKey, IEnumerable(Of TElement), TResult)
Dim returnValue As IEnumerable(Of TResult)

returnValue = source.GroupBy(keySelector, _
	elementSelector, resultSelector)

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the elements in each IGrouping(Of TKey, TElement).


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 TSource, TElement)

A function to map each source element to an element in an IGrouping(Of TKey, TElement).

Type: System.Func(Of TKey, IEnumerable(Of TElement), 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, TElement, TResult)(IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement), Func(Of TKey, IEnumerable(Of TElement), TResult)) to group the projected elements of a sequence and then project a sequence of results of type TResult.

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

Public Sub GroupByEx4()
    ' 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.Age values by the Math.Floor of the 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(pet) pet.Age, _
        Function(baseAge, ages) New With _
            {.Key = baseAge, _
            .Count = ages.Count(), _
            .Min = ages.Min(), _
            .Max = ages.Max()} _

    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

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

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