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)

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.

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.
      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


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

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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