GroupBy(TSource, TKey, TElement) Method (IQueryable(TSource), Expression(Func(TSource, TKey)), Expression(Func(TSource, TElement)))

Queryable.GroupBy(Of TSource, TKey, TElement) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)))


Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function.

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

Public Shared Function GroupBy(Of TSource, TKey, TElement) (
	source As IQueryable(Of TSource),
	keySelector As Expression(Of Func(Of TSource, TKey)),
	elementSelector As Expression(Of Func(Of TSource, TElement))
) As IQueryable(Of IGrouping(Of TKey, TElement))


Type: System.Linq.IQueryable(Of TSource)

An IQueryable(Of T) whose elements to group.

Type: System.Linq.Expressions.Expression(Of Func(Of TSource, TKey))

A function to extract the key for each element.

Type: System.Linq.Expressions.Expression(Of Func(Of TSource, TElement))

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

Return Value

Type: System.Linq.IQueryable(Of IGrouping(Of TKey, TElement))

An IQueryable<IGrouping<TKey, TElement>> in C# or IQueryable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping(Of TKey, TElement) contains a sequence of objects of type TElement and a key.

Type Parameters


The type of the elements of source.


The type of the key returned by the function represented in keySelector.


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

Exception Condition

source or keySelector or elementSelector is null.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The GroupBy(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement))) method generates a MethodCallExpression that represents calling GroupBy(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement))) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of TElement)(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling GroupBy(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement))) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by a key value that is obtained by invoking keySelector on each element. It invokes elementSelector on each element to obtain a result element.

The following code example demonstrates how to use GroupBy(Of TSource, TKey, TElement)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement))) to group the elements of a sequence.

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

Shared Sub GroupByEx2()
    ' Create a list of Pet objects.
    Dim pets As New List(Of Pet)(New Pet() { _
                    New Pet With {.Name = "Barley", .Age = 8}, _
                    New Pet With {.Name = "Boots", .Age = 4}, _
                    New Pet With {.Name = "Whiskers", .Age = 1}, _
                    New Pet With {.Name = "Daisy", .Age = 4}})

    ' Group the pets using Pet.Age as the key.
    ' Use Pet.Name as the value for each entry.
    Dim query As IEnumerable(Of IGrouping(Of Integer, String)) = _
        pets.AsQueryable().GroupBy(Function(pet) pet.Age, Function(pet) pet.Name)

    Dim output As New System.Text.StringBuilder
    ' Iterate over each IGrouping in the collection.
    For Each petGroup As IGrouping(Of Integer, String) In query
        ' Print the key value of the IGrouping.
        ' Iterate over each value in the 
        ' IGrouping and print the value.
        For Each name As String In petGroup
            output.AppendLine(String.Format("  {0}", name))

    ' Display the output.
End Sub

' This code produces the following output:
' 8
'  Barley
' 4
'  Boots
'  Daisy
' 1
'  Whiskers

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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