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


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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)

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


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).

Type: System.Linq.Expressions.Expression(Of Func(Of TKey, IEnumerable(Of TElement), TResult))

A function to create a result value from each group.

Return Value

Type: System.Linq.IQueryable(Of TResult)

An T:System.Linq.IQueryable`1 that has a type argument of TResult and where each element represents a projection over a group and its 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).


The type of the result value returned by resultSelector.

Exception Condition

source or keySelector or elementSelector or resultSelector 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, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), Expression(Of Func(Of TKey, IEnumerable(Of TElement), TResult))) method generates a MethodCallExpression that represents calling GroupBy(Of TSource, TKey, TElement, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), Expression(Of Func(Of TKey, IEnumerable(Of TElement), TResult))) 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, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), Expression(Of Func(Of TSource, TElement)), Expression(Of Func(Of TKey, IEnumerable(Of TElement), TResult))) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by key values that are obtained by invoking keySelector on each element. The elementSelector parameter is used to project the elements of each group, and the resultSelector parameter is used to obtain a result value from each group and its key.

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

Shared 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 valuesby 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.AsQueryable().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 with this age: " & result.Count)
        output.AppendLine("Minimum age: " & result.Min)
        output.AppendLine("Maximum age: " & result.Max)

    ' Display the output.

    ' This code produces the following output:

    '  Age group: 8
    '  Number of pets with this age: 1
    '  Minimum age: 8.3
    '  Maximum age: 8.3

    '  Age group: 4
    '  Number of pets with this age: 2
    '  Minimum age: 4.3
    '  Maximum age: 4.9

    '  Age group: 1
    '  Number of pets with this age: 1
    '  Minimum age: 1.5
    '  Maximum age: 1.5
End Sub

Universal Windows Platform
Available since 8
.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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