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Enumerable.SelectMany(Of TSource, TResult) Method (IEnumerable(Of TSource), Func(Of TSource, Int32, IEnumerable(Of TResult)))


Projects each element of a sequence to an IEnumerable(Of T), and flattens the resulting sequences into one sequence. The index of each source element is used in the projected form of that element.

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

Public Shared Function SelectMany(Of TSource, TResult) (
	source As IEnumerable(Of TSource),
	selector As Func(Of TSource, Integer, IEnumerable(Of TResult))
) As IEnumerable(Of TResult)


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

A sequence of values to project.

Type: System.Func(Of TSource, Int32, IEnumerable(Of TResult))

A transform function to apply to each source element; the second parameter of the function represents the index of the source element.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TResult)

An IEnumerable(Of T) whose elements are the result of invoking the one-to-many transform function on each element of an input sequence.

Type Parameters


The type of the elements of source.


The type of the elements of the sequence returned by selector.

Exception Condition

source or selector is null.

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The SelectMany(Of TSource, TResult)(IEnumerable(Of TSource), Func(Of TSource, Int32, IEnumerable(Of TResult))) method enumerates the input sequence, uses a transform function to map each element to an IEnumerable(Of T), and then enumerates and yields the elements of each such IEnumerable(Of T) object. That is, for each element of source, selector is invoked and a sequence of values is returned. SelectMany(Of TSource, TResult)(IEnumerable(Of TSource), Func(Of TSource, Int32, IEnumerable(Of TResult))) then flattens this two-dimensional collection of collections into a one-dimensional IEnumerable(Of T) and returns it. For example, if a query uses SelectMany(Of TSource, TResult)(IEnumerable(Of TSource), Func(Of TSource, Int32, IEnumerable(Of TResult))) to obtain the orders (of type Order) for each customer in a database, the result is of type IEnumerable<Order> in C# or IEnumerable(Of Order) in Visual Basic. If instead the query uses Select(Of TSource, TResult) to obtain the orders, the collection of collections of orders is not combined and the result is of type IEnumerable<List<Order>> in C# or IEnumerable(Of List(Of Order)) in Visual Basic.

The first argument to selector represents the element to process. The second argument to selector represents the zero-based index of that element in the source sequence. This can be useful if the elements are in a known order and you want to do something with an element at a particular index, for example. It can also be useful if you want to retrieve the index of one or more elements.

The following code example demonstrates how to use SelectMany(Of TSource, TResult)(IEnumerable(Of TSource), Func(Of TSource, Int32, IEnumerable(Of TResult))) to perform a one-to-many projection over an array and use the index of each outer element.

Structure PetOwner
    Public Name As String
    Public Pets() As String
End Structure

Sub SelectManyEx2()
    ' Create an array of PetOwner objects.
    Dim petOwners() As PetOwner =
{New PetOwner With
 {.Name = "Higa, Sidney", .Pets = New String() {"Scruffy", "Sam"}},
 New PetOwner With
 {.Name = "Ashkenazi, Ronen", .Pets = New String() {"Walker", "Sugar"}},
 New PetOwner With
 {.Name = "Price, Vernette", .Pets = New String() {"Scratches", "Diesel"}},
 New PetOwner With
 {.Name = "Hines, Patrick", .Pets = New String() {"Dusty"}}}

    ' Project the items in the array by appending the index 
    ' of each PetOwner to each pet's name in that petOwner's 
    ' array of pets.
    Dim query As IEnumerable(Of String) =
petOwners.SelectMany(Function(petOwner, index) _
                         petOwner.Pets.Select(Function(pet) _
                                                  index.ToString() + pet))

    Dim output As New System.Text.StringBuilder
    For Each pet As String In query

    ' Display the output.
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 7.0
Windows Phone
Available since 8.1
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