Duration (Pacific Standard Time):
To (Pacific Standard Time):
  • None
User Action:
  • None

Enumerable.SelectMany<TSource, TResult> Method (IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>)


Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence.

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, IEnumerable(Of TResult))
) As IEnumerable(Of TResult)


Type: System.Collections.Generic.IEnumerable<TSource>

A sequence of values to project.

Type: System.Func<TSourceIEnumerable<TResult>>

A transform function to apply to each element.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>

An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of the 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<TSource, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>) method enumerates the input sequence, uses a transform function to map each element to an IEnumerable<T>, and then enumerates and yields the elements of each such IEnumerable<T> object. That is, for each element of source, selector is invoked and a sequence of values is returned. SelectMany<TSource, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>) then flattens this two-dimensional collection of collections into a one-dimensional IEnumerable<T> and returns it. For example, if a query uses SelectMany<TSource, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<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<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.

In query expression syntax, each from clause (Visual C#) or From clause (Visual Basic) after the initial one translates to an invocation of SelectMany<TSource, TResult>.

The following code example demonstrates how to use SelectMany<TSource, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>) to perform a one-to-many projection over an array.

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

Sub SelectManyEx1()
    ' 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"}}}

    ' Call SelectMany() to gather all pets into a "flat" sequence.
    Dim query1 As IEnumerable(Of String) = _
        petOwners.SelectMany(Function(petOwner) petOwner.Pets)

    Dim output As New System.Text.StringBuilder("Using SelectMany():" & vbCrLf)
    ' Only one foreach loop is required to iterate through 
    ' the results because it is a one-dimensional collection.
    For Each pet As String In query1

    ' This code demonstrates how to use Select() instead 
    ' of SelectMany() to get the same result.
    Dim query2 As IEnumerable(Of String()) = _
        petOwners.Select(Function(petOwner) petOwner.Pets)
    output.AppendLine(vbCrLf & "Using Select():")
    ' Notice that two foreach loops are required to iterate through 
    ' the results because the query returns a collection of arrays.
    For Each petArray() As String In query2
        For Each pet As String In petArray

    ' Display the output.
End Sub

' This code produces the following output:
' Using SelectMany():
' Scruffy
' Sam
' Walker
' Sugar
' Scratches
' Diesel
' Using Select():
' Scruffy
' Sam
' Walker
' Sugar
' Scratches
' Diesel

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 7.0
Windows Phone
Available since 8.1
Return to top