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

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Projects each element of a sequence to an IEnumerable<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)

Syntax

'Declaration
<ExtensionAttribute> _
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)

public static IEnumerable<TResult> SelectMany<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, int, IEnumerable<TResult>> selector
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TResult
    The type of the elements of the sequence returned by selector.

Parameters

  • selector
    Type: System.Func<TSource, Int32, IEnumerable<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<TResult>
An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of an input sequence.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source or selector is nulla null reference (Nothing in Visual Basic).

Remarks

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, Int32, 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, Int32, 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, Int32, 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 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.

Examples

The following code example demonstrates how to use SelectMany<TSource, TResult>(IEnumerable<TSource>, Func<TSource, Int32, IEnumerable<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
      output.AppendLine(pet)
   Next

   ' Display the output.
   outputBlock.Text &= output.ToString() & vbCrLf
End Sub

      class PetOwner
      {
         public string Name { get; set; }
         public List<string> Pets { get; set; }
      }

      public static void SelectManyEx2()
      {
         PetOwner[] petOwners = 
                 { new PetOwner { Name="Higa, Sidney", 
                       Pets = new List<string>{ "Scruffy", "Sam" } },
                   new PetOwner { Name="Ashkenazi, Ronen", 
                       Pets = new List<string>{ "Walker", "Sugar" } },
                   new PetOwner { Name="Price, Vernette", 
                       Pets = new List<string>{ "Scratches", "Diesel" } },
                   new PetOwner { Name="Hines, Patrick", 
                       Pets = new List<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.
         IEnumerable<string> query =
             petOwners.SelectMany((petOwner, index) =>
                                      petOwner.Pets.Select(pet => index + pet));

         foreach (string pet in query)
         {
            outputBlock.Text += pet + "\n";
         }
      }

      // This code produces the following output:
      //
      // 0Scruffy
      // 0Sam
      // 1Walker
      // 1Sugar
      // 2Scratches
      // 2Diesel
      // 3Dusty

Version Information

Silverlight

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

Platforms

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