• Article

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

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Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function SelectMany(Of TSource, TCollection, TResult) ( _
    source As IEnumerable(Of TSource), _
    collectionSelector As Func(Of TSource, IEnumerable(Of TCollection)), _
    resultSelector As Func(Of TSource, TCollection, TResult) _
) As IEnumerable(Of TResult)

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TCollection>> collectionSelector,
    Func<TSource, TCollection, TResult> resultSelector
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TCollection
    The type of the intermediate elements collected by collectionSelector.
  • TResult
    The type of the elements of the resulting sequence.

Parameters

  • collectionSelector
    Type: System.Func<TSource, IEnumerable<TCollection>>
    A transform function to apply to each element of the input sequence.
  • resultSelector
    Type: System.Func<TSource, TCollection, TResult>
    A transform function to apply to each element of the intermediate sequence.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>
An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function collectionSelector on each element of source and then mapping each of those sequence elements and their corresponding source element to a result element.

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 collectionSelector or resultSelector 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, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>) method is useful when you have to keep the elements of source in scope for query logic that occurs after the call to SelectMany<TSource, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>). See the Example section for a code example. If there is a bidirectional relationship between objects of type TSource and objects of type TCollection, that is, if an object of type TCollection provides a property to retrieve the TSource object that produced it, you do not need this overload of SelectMany<TSource, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>). Instead, you can use SelectMany<TSource, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>) and navigate back to the TSource object through the TCollection object.

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

Examples

The following code example demonstrates how to use SelectMany<TSource, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>) to perform a one-to-many projection over an array and use a result selector function to keep each corresponding element from the source sequence in scope for the final call to Select.

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

   Sub SelectManyEx3()
      ' Create an array of PetOwner objects.
      Dim petOwners() As PetOwner = _
          {New PetOwner With _
           {.Name = "Higa", .Pets = New String() {"Scruffy", "Sam"}}, _
           New PetOwner With _
           {.Name = "Ashkenazi", .Pets = New String() {"Walker", "Sugar"}}, _
           New PetOwner With _
           {.Name = "Price", .Pets = New String() {"Scratches", "Diesel"}}, _
           New PetOwner With _
           {.Name = "Hines", .Pets = New String() {"Dusty"}}}

      ' Project an anonymous type that consists of
      ' the owner's name and the pet's name (string).
      Dim query = _
          petOwners _
          .SelectMany( _
              Function(petOwner) petOwner.Pets, _
              Function(petOwner, petName) New With {petOwner, petName}) _
          .Where(Function(ownerAndPet) ownerAndPet.petName.StartsWith("S")) _
          .Select(Function(ownerAndPet) _
                 New With {.Owner = ownerAndPet.petOwner.Name, _
                           .Pet = ownerAndPet.petName _
                 })

      Dim output As New System.Text.StringBuilder
      For Each obj In query
         output.AppendLine(String.Format("Owner={0}, Pet={1}", obj.Owner, obj.Pet))
      Next

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

   ' This code produces the following output:
   '
   ' Owner=Higa, Pet=Scruffy
   ' Owner=Higa, Pet=Sam
   ' Owner=Ashkenazi, Pet=Sugar
   ' Owner=Price, Pet=Scratches


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

      public static void SelectManyEx3()
      {
         PetOwner[] petOwners =
                 { new PetOwner { Name="Higa", 
                       Pets = new List<string>{ "Scruffy", "Sam" } },
                   new PetOwner { Name="Ashkenazi", 
                       Pets = new List<string>{ "Walker", "Sugar" } },
                   new PetOwner { Name="Price", 
                       Pets = new List<string>{ "Scratches", "Diesel" } },
                   new PetOwner { Name="Hines", 
                       Pets = new List<string>{ "Dusty" } } };

         // Project the pet owner's name and the pet's name.
         var query =
             petOwners
             .SelectMany(petOwner => petOwner.Pets, (petOwner, petName) => new { petOwner, petName })
             .Where(ownerAndPet => ownerAndPet.petName.StartsWith("S"))
             .Select(ownerAndPet =>
                     new
                     {
                        Owner = ownerAndPet.petOwner.Name,
                        Pet = ownerAndPet.petName
                     }
             );

         // Print the results.
         foreach (var obj in query)
         {
            outputBlock.Text += obj + "\n";
         }
      }

      // This code produces the following output:
      //
      // {Owner=Higa, Pet=Scruffy}
      // {Owner=Higa, Pet=Sam}
      // {Owner=Ashkenazi, Pet=Sugar}
      // {Owner=Price, Pet=Scratches}

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.