Queryable.SelectMany<TSource, TCollection, TResult> Method (IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>)

Projects each element of a sequence to an IEnumerable<T> and invokes a result selector function on each element therein. The resulting values from each intermediate sequence are combined into a single, one-dimensional sequence and returned.

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

Syntax

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

Type Parameters

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

Parameters

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> whose elements are the result of invoking the one-to-many projection 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 IQueryable<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 has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) method generates a MethodCallExpression that represents calling SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(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 SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) depends on the implementation of the type of the source parameter. The expected behavior is that it invokes collectionSelector on each element of source to project it into an enumerable form. Then the function represented by resultSelector is invoked on each element in each intermediate sequence. The resulting values are concatenated into a single, one-dimensional sequence.

Examples

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

   Structure PetOwner
      Public Name As String
      Public Pets As List(Of Pet)
   End Structure

   Structure Pet
      Public Name As String
      Public Breed As String
   End Structure

   Shared Sub SelectManyEx3()
      Dim petOwners() As PetOwner = _
                  {New PetOwner With {.Name = "Higa", _
                        .Pets = New List(Of Pet)(New Pet() { _
                            New Pet With {.Name = "Scruffy", .Breed = "Poodle"}, _
                            New Pet With {.Name = "Sam", .Breed = "Hound"}})}, _
                    New PetOwner With {.Name = "Ashkenazi", _
                        .Pets = New List(Of Pet)(New Pet() { _
                            New Pet With {.Name = "Walker", .Breed = "Collie"}, _
                            New Pet With {.Name = "Sugar", .Breed = "Poodle"}})}, _
                    New PetOwner With {.Name = "Price", _
                        .Pets = New List(Of Pet)(New Pet() { _
                            New Pet With {.Name = "Scratches", .Breed = "Dachshund"}, _
                            New Pet With {.Name = "Diesel", .Breed = "Collie"}})}, _
                    New PetOwner With {.Name = "Hines", _
                        .Pets = New List(Of Pet)(New Pet() { _
                            New Pet With {.Name = "Dusty", .Breed = "Collie"}})} _
                  }

      ' This query demonstrates how to obtain a sequence of
      ' the names of all the pets whose breed is "Collie", while
      ' keeping an association with the owner that owns the pet.
      Dim query = petOwners.AsQueryable() _
          .SelectMany(Function(owner) owner.Pets, _
                 Function(owner, pet) New With {owner, pet}) _
          .Where(Function(ownerAndPet) ownerAndPet.pet.Breed = "Collie") _
          .Select(Function(ownerAndPet) New With { _
              .Owner = ownerAndPet.owner.Name, _
              .Pet = ownerAndPet.pet.Name})

      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=Ashkenazi, Pet=Walker
   ' Owner=Price, Pet=Diesel
   ' Owner=Hines, Pet=Dusty

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

         class Pet
         {
            public string Name { get; set; }
            public string Breed { get; set; }
         }

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

            // This query demonstrates how to obtain a sequence of
            // the names of all the pets whose breed is "Collie", while
            // keeping an association with the owner that owns the pet.
            var query =
                petOwners.AsQueryable()
               // Create a sequence of ALL the Pet objects. Then
               // project an anonymous type that consists of each
               // Pet in the new sequence and the PetOwner object
               // from the initial array that corresponds to that pet.
               .SelectMany(owner => owner.Pets,
                           (owner, pet) => new { owner, pet })
               // Filter the sequence of anonymous types to only
               // keep pets whose breed is "Collie".
                .Where(ownerAndPet => ownerAndPet.pet.Breed == "Collie")
               // Project an anonymous type that consists
               // of the pet owner's name and the pet's name.
                .Select(ownerAndPet => new
                {
                   Owner = ownerAndPet.owner.Name,
                   Pet = ownerAndPet.pet.Name
                });

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

         /* This code produces the following output:

             { Owner = Ashkenazi, Pet = Walker }
             { Owner = Price, Pet = Diesel }
             { Owner = Hines, Pet = Dusty }
         */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

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