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

 
System_CAPS_noteNote

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Projects each element of a sequence to an IEnumerable(Of 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)

<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)

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

A sequence of values to project.

collectionSelector
Type: System.Linq.Expressions.Expression(Of Func(Of TSource, IEnumerable(Of TCollection)))

A projection function to apply to each element of the input sequence.

resultSelector
Type: System.Linq.Expressions.Expression(Of Func(Of TSource, TCollection, TResult))

A projection function to apply to each element of each intermediate sequence.

Return Value

Type: System.Linq.IQueryable(Of TResult)

An IQueryable(Of 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.

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.

Exception Condition
ArgumentNullException

source or collectionSelector or resultSelector is null.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The SelectMany(Of TSource, TCollection, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, IEnumerable(Of TCollection))), Expression(Of Func(Of TSource, TCollection, TResult))) method generates a MethodCallExpression that represents calling SelectMany(Of TSource, TCollection, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, IEnumerable(Of TCollection))), Expression(Of Func(Of 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(Of TSource, TCollection, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, IEnumerable(Of TCollection))), Expression(Of Func(Of 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.

The following code example demonstrates how to use SelectMany(Of TSource, TCollection, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, IEnumerable(Of TCollection))), Expression(Of Func(Of 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.
    MsgBox(output.ToString())
End Sub

' This code produces the following output:

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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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