Enumerable.SelectMany Enumerable.SelectMany Enumerable.SelectMany Enumerable.SelectMany Method

定義

將序列的每個項目都投影成 IEnumerable<T>,並將產生的序列簡化成單一序列。Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence.

多載

SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)

將序列的每個項目投影為 IEnumerable<T>、將產生的序列簡化成單一序列,並對其中的每個項目叫用結果選取器函式。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.

SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)

將序列的每個項目投影為 IEnumerable<T>、將產生的序列簡化成單一序列,並對其中的每個項目叫用結果選取器函式。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. 各來源項目的索引是在該項目的中繼投影表單中使用。The index of each source element is used in the intermediate projected form of that element.

SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>)

將序列的每個項目都投影成 IEnumerable<T>,並將產生的序列簡化成單一序列。Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence.

SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>)

將序列的每個項目都投影成 IEnumerable<T>,並將產生的序列簡化成單一序列。Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. 各來源項目的索引是在該項目的投影表單中使用。The index of each source element is used in the projected form of that element.

SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)

將序列的每個項目投影為 IEnumerable<T>、將產生的序列簡化成單一序列,並對其中的每個項目叫用結果選取器函式。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.

public:
generic <typename TSource, typename TCollection, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, System::Collections::Generic::IEnumerable<TCollection> ^> ^ collectionSelector, Func<TSource, TCollection, TResult> ^ resultSelector);
public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TCollection,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,System.Collections.Generic.IEnumerable<TCollection>> collectionSelector, Func<TSource,TCollection,TResult> resultSelector);
static member SelectMany : seq<'Source> * Func<'Source, seq<'Collection>> * Func<'Source, 'Collection, 'Result> -> seq<'Result>
<Extension()>
Public 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)

類型參數

TSource

source 項目的類型。The type of the elements of source.

TCollection

collectionSelector 所收集之中繼項目的型別。The type of the intermediate elements collected by collectionSelector.

TResult

產生的序列之項目型別。The type of the elements of the resulting sequence.

參數

source
IEnumerable<TSource>

要投影的值序列。A sequence of values to project.

collectionSelector
Func<TSource,IEnumerable<TCollection>>

要套用到輸入序列中各個項目的轉換函式。A transform function to apply to each element of the input sequence.

resultSelector
Func<TSource,TCollection,TResult>

要套用到中繼序列中各個項目的轉換函式。A transform function to apply to each element of the intermediate sequence.

傳回

IEnumerable<TResult>

IEnumerable<T>,其項目是執行下列動作後所產生的結果:對 collectionSelector 的各個項目叫用一對多轉換函式 source,然後再將每個序列項目及其對應的來源項目對應到結果項目。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.

例外狀況

sourcecollectionSelectorresultSelectornullsource or collectionSelector or resultSelector is null.

範例

下列程式碼範例示範如何使用SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)在陣列上執行一對多投影, 並使用結果選取器函式, 將來源序列中的每個對應專案保留在最後Select呼叫的範圍中。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.

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)
    {
        Console.WriteLine(obj);
    }
}

// This code produces the following output:
//
// {Owner=Higa, Pet=Scruffy}
// {Owner=Higa, Pet=Sam}
// {Owner=Ashkenazi, Pet=Sugar}
// {Owner=Price, Pet=Scratches}
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.
    MsgBox(output.ToString())
End Sub

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

備註

這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件, 它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 這個方法所表示的查詢, 必須等到透過直接GetEnumerator呼叫其方法, 或是在 Visual C#或For Each Visual Basic 中使用foreach來列舉物件之後, 才會執行。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.

當您必須將source中的元素保留在呼叫之後SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)發生的查詢邏輯範圍時,方法就很有用。SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)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>). 如需程式碼範例,請參閱<範例>一節。See the Example section for a code example. 如果TSource類型的物件與類型TCollection的物件之間有雙向關聯性, 也就是, 如果類型TCollection的物件提供屬性來抓取產生它的TSource物件, 您就不需要這麼做。的SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)多載。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>). 相反地, 您可以SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>)使用, 並透過TCollection物件TSource流覽回到物件。Instead, you can use SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) and navigate back to the TSource object through the TCollection object.

在查詢運算式語法中, from第一個子句C#(Visual From ) 或子句 (Visual Basic) 會在SelectMany初始一之後轉譯成的調用。In query expression syntax, each from clause (Visual C#) or From clause (Visual Basic) after the initial one translates to an invocation of SelectMany.

另請參閱

SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)

將序列的每個項目投影為 IEnumerable<T>、將產生的序列簡化成單一序列,並對其中的每個項目叫用結果選取器函式。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. 各來源項目的索引是在該項目的中繼投影表單中使用。The index of each source element is used in the intermediate projected form of that element.

public:
generic <typename TSource, typename TCollection, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, int, System::Collections::Generic::IEnumerable<TCollection> ^> ^ collectionSelector, Func<TSource, TCollection, TResult> ^ resultSelector);
public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TCollection,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,int,System.Collections.Generic.IEnumerable<TCollection>> collectionSelector, Func<TSource,TCollection,TResult> resultSelector);
static member SelectMany : seq<'Source> * Func<'Source, int, seq<'Collection>> * Func<'Source, 'Collection, 'Result> -> seq<'Result>
<Extension()>
Public Function SelectMany(Of TSource, TCollection, TResult) (source As IEnumerable(Of TSource), collectionSelector As Func(Of TSource, Integer, IEnumerable(Of TCollection)), resultSelector As Func(Of TSource, TCollection, TResult)) As IEnumerable(Of TResult)

類型參數

TSource

source 項目的類型。The type of the elements of source.

TCollection

collectionSelector 所收集之中繼項目的型別。The type of the intermediate elements collected by collectionSelector.

TResult

產生的序列之項目型別。The type of the elements of the resulting sequence.

參數

source
IEnumerable<TSource>

要投影的值序列。A sequence of values to project.

collectionSelector
Func<TSource,Int32,IEnumerable<TCollection>>

要套用到每個來源項目的轉換函式;此函式的第二個參數代表來源項目的索引。A transform function to apply to each source element; the second parameter of the function represents the index of the source element.

resultSelector
Func<TSource,TCollection,TResult>

要套用到中繼序列中各個項目的轉換函式。A transform function to apply to each element of the intermediate sequence.

傳回

IEnumerable<TResult>

IEnumerable<T>,其項目是執行下列動作後所產生的結果:對 collectionSelector 的各個項目叫用一對多轉換函式 source,然後再將每個序列項目及其對應的來源項目對應到結果項目。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.

例外狀況

sourcecollectionSelectorresultSelectornullsource or collectionSelector or resultSelector is null.

備註

這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件, 它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 這個方法所表示的查詢, 必須等到透過直接GetEnumerator呼叫其方法, 或是在 Visual C#或For Each Visual Basic 中使用foreach來列舉物件之後, 才會執行。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.

當您必須將source中的元素保留在呼叫之後SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)發生的查詢邏輯範圍時,方法就很有用。SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)The SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,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,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>). 如需程式碼範例,請參閱<範例>一節。See the Example section for a code example. 如果TSource類型的物件與類型TCollection的物件之間有雙向關聯性, 也就是, 如果類型TCollection的物件提供屬性來抓取產生它的TSource物件, 您就不需要這麼做。的SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)多載。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,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>). 相反地, 您可以SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>)使用, 並透過TCollection物件TSource流覽回到物件。Instead, you can use SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) and navigate back to the TSource object through the TCollection object.

SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>)

將序列的每個項目都投影成 IEnumerable<T>,並將產生的序列簡化成單一序列。Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence.

public:
generic <typename TSource, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, System::Collections::Generic::IEnumerable<TResult> ^> ^ selector);
public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,System.Collections.Generic.IEnumerable<TResult>> selector);
static member SelectMany : seq<'Source> * Func<'Source, seq<'Result>> -> seq<'Result>
<Extension()>
Public Function SelectMany(Of TSource, TResult) (source As IEnumerable(Of TSource), selector As Func(Of TSource, IEnumerable(Of TResult))) As IEnumerable(Of TResult)

類型參數

TSource

source 項目的類型。The type of the elements of source.

TResult

selector 所傳回序列之項目的型別。The type of the elements of the sequence returned by selector.

參數

source
IEnumerable<TSource>

要投影的值序列。A sequence of values to project.

selector
Func<TSource,IEnumerable<TResult>>

要套用至每個項目的轉換函式。A transform function to apply to each element.

傳回

IEnumerable<TResult>

IEnumerable<T>,其項目是在輸入序列的各個項目上叫用一對多轉換函式所產生的結果。An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of the input sequence.

例外狀況

sourceselectornullsource or selector is null.

範例

下列程式碼範例示範如何使用SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) , 在陣列上執行一對多投射。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.

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

public static void SelectManyEx1()
{
    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" } } };

    // Query using SelectMany().
    IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets);

    Console.WriteLine("Using SelectMany():");

    // Only one foreach loop is required to iterate 
    // through the results since it is a
    // one-dimensional collection.
    foreach (string pet in query1)
    {
        Console.WriteLine(pet);
    }

    // This code shows how to use Select() 
    // instead of SelectMany().
    IEnumerable<List<String>> query2 =
        petOwners.Select(petOwner => petOwner.Pets);

    Console.WriteLine("\nUsing Select():");

    // Notice that two foreach loops are required to 
    // iterate through the results
    // because the query returns a collection of arrays.
    foreach (List<String> petList in query2)
    {
        foreach (string pet in petList)
        {
            Console.WriteLine(pet);
        }
        Console.WriteLine();
    }
}

/*
 This code produces the following output:

 Using SelectMany():
 Scruffy
 Sam
 Walker
 Sugar
 Scratches
 Diesel

 Using Select():
 Scruffy
 Sam

 Walker
 Sugar

 Scratches
 Diesel
*/
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
        output.AppendLine(pet)
    Next

    ' 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
            output.AppendLine(pet)
        Next
    Next

    ' Display the output.
    MsgBox(output.ToString())
End Sub

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

備註

這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件, 它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 這個方法所表示的查詢, 必須等到透過直接GetEnumerator呼叫其方法, 或是在 Visual C#或For Each Visual Basic 中使用foreach來列舉物件之後, 才會執行。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.

方法會列舉輸入序列、使用轉換函式IEnumerable<T>將每個專案對應至, 然後列舉並產生每個這類IEnumerable<T>物件的元素。 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>)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. 亦即source, selector針對的每個專案叫用, 並傳回一系列的值。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>>)然後將這二維的集合集合簡維IEnumerable<T> , 並將其傳回。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. 例如SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) , 如果查詢使用來取得資料庫中每個客戶的訂單 (類型Order為), 則結果會是 in C#或IEnumerable(Of Order) Visual Basic 中IEnumerable<Order>的類型。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. 如果查詢使用Select來取得訂單, 則不會結合訂單集合的集合, 且結果會在或IEnumerable(Of List(Of Order)) Visual Basic 中C#的類型IEnumerable<List<Order>>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.

在查詢運算式語法中, from第一個子句C#(Visual From ) 或子句 (Visual Basic) 會在SelectMany初始一之後轉譯成的調用。In query expression syntax, each from clause (Visual C#) or From clause (Visual Basic) after the initial one translates to an invocation of SelectMany.

另請參閱

SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>)

將序列的每個項目都投影成 IEnumerable<T>,並將產生的序列簡化成單一序列。Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. 各來源項目的索引是在該項目的投影表單中使用。The index of each source element is used in the projected form of that element.

public:
generic <typename TSource, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, int, System::Collections::Generic::IEnumerable<TResult> ^> ^ selector);
public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,int,System.Collections.Generic.IEnumerable<TResult>> selector);
static member SelectMany : seq<'Source> * Func<'Source, int, seq<'Result>> -> seq<'Result>
<Extension()>
Public Function SelectMany(Of TSource, TResult) (source As IEnumerable(Of TSource), selector As Func(Of TSource, Integer, IEnumerable(Of TResult))) As IEnumerable(Of TResult)

類型參數

TSource

source 項目的類型。The type of the elements of source.

TResult

selector 所傳回序列之項目的型別。The type of the elements of the sequence returned by selector.

參數

source
IEnumerable<TSource>

要投影的值序列。A sequence of values to project.

selector
Func<TSource,Int32,IEnumerable<TResult>>

要套用到每個來源項目的轉換函式;此函式的第二個參數代表來源項目的索引。A transform function to apply to each source element; the second parameter of the function represents the index of the source element.

傳回

IEnumerable<TResult>

IEnumerable<T>,其項目是對輸入序列中各個項目叫用一對多轉換函式後所產生的結果。An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of an input sequence.

例外狀況

sourceselectornullsource or selector is null.

範例

下列程式碼範例示範如何使用SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>)對陣列執行一對多投影, 並使用每個專用項目的索引。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.

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)
    {
        Console.WriteLine(pet);
    }
}

// This code produces the following output:
//
// 0Scruffy
// 0Sam
// 1Walker
// 1Sugar
// 2Scratches
// 2Diesel
// 3Dusty
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.
    MsgBox(output.ToString())
End Sub

備註

這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件, 它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 這個方法所表示的查詢, 必須等到透過直接GetEnumerator呼叫其方法, 或是在 Visual C#或For Each Visual Basic 中使用foreach來列舉物件之後, 才會執行。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.

方法會列舉輸入序列、使用轉換函式IEnumerable<T>將每個專案對應至, 然後列舉並產生每個這類IEnumerable<T>物件的元素。 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>)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. 亦即source, selector針對的每個專案叫用, 並傳回一系列的值。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>>)然後將這二維的集合集合簡維IEnumerable<T> , 並將其傳回。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. 例如SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) , 如果查詢使用來取得資料庫中每個客戶的訂單 (類型Order為), 則結果會是 in C#或IEnumerable(Of Order) Visual Basic 中IEnumerable<Order>的類型。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. 如果查詢使用Select來取得訂單, 則不會結合訂單集合的集合, 且結果會在或IEnumerable(Of List(Of Order)) Visual Basic 中C#的類型IEnumerable<List<Order>>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.

第一個引數selector , 表示要處理的元素。The first argument to selector represents the element to process. 第二個自selector變數代表來源序列中該元素以零為起始的索引。The second argument to selector represents the zero-based index of that element in the source sequence. 如果專案是以已知的順序, 而您想要使用特定索引的專案來執行某個動作, 這項功能就很有用, 例如。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. 如果您想要取出一或多個專案的索引, 它也很有用。It can also be useful if you want to retrieve the index of one or more elements.

適用於