Enumerable.SelectMany Method

Definition

将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence.

Overloads

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

将序列的每个元素投影到 IEnumerable<T>,并将结果序列合并为一个序列,并对其中每个元素调用结果选择器函数。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>)

将序列的每个元素投影到 IEnumerable<T>,并将结果序列合并为一个序列,并对其中每个元素调用结果选择器函数。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. 每个源元素的索引用于该元素的中间投影表。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>>)

将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。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>>)

将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. 每个源元素的索引用于该元素的投影表。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>)

将序列的每个元素投影到 IEnumerable<T>,并将结果序列合并为一个序列,并对其中每个元素调用结果选择器函数。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)

Type Parameters

TSource

source 的元素类型。The type of the elements of source.

TCollection

collectionSelector 收集的中间元素的类型。The type of the intermediate elements collected by collectionSelector.

TResult

结果序列的元素的类型。The type of the elements of the resulting sequence.

Parameters

source
IEnumerable<TSource>

一个要投影的值序列。A sequence of values to project.

collectionSelector
Func<TSource,IEnumerable<TCollection>>

应用于输入序列的每个元素的转换函数。A transform function to apply to each element of the input sequence.

resultSelector
Func<TSource,TCollection,TResult>

应用于中间序列的每个元素的转换函数。A transform function to apply to each element of the intermediate sequence.

Returns

IEnumerable<TResult>

一个 IEnumerable<T>,其元素是通过以下方法得到的:对 source 的每个元素调用一对多转换函数 collectionSelector,然后将这些序列元素中的每一个元素及其相应的源元素映射到一个结果元素。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.

Exceptions

sourcecollectionSelectorresultSelectornullsource or collectionSelector or resultSelector is null.

Examples

下面的代码示例演示如何使用 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) 对数组执行一对多投影,并使用结果选择器函数将源序列中的每个相应元素保留在对 Select的最终调用范围内。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.
    Console.WriteLine(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

Remarks

此方法是使用延迟执行实现的。This method is implemented by using deferred execution. 即时返回值是一个对象,该对象存储执行操作所需的所有信息。The immediate return value is an object that stores all the information that is required to perform the action. 此方法表示的查询在枚举对象之前不会执行,方法是通过直接调用 GetEnumerator 方法或在 Visual Basic 中使用视觉C#对象中的 foreachFor Each 来枚举对象。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.

当必须将 source 的元素保留在调用 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)之后发生的查询逻辑的范围内时,SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) 方法非常有用。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>). 有关代码示例,请参见“示例”部分。See the Example section for a code example. 如果 TSource 类型的对象与 TCollection类型的对象之间存在双向关系,即,如果类型 TCollection 的对象提供了属性来检索生成它的 TSource 对象,则不需要 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)的重载。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,你可以使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>),并通过 TCollection 对象向后导航到 TSource 对象。Instead, you can use SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) and navigate back to the TSource object through the TCollection object.

在查询表达式语法中,第一个 from 子句C#(Visual)或 From 子句(Visual Basic)在第一次调用后转换为对 SelectMany的调用。In query expression syntax, each from clause (Visual C#) or From clause (Visual Basic) after the initial one translates to an invocation of SelectMany.

See also

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

将序列的每个元素投影到 IEnumerable<T>,并将结果序列合并为一个序列,并对其中每个元素调用结果选择器函数。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. 每个源元素的索引用于该元素的中间投影表。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)

Type Parameters

TSource

source 的元素类型。The type of the elements of source.

TCollection

collectionSelector 收集的中间元素的类型。The type of the intermediate elements collected by collectionSelector.

TResult

结果序列的元素的类型。The type of the elements of the resulting sequence.

Parameters

source
IEnumerable<TSource>

一个要投影的值序列。A sequence of values to project.

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

一个应用于每个源元素的转换函数;函数的第二个参数表示源元素的索引。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>

应用于中间序列的每个元素的转换函数。A transform function to apply to each element of the intermediate sequence.

Returns

IEnumerable<TResult>

一个 IEnumerable<T>,其元素是通过以下方法得到的:对 source 的每个元素调用一对多转换函数 collectionSelector,然后将这些序列元素中的每一个元素及其相应的源元素映射到一个结果元素。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.

Exceptions

sourcecollectionSelectorresultSelectornullsource or collectionSelector or resultSelector is null.

Remarks

此方法是使用延迟执行实现的。This method is implemented by using deferred execution. 即时返回值是一个对象,该对象存储执行操作所需的所有信息。The immediate return value is an object that stores all the information that is required to perform the action. 此方法表示的查询在枚举对象之前不会执行,方法是通过直接调用 GetEnumerator 方法或在 Visual Basic 中使用视觉C#对象中的 foreachFor Each 来枚举对象。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.

当必须将 source 的元素保留在调用 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)之后发生的查询逻辑的范围内时,SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) 方法非常有用。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>). 有关代码示例,请参见“示例”部分。See the Example section for a code example. 如果 TSource 类型的对象与 TCollection类型的对象之间存在双向关系,即,如果类型 TCollection 的对象提供了属性来检索生成它的 TSource 对象,则不需要 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)的重载。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,你可以使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>),并通过 TCollection 对象向后导航到 TSource 对象。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>>)

将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。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)

Type Parameters

TSource

source 的元素类型。The type of the elements of source.

TResult

selector 返回的序列元素的类型。The type of the elements of the sequence returned by selector.

Parameters

source
IEnumerable<TSource>

一个要投影的值序列。A sequence of values to project.

selector
Func<TSource,IEnumerable<TResult>>

应用于每个元素的转换函数。A transform function to apply to each element.

Returns

IEnumerable<TResult>

一个 IEnumerable<T>,其元素是对输入序列的每个元素调用一对多转换函数得到的结果。An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of the input sequence.

Exceptions

sourceselectornullsource or selector is null.

Examples

下面的代码示例演示如何使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 对数组执行一对多投影。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.
    Console.WriteLine(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

Remarks

此方法是使用延迟执行实现的。This method is implemented by using deferred execution. 即时返回值是一个对象,该对象存储执行操作所需的所有信息。The immediate return value is an object that stores all the information that is required to perform the action. 此方法表示的查询在枚举对象之前不会执行,方法是通过直接调用 GetEnumerator 方法或在 Visual Basic 中使用视觉C#对象中的 foreachFor Each 来枚举对象。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.

SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 方法枚举输入序列,使用转换函数将每个元素映射到 IEnumerable<T>,然后枚举并生成每个此类 IEnumerable<T> 对象的元素。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. 也就是说,对于 source的每个元素,都会调用 selector 并返回一系列值。That is, for each element of source, selector is invoked and a sequence of values is returned. 然后 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 将此两维集合合并为一维 IEnumerable<T> 并返回它。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,如果查询使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 获取数据库中每个客户的订单(类型 Order),则结果为类型 IEnumerable<Order> 在C# Visual Basic 中或 IEnumerable(Of Order)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. 如果查询使用 Select 获取订单,则不会合并订单集合的集合,并且结果是 IEnumerable<List<Order>> 在 Visual Basic 中C#的类型或 IEnumerable(Of List(Of 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.

在查询表达式语法中,第一个 from 子句C#(Visual)或 From 子句(Visual Basic)在第一次调用后转换为对 SelectMany的调用。In query expression syntax, each from clause (Visual C#) or From clause (Visual Basic) after the initial one translates to an invocation of SelectMany.

See also

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

将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. 每个源元素的索引用于该元素的投影表。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)

Type Parameters

TSource

source 的元素类型。The type of the elements of source.

TResult

selector 返回的序列元素的类型。The type of the elements of the sequence returned by selector.

Parameters

source
IEnumerable<TSource>

一个要投影的值序列。A sequence of values to project.

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

一个应用于每个源元素的转换函数;函数的第二个参数表示源元素的索引。A transform function to apply to each source element; the second parameter of the function represents the index of the source element.

Returns

IEnumerable<TResult>

一个 IEnumerable<T>,其元素是对输入序列的每个元素调用一对多转换函数得到的结果。An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function on each element of an input sequence.

Exceptions

sourceselectornullsource or selector is null.

Examples

下面的代码示例演示如何使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 对数组执行一对多投影,并使用每个外部元素的索引。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.
    Console.WriteLine(output.ToString())
End Sub

Remarks

此方法是使用延迟执行实现的。This method is implemented by using deferred execution. 即时返回值是一个对象,该对象存储执行操作所需的所有信息。The immediate return value is an object that stores all the information that is required to perform the action. 此方法表示的查询在枚举对象之前不会执行,方法是通过直接调用 GetEnumerator 方法或在 Visual Basic 中使用视觉C#对象中的 foreachFor Each 来枚举对象。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.

SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 方法枚举输入序列,使用转换函数将每个元素映射到 IEnumerable<T>,然后枚举并生成每个此类 IEnumerable<T> 对象的元素。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. 也就是说,对于 source的每个元素,都会调用 selector 并返回一系列值。That is, for each element of source, selector is invoked and a sequence of values is returned. 然后 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 将此两维集合合并为一维 IEnumerable<T> 并返回它。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,如果查询使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 获取数据库中每个客户的订单(类型 Order),则结果为类型 IEnumerable<Order> 在C# Visual Basic 中或 IEnumerable(Of Order)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. 如果查询使用 Select 获取订单,则不会合并订单集合的集合,并且结果是 IEnumerable<List<Order>> 在 Visual Basic 中C#的类型或 IEnumerable(Of List(Of 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.

selector 的第一个参数表示要处理的元素。The first argument to selector represents the element to process. selector 的第二个参数表示源序列中该元素从零开始的索引。The second argument to selector represents the zero-based index of that element in the source sequence. 例如,如果元素处于已知顺序,并且你想要对特定索引处的元素执行某些操作,则这会很有用。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. 如果要检索一个或多个元素的索引,此方法也会很有用。It can also be useful if you want to retrieve the index of one or more elements.

Applies to