Enumerable.SelectMany 方法

Source:

SelectMany.cs

Source:

SelectMany.cs

Source:

SelectMany.cs

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)

类型参数

参数

返回

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。

例外

示例

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 的最终调用 Select的范围内。

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

注解

此方法通过使用延迟执行来实现。 即时返回值是一个对象，用于存储执行操作所需的所有信息。 在通过直接调用GetEnumerator其方法或通过在 C# For Each 或 foreach Visual Basic 中使用 来枚举对象之前，不会执行此方法表示的查询。

SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)当必须将 的source元素保留在调用 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)后发生的查询逻辑的范围内时， 方法非常有用。 有关代码示例，请参见“示例”部分。 如果 类型的对象与 类型的TSource对象之间存在双向关系，也就是说，如果 类型的TCollection对象提供属性来检索TSource生成它的 对象，则不需要的此重载SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)TCollection。 相反，可以使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 并通过 对象TCollection导航回 TSource 对象。

在查询表达式语法中，在初始子句转换为 调用SelectMany后，每个from子句 (C#) 或 From 子句 (Visual Basic) 。

另请参阅

• from 子句（C# 参考）
• From 子句 (Visual Basic)

Source:

SelectMany.cs

Source:

SelectMany.cs

Source:

SelectMany.cs

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)

类型参数

参数

返回

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。

例外

注解

此方法通过使用延迟执行来实现。 即时返回值是一个对象，用于存储执行操作所需的所有信息。 在通过直接调用GetEnumerator其方法或通过在 C# For Each 或 foreach Visual Basic 中使用 来枚举对象之前，不会执行此方法表示的查询。

SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)当必须将 的source元素保留在调用 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)后发生的查询逻辑的范围内时， 方法非常有用。 有关代码示例，请参见“示例”部分。 如果 类型的对象与 类型的TSource对象之间存在双向关系，也就是说，如果 类型的TCollection对象提供属性来检索TSource生成它的 对象，则不需要的此重载SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)TCollection。 相反，可以使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 并通过 对象TCollection导航回 TSource 对象。

Source:

SelectMany.cs

Source:

SelectMany.cs

Source:

SelectMany.cs

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)

类型参数

参数

返回

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。

例外

示例

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。

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

注解

此方法通过使用延迟执行来实现。 即时返回值是一个对象，用于存储执行操作所需的所有信息。 在通过直接调用GetEnumerator其方法或通过在 C# For Each 或 foreach Visual Basic 中使用 来枚举对象之前，不会执行此方法表示的查询。

方法 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 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。 也就是说，对于 的每个元素 source， selector 将调用 并返回值序列。 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 然后，将此集合的二维集合平展为一维 IEnumerable<T> 集合并返回它。 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) 类型的 Order 订单 (，则结果的类型为 IEnumerable<Order> C# 或 IEnumerable(Of Order) Visual Basic。 如果查询改用 Select 来获取订单，则订单集合的集合不会合并，并且结果的类型为 IEnumerable<List<Order>> C# 或 IEnumerable(Of List(Of Order)) Visual Basic。

在查询表达式语法中，在初始子句转换为 调用SelectMany后，每个from子句 (C#) 或 From 子句 (Visual Basic) 。

另请参阅

• from 子句（C# 参考）
• From 子句 (Visual Basic)

Source:

SelectMany.cs

Source:

SelectMany.cs

Source:

SelectMany.cs

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)

类型参数

参数

返回

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。

例外

示例

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。

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

注解

此方法通过使用延迟执行来实现。 即时返回值是一个对象，用于存储执行操作所需的所有信息。 在通过直接调用GetEnumerator其方法或通过在 C# For Each 或 foreach Visual Basic 中使用 来枚举对象之前，不会执行此方法表示的查询。

方法 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 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。 也就是说，对于 的每个元素 source， selector 将调用 并返回值序列。 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 然后，将此集合的二维集合平展为一维 IEnumerable<T> 集合并返回它。 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) 类型的 Order 订单 (，则结果的类型为 IEnumerable<Order> C# 或 IEnumerable(Of Order) Visual Basic。 如果查询改用 Select 来获取订单，则订单集合的集合不会合并，并且结果的类型为 IEnumerable<List<Order>> C# 或 IEnumerable(Of List(Of Order)) Visual Basic。

的第一个 selector 参数表示要处理的元素。 第二个参数表示 selector 源序列中该元素的从零开始的索引。 例如，如果元素以已知顺序排列，并且你想要使用特定索引处的元素执行某些操作，则这非常有用。 如果要检索一个或多个元素的索引，它也很有用。