投影（Projection）运算

投影是指将对象转换为一种新形式的操作，该形式通常只包含那些将随后使用的属性。通过使用投影，您可以构建依据每个对象生成的新类型。您可以映射属性，并对该属性执行数学函数。还可以在不更改原始对象的情况下映射该对象。简单地说，便是在关系中选择某些属性列。

下面一节中列出了执行投影的标准查询运算符方法。

方法

方法名

说明

C# 查询表达式语法

Visual Basic 查询表达式语法

更多信息

Select

映射基于转换函数的值。

select

Select

Enumerable..::.Select

Queryable..::.Select

SelectMany

映射基于转换函数的值序列，然后将它们展平为一个序列。

使用多个 from 子句

使用多个 From 子句

Enumerable..::.SelectMany

Queryable..::.SelectMany

查询表达式语法示例

Select

下面的示例使用 C# 中的 select 子句或 Visual Basic 中的 Select 子句来映射字符串列表中每个字符串的第一个字母。

Visual Basic 复制代码Dim words As New List(Of String)(New String() {"an", "apple", "a", "day"})

Dim query = From word In words _

Select word.Substring(0, 1)

Dim sb As New System.Text.StringBuilder()

For Each letter As String In query

sb.AppendLine(letter)

Next

' Display the output.

MsgBox(sb.ToString())

' This code produces the following output:

' a

' a

' a

' d

C# 复制代码List<string> words = new List<string>() { "an", "apple", "a", "day" };

var query = from word in words

select word.Substring(0, 1);

foreach (string s in query)

Console.WriteLine(s);

/* This code produces the following output:

a

a

a

d

*/

SelectMany

下面的示例使用多个 from 子句（在 C# 中）或 From 子句（在 Visual Basic 中）来映射字符串列表中每个字符串中的每个单词。

Visual Basic 复制代码Dim phrases As New List(Of String)(New String() {"an apple a day", "the quick brown fox"})

Dim query = From phrase In phrases _

From word In phrase.Split(" "c) _

Select word

Dim sb As New System.Text.StringBuilder()

For Each str As String In query

sb.AppendLine(str)

Next

' Display the output.

MsgBox(sb.ToString())

' This code produces the following output:

' an

' apple

' a

' day

' the

' quick

' brown

' fox

C# 复制代码List<string> phrases = new List<string>() { "an apple a day", "the quick brown fox" };

var query = from phrase in phrases

from word in phrase.Split(' ')

select word;

foreach (string s in query)

Console.WriteLine(s);

/* This code produces the following output:

an

apple

a

day

the

quick

brown

fox

*/

Select 与 SelectMany

Select() 和 SelectMany() 的工作都是依据源值生成一个或多个结果值。Select() 为每个源值生成一个结果值。因此，总体结果是一个与源集合具有相同元素数目的集合。与之相反，SelectMany() 将生成单一总体结果，其中包含来自每个源值的串联子集合。作为参数传递到 SelectMany() 的转换函数必须为每个源值返回一个可枚举值序列。然后，SelectMany() 将串联这些可枚举序列以创建一个大的序列。

下面两个插图演示了这两个方法的操作之间的概念性区别。在每种情况下，假定选择器（转换）函数从每个源值中选择一个由花卉数据组成的数组。

下图描述 Select() 如何返回一个与源集合具有相同元素数目的集合。

下图描述 SelectMany() 如何将中间数组序列串联为一个最终结果值，其中包含每个中间数组中的每个值。

代码示例

下面的示例比较 Select() 和 SelectMany() 的行为。代码将通过从源集合的每个花卉名称列表中提取前两项来创建一个“花束”。在此示例中，转换函数 Select<(Of <(TSource, TResult>)>)(IEnumerable<(Of <(TSource>)>), Func<(Of <(TSource, TResult>)>)) 使用的“单一值”本身就是一个值集合。这需要额外的 foreach（Visual Basic 中为 For Each）循环，以便枚举每个子序列中的每个字符串。

Visual Basic 复制代码Class Bouquet

Public Flowers As List(Of String)

End Class

Sub SelectVsSelectMany()

Dim bouquets As New List(Of Bouquet)(New Bouquet() { _

New Bouquet With {.Flowers = New List(Of String)(New String() {"sunflower", "daisy", "daffodil", "larkspur"})}, _

New Bouquet With {.Flowers = New List(Of String)(New String() {"tulip", "rose", "orchid"})}, _

New Bouquet With {.Flowers = New List(Of String)(New String() {"gladiolis", "lily", "snapdragon", "aster", "protea"})}, _

New Bouquet With {.Flowers = New List(Of String)(New String() {"larkspur", "lilac", "iris", "dahlia"})}})

Dim output As New System.Text.StringBuilder

' Select()

Dim query1 = bouquets.Select(Function(b) b.Flowers)

output.AppendLine("Using Select():")

For Each flowerList In query1

For Each str As String In flowerList

output.AppendLine(str)

Next

Next

' SelectMany()

Dim query2 = bouquets.SelectMany(Function(b) b.Flowers)

output.AppendLine(vbCrLf & "Using SelectMany():")

For Each str As String In query2

output.AppendLine(str)

Next

' Display the output

MsgBox(output.ToString())

' This code produces the following output:

'

' Using Select():

' sunflower

' daisy

' daffodil

' larkspur

' tulip

' rose

' orchid

' gladiolis

' lily

' snapdragon

' aster

' protea

' larkspur

' lilac

' iris

' dahlia

' Using SelectMany()

' sunflower

' daisy

' daffodil

' larkspur

' tulip

' rose

' orchid

' gladiolis

' lily

' snapdragon

' aster

' protea

' larkspur

' lilac

' iris

' dahlia

End Sub

C# 复制代码class Bouquet

{

public List<string> Flowers { get; set; }

}

static void SelectVsSelectMany()

{

List<Bouquet> bouquets = new List<Bouquet>() {

new Bouquet { Flowers = new List<string> { "sunflower", "daisy", "daffodil", "larkspur" }},

new Bouquet{ Flowers = new List<string> { "tulip", "rose", "orchid" }},

new Bouquet{ Flowers = new List<string> { "gladiolis", "lily", "snapdragon", "aster", "protea" }},

new Bouquet{ Flowers = new List<string> { "larkspur", "lilac", "iris", "dahlia" }}

};

// *********** Select ***********

IEnumerable<List<string>> query1 = bouquets.Select(bq => bq.Flowers);

// ********* SelectMany *********

IEnumerable<string> query2 = bouquets.SelectMany(bq => bq.Flowers);

Console.WriteLine("Results by using Select():");

// Note the extra foreach loop here.

foreach (IEnumerable<String> collection in query1)

foreach (string item in collection)

Console.WriteLine(item);

Console.WriteLine("\Results by using SelectMany():");

foreach (string item in query2)

Console.WriteLine(item);

/* This code produces the following output:

Results by using Select():

sunflower

daisy

daffodil

larkspur

tulip

rose

orchid

gladiolis

lily

snapdragon

aster

protea

larkspur

lilac

iris

dahlia

Results by using SelectMany():

sunflower

daisy

daffodil

larkspur

tulip

rose

orchid

gladiolis

lily

snapdragon

aster

protea

larkspur

lilac

iris

dahlia

*/

}