基本信息
投影（Projection）運算是指將對象轉換為一種新形式的操作，該形式通常只包含那些將隨後使用的屬性。通過使用投影，您可以構建依據每個對象生成的新類型。您可以映射屬性，並對該屬性執行數學函式。還可以在不更改原始對象的情況下映射該對象。簡單地說，便是在關係中選擇某些屬性列。
下面一節中列出了執行投影的標準查詢運算符方法。
方法
方法名
說明
C# 查詢表達式語法
Visual Basic 查詢表達式語法
更多信息
Select
映射基於轉換函式的值。
select
Select
Enumerable..::.Select
Queryable..::.Select
SelectMany
映射基於轉換函式的值序列，然後將它們展平為一個序列。
使用多個 from 子句
使用多個 From 子句
Enumerable..::.SelectMany
Queryable..::.SelectMany
查詢表達式語法示例
Select
下面的示例使用 C# 中的 select 子句或 Visual Basic 中的 Select 子句來映射字元串列表中每個字元串的第一個字母。
Visual Basic 複製代碼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# 複製代碼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
下面的示例使用多個 from 子句（在 C# 中）或 From 子句（在 Visual Basic 中）來映射字元串列表中每個字元串中的每個單詞。
Visual Basic 複製代碼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# 複製代碼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 與 SelectMany
Select() 和 SelectMany() 的工作都是依據源值生成一個或多個結果值。Select() 為每個源值生成一個結果值。因此，總體結果是一個與源集合具有相同元素數目的集合。與之相反，SelectMany() 將生成單一總體結果，其中包含來自每個源值的串聯子集合。作為參數傳遞到 SelectMany() 的轉換函式必須為每個源值返回一個可枚舉值序列。然後，SelectMany() 將串聯這些可枚舉序列以創建一個大的序列。
下面兩個插圖演示了這兩個方法的操作之間的概念性區別。在每種情況下，假定選擇器（轉換）函式從每個源值中選擇一個由花卉數據組成的數組。
下圖描述 Select() 如何返回一個與源集合具有相同元素數目的集合。
下圖描述 SelectMany() 如何將中間數組序列串聯為一個最終結果值，其中包含每箇中間數組中的每個值。
代碼示例
下面的示例比較 Select() 和 SelectMany() 的行為。代碼將通過從源集合的每個花卉名稱列表中提取前兩項來創建一個“花束”。在此示例中，轉換函式 Select<(Of <(TSource, TResult>)>)(IEnumerable<(Of <(TSource>)>), Func<(Of <(TSource, TResult>)>)) 使用的“單一值”本身就是一個值集合。這需要額外的 foreach（Visual Basic 中為 For Each）循環，以便枚舉每個子序列中的每個字元串。
Visual Basic 複製代碼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
其他信息
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# 複製代碼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("\nResults 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
*/
}
