開始使用 LINQ（三）- 使用 LINQ 進行數據轉換

　　語言集成查詢 (LINQ) 不僅可用於檢索數據，  而且還是一個功能強大的數據轉換工具。  通過使用 LINQ 查詢，您可以將源序列用作輸入，並採用多種方式修改它以創建新的輸出序列。您可以通過排序和分組來修改該序列，而不必修改元素本身。但是，LINQ 查詢的最強大的功能是能夠創建新類型。這一功能在 select 子句中實現。 例如，可以執行下列任務：  

一、 將多個輸入聯接到一個輸出序列

 1     class Student
 2     {
 3         public string Name { get; set; }
 4 
 5         public int Age { get; set; }
 6 
 7         public string City { get; set; }
 8 
 9         public List<int> Scores { get; set; }
10     }
11 
12     class Teacher
13     {
14         public int Id { get; set; }
15 
16         public string Name { get; set; }
17 
18         public int Age { get; set; }
19 
20         public string City { get; set; }
21 
22     }

 1     internal class Program
 2     {
 3         private static void Main(string[] args)
 4         {
 5             //創建第一個數據源
 6             var students = new List<Student>()
 7             {
 8                 new Student()
 9                 {
10                     Age = 23,
11                     City = "廣州",
12                     Name = "小C",
13                     Scores = new List<int>(){85,88,83,97}
14                 },
15                 new Student()
16                 {
17                     Age = 18,
18                     City = "廣西",
19                     Name = "小明",
20                     Scores = new List<int>(){86,78,85,90}
21                 },
22                 new Student()
23                 {
24                     Age = 33,
25                     City = "夢裡",
26                     Name = "小叄",
27                     Scores = new List<int>(){86,68,73,97}
28                 }
29             };
30 
31             //創建第二個數據源
32             var teachers = new List<Teacher>()
33             {
34                 new Teacher()
35                 {
36                     Age = 35,
37                     City = "夢裡",
38                     Name = "啵哆"
39                 },
40                 new Teacher()
41                 {
42                     Age = 28,
43                     City = "雲南",
44                     Name = "小紅"
45                 },
46                 new Teacher()
47                 {
48                     Age = 38,
49                     City = "河南",
50                     Name = "麗麗"
51                 }
52             };
53 
54             //創建查詢
55             var peopleInDreams = (from student in students
56                             where student.City == "夢裡"
57                             select student.Name)
58                             .Concat(from teacher in teachers
59                                     where teacher.City == "夢裡"
60                                     select teacher.Name);
61 
62             //執行查詢
63             foreach (var person in peopleInDreams)
64             {
65                 Console.WriteLine(person);
66             }
67 
68             Console.Read();
69         }
70     }

二、 選擇各個源元素的子集

　　1. 若要只選擇源元素的一個成員，請使用點運算。

1 var query = from cust in Customers
2             select cust.City;

　　2. 若要創建包含源元素的多個屬性的元素，可以使用具有命名對象或匿名類型的對象初始值設定項。

1 var query = from cust in Customer
2             select new {Name = cust.Name, City = cust.City};

三、將記憶體中的對象轉換為 XML

 1             //創建數據源
 2             var students = new List<Student>()
 3             {
 4                 new Student()
 5                 {
 6                     Age = 18,
 7                     Name = "小A",
 8                     Scores = new List<int>() {88,85,74,66 }
 9                 },
10                 new Student()
11                 {
12                     Age = 35,
13                     Name = "小B",
14                     Scores = new List<int>() {88,85,74,66 }
15                 },
16                 new Student()
17                 {
18                     Age = 28,
19                     Name = "小啥",
20                     Scores = new List<int>() {88,85,74,66 }
21                 }
22             };
23 
24             //創建查詢
25             var studentsToXml = new XElement("Root",
26                 from student in students
27                 let x = $"{student.Scores[0]},{student.Scores[1]},{student.Scores[2]},{student.Scores[3]}"
28                 select new XElement("student",
29                 new XElement("Name", student.Name),
30                 new XElement("Age", student.Age),
31                 new XElement("Scores", x))
32             );
33 
34             //執行查詢
35             Console.WriteLine(studentsToXml);            

四、對源元素執行操作

　　輸出序列可能不包含源序列的任何元素或元素屬性。輸出可能是通過將源元素用作輸入參數計算出的值的序列。

 1             //數據源
 2             double[] radii = {1, 2, 3};
 3 
 4             //創建查詢
 5             var query = from radius in radii
 6                 select $"{radius * radius * 3.14}";
 7 
 8             //執行查詢
 9             foreach (var i in query)
10             {
11                 Console.WriteLine(i);
12             }

【來源】本文文字主要摘自微軟官方文檔