Linq101-QueryExecution

 1 using System;
 2 using System.Linq;
 3 
 4 namespace Linq101
 5 {
 6     class QueryExecution
 7     {
 8         /// <summary>
 9         /// The following sample shows how query execution is deferred until the query is enumerated at a foreach statement.
10         /// </summary>
11         public void Linq99()
12         {
13             int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
14 
15             int i = 0;
16 
17             //延迟执行
18             var q = from n in numbers select ++i;
19 
20             foreach (int n in q)
21             {
22                 Console.WriteLine("n={0},i={1}", n, i);
23             }
24         }
25 
26         /// <summary>
27         /// The following sample shows how queries can be executed immediately with operators such as ToList().
28         /// </summary>
29         public void Linq100()
30         {
31             int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
32 
33             int i = 0;
34 
35             //ToList->立即执行
36             var q = (from n in numbers select ++i).ToList();
37 
38             foreach (int n in q)
39             {
40                 Console.WriteLine("n={0},i={1}", n, i);
41             }
42         }
43 
44         /// <summary>
45         /// The following sample shows how, because of deferred execution, queries can be used again after data changes and will then operate on the new data.
46         /// </summary>
47         public void Linq101()
48         {
49             int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
50 
51             //可重用
52             var lowNumbers = from n in numbers
53                              where n <= 3
54                              select n;
55 
56             Console.WriteLine("First run numbers <=3:");
57             foreach (int number in lowNumbers)
58             {
59                 Console.WriteLine(number);
60             }
61 
62             lowNumbers = (from n in numbers select -n).ToArray();
63 
64             Console.WriteLine("Second run numbers <=3:");
65             foreach (int number in lowNumbers)
66             {
67                 Console.WriteLine(number);
68             }
69         }
70     }
71 }