Linq延迟求值

Linq语句只有在调用结果的时候才会真正执行，如下

 1 static void Main(string[] args)
 2         {
 3             PersonList personList = new PersonList();
 4             personList.CurrentIndex = 0;
 5             var r1 = from p in personList where p.Age == 18 select p ;
 6             Console.WriteLine(personList.CurrentIndex.ToString());
 7             personList.CurrentIndex = 0;
 8             var r2 = (from p in personList where p.Age >= 20 select p).First();
 9             Console.WriteLine(personList.CurrentIndex.ToString());
10             personList.CurrentIndex = 0;
11             var r3 = (from p in personList select p).Take(2);
12             Console.WriteLine(personList. CurrentIndex.ToString());
13 
14             Console.ReadLine();
15         }
16     }
17     class Person
18     {
19         public string Name { get; set; }
20         public int Age { get; set; }
21         public Person(string name, int age)
22         {
23             Name = name;
24             Age = age;
25         }
26     }
27     class PersonList : IEnumerable<Person>
28     {
29         List<Person> pList = new List<Person>()
30     {
31         new Person("Mike",20),
32         new Person("John",23),
33         new Person("Jack",18),
34         new Person("Tom",34)
35     };
36         public void Add(Person person)
37         {
38             pList.Add(person);
39         }
40         public int CurrentIndex { get; set; }
41         public IEnumerator<Person> GetEnumerator()
42         {
43             foreach (var p in pList)
44             {
45                 CurrentIndex++;
46                 yield return p;
47             }
48         }
49          IEnumerator IEnumerable.GetEnumerator()
50         {
51             return GetEnumerator();
52         }
53     }

结果：

1，

第5行和第11行只有在调用r1,r3的时候才会真正执行，而我们并没有调用r1和r3,所以CurrentIndex为0，而第8行则直接执行。

如果linq语句立刻执行，可以在后边加上ToList<>等方法。