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

static void Main(string[] args)
        {
            PersonList personList = new PersonList();
            personList.CurrentIndex = 0;
            var r1 = from p in personList where p.Age == 18 select p ;
            Console.WriteLine(personList.CurrentIndex.ToString());
            personList.CurrentIndex = 0;
            var r2 = (from p in personList where p.Age >= 20 select p).First();
            Console.WriteLine(personList.CurrentIndex.ToString());
            personList.CurrentIndex = 0;
            var r3 = (from p in personList select p).Take(2);
            Console.WriteLine(personList. CurrentIndex.ToString());

            Console.ReadLine();
        }
    }
    class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public Person(string name, int age)
        {
            Name = name;
            Age = age;
        }
    }
    class PersonList : IEnumerable<Person>
    {
        List<Person> pList = new List<Person>()
    {
        new Person("Mike",20),
        new Person("John",23),
        new Person("Jack",18),
        new Person("Tom",34)
    };
        public void Add(Person person)
        {
            pList.Add(person);
        }
        public int CurrentIndex { get; set; }
        public IEnumerator<Person> GetEnumerator()
        {
            foreach (var p in pList)
            {
                CurrentIndex++;
                yield return p;
            }
        }
         IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

结果：

0,

1，

0.

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。