LINQ 学习路程 -- 查询操作 Deferred Execution of LINQ Query 延迟执行

延迟执行是指一个表达式的值延迟获取，知道它的值真正用到。

当你用foreach循环时，表达式才真正的执行。

延迟执行有个最重要的好处：它总是给你最新的数据

实现延迟运行

你可以使用yield关键字实现延迟加载

public static class EnumerableExtensionMethods
{
    public static IEnumerable<Student> GetTeenAgerStudents(this IEnumerable<Student> source)
    {

        foreach (Student std in source)
        {
            Console.WriteLine("Accessing student {0}", std.StudentName);

            if (std.age > 12 && std.age < 20)
                yield return std;
        }
    }
}

IList<Student> studentList = new List<Student>() {
            new Student() { StudentID = 1, StudentName = "John", age = 13 } ,
            new Student() { StudentID = 2, StudentName = "Steve",  age = 15 } ,
            new Student() { StudentID = 3, StudentName = "Bill",  age = 18 } ,
            new Student() { StudentID = 4, StudentName = "Ram" , age = 12 } ,
            new Student() { StudentID = 5, StudentName = "Ron" , age = 21 }
        };

var teenAgerStudents = from s in studentList.GetTeenAgerStudents()
                        select s;

foreach (Student teenStudent in teenAgerStudents)
    Console.WriteLine("Student Name: {0}", teenStudent.StudentName);

从上面输出的结果看出：当你用foreach循环遍历时，GetTeenAgerStudents（）方法才会被调用

LINQ查询的立即执行

立即执行和延迟执行相反，它迫使LINQ查询立即执行并返回结果，

IList<Student> teenAgerStudents =
                studentList.Where(s => s.age > 12 && s.age < 20).ToList();

IList<Student> teenAgerStudents = (from s in studentList
                                    where s.age > 12 && s.age < 20
                                    select s).ToList();