在使用Linq中 遇到问题的一点笔记

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    /// <summary>
    /// 只作为快速的测试，没考虑编码规范的问题
    /// </summary>
    class Program  
    {
        static void Main(string[] args)
        {
            List<Student> student_List = new List<Student>();
            for (int i = 0; i <= 3;i++ )
            {
                Student studen = new Student();
                studen.Age = i + 20;
                studen.Name = (i + 1).ToString();
                student_List.Add(studen);
            }
            //查询，本人曾经出错的地方
            //Student test = (Student)(from r in student_List where r.Age == 21 select r);//错误的写法，因为linq返回的是IEnumerable的 不能强制转换。
            //正确的写法
            var tmp = (from r in student_List where r.Age == 21 select r);
            Student test = tmp.FirstOrDefault();//或者这样写Student test = tmp.First();
            //查询多条
            List<Student> tmp_List = (from r in student_List where r.Age >= 20 select r).ToList();

        }
    }
    class Student
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}