MVC方法使用与集合：

First()与FirstOrDefault()的区别

First()当代象为NULL或没有数据时就会报错，FirstOrDefault()则不会报错而是返回NULL

异常是 IQuerable<T>.First() 方法报的，当查询的结果集为空时，调用 First() 方法将报此异常，换用 FirstOrDefault() 方法，则不报异常

 

 

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            List<Animal> animals=new List<Animal>();
            Animal animal=new Animal(){Age = 45,Name = "小明",Type = "人",Sex = true};
            Animal animal1 = new Animal() { Age = 45, Name = "小F", Type = "妖", Sex = true };
            Animal animal2 = new Animal() { Age = 45, Name = "小D", Type = "人", Sex = true };
            Animal animal3 = new Animal() { Age = 45, Name = "小E", Type = "妖", Sex = true };
            Animal animal4 = new Animal() { Age = 45, Name = "小A", Type = "妖", Sex = true };
            animals.Add(animal);
            animals.Add(animal1);
            animals.Add(animal2);
            animals.Add(animal3);

            List<Person> persons = animals.Where(item => item.Type == "人").Select(item => new Person()
            {
                Name = item.Name,
                Age = item.Age,
                //Sex = GetSex(item.Sex),//此处注册一个方法则可执行返回结果要 也可以创建一个对象直接调用其方法： Sex = new Helper().GetSex(item.Sex)
                Sex = new Helper().GetSex(item.Sex)
            }).ToList();



        }


        public static string GetSex(bool sex)
        {
            if (sex)
            {
                return "男人";
            }
            else
            {
                return "女人"; 
            }
        }


       
    }


    public class Helper
    {

        public  string GetSex(bool sex)
        {
            if (sex)
            {
                return "男人";
            }
            else
            {
                return "女人";
            }
        }

    }


    public class Person
    {
        public string Sex { get; set; }
        public int Age { get; set; }
        public string Name { get; set; }
       
    }



    public class Animal
    {      
       public bool Sex { get; set; }
       public int Age { get; set; }
       public string Name { get; set; }
       public string Type { get; set; }
    }
}