lambda表达式自学笔记1

Lambda表达式

Sample 1
[code]
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public delegate int mydg(int a,int b);
    public static class LambdaTest
    {
        public static int oper(this int a, int b, mydg dg)
        {
            return dg(a, b);
        }
    }
    class Program
    {

        static void Main(string[] args)
        {
            Console.WriteLine(1.oper(2, (a, b) => a + b));

            Console.WriteLine(2.oper(1, (a, b) => a - b));

            Console.ReadLine();
        }
    }
}

[/code]
运行结果：
[code]
3
1
[/code]

分析：
1、    public delegate int mydg(int a, int b);
非常重要，代理，Lambda表达式就是delegate
2、 public static int oper(this int a, int b, mydg dg)
静态方法，结合this的使用，可以通过这种方式对已有类添加方法。
不过这里添加的方法是Lambda表达式。
3、1.oper(2, (a, b) => a + b)
实际执行的是LambdaTest.oper(1,2,(a,b)=>a+b);
补充，所有的类的方法基本上都是用这种方式来执行的。

Sample 2
[code]
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public class Person
    {

        public string username { get; set; }

        public int age { get; set; }

        public override string ToString()
        {

            return string.Format("username:{0} age:{1}", this.username, this.age);

        }

    }

    class Program
    {

        static void Main(string[] args)
        {
            var persons = new List<Person> {

    new Person {username = "a", age=19},

    new Person {username = "b", age=20},

    new Person {username = "c", age=21},

    };
            var selectPerson = from p in persons where p.age >= 20 select p.username.ToUpper();
            foreach (var p in selectPerson)
                Console.WriteLine(p);
            Console.ReadLine();

        }
    }
}
[/code]

分析：
  var selectPerson = from p in persons where p.age >= 20 select p.username.ToUpper();
等价于：
var selectPerson = persons.Where(p => p.age >= 20).Select(p => p.username.ToUpper());  

2011-6-2 10:17 danny