学习lamda表达式

Posted 2020-07-19 拒马河

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

namespace 阐述lambda
{
    class Program
    {
        delegate int GuangChaoshi(int a);
        /// <summary>
        /// 委托  逛超市
        /// </summary>
        /// <param name="a">花费</param>
        /// <param name="b">付钱</param>
        /// <returns>找零</returns>
        delegate int GuangChaoshi2(int a, int b);

        //结账
        public static int JieZhang(int a)
        {
            return a + 10;
        }


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


        public static List<Person> PersonsList()
        {
            List<Person> persons = new List<Person>();
            for (int i = 0; i < 7; i++)
            {
                Person p = new Person() { Name = i + "儿子", Age = 8 - i, };
                persons.Add(p);
            }
            return persons;
        }

        static void Main(string[] args)
        {
            List<Person> persons = PersonsList();
            persons = persons.Where(p => p.Age > 6).ToList();       //所有Age>6的Person的集合
            Person per = persons.SingleOrDefault(p => p.Age == 1);  //Age=1的单个people类
            persons = persons.Where(p => p.Name.Contains("儿子")).ToList();   //所有Name包含儿子的Person的集合
            //1
            GuangChaoshi gwl = JieZhang;
            Console.WriteLine(gwl(10) + "");   //打印20，委托的应用
            Console.ReadKey();
            //2
            GuangChaoshi gw2 = p => p + 10;
            Console.WriteLine(gw2(10) + "");   //打印20，表达式的应用
            Console.ReadKey();

            //3
            GuangChaoshi2 gw3 = (p, z) =>
            {
                int zuidixiaofei = 10;
                if (p < zuidixiaofei)
                {
                    return 100;
                }
                else
                {
                    return z - p - 10;
                }

            };
            Console.WriteLine(gw3(10, 100) + "");   //打印80，z对应参数b，p对应参数a
            Console.ReadKey();

            //4
            //i*j+w*x
            ParameterExpression a = Expression.Parameter(typeof(int), "i");   //创建一个表达式树中的参数，作为一个节点，这里是最下层的节点
            ParameterExpression b = Expression.Parameter(typeof(int), "j");
            BinaryExpression be = Expression.Multiply(a, b);    //这里i*j,生成表达式树中的一个节点，比上面节点高一级

            ParameterExpression c = Expression.Parameter(typeof(int), "w");
            ParameterExpression d = Expression.Parameter(typeof(int), "x");
            BinaryExpression be1 = Expression.Multiply(c, d);

            BinaryExpression su = Expression.Add(be, be1);   //运算两个中级节点，产生终结点

            Expression<Func<int, int, int, int, int>> lambda = Expression.Lambda<Func<int, int, int, int, int>>(su, a, b, c, d);

            Console.WriteLine(lambda + "");   //打印‘(i,j,w,x)=>((i*j)+(w*x))’，z对应参数b，p对应参数a

            Func<int, int, int, int, int> f = lambda.Compile();  //将表达式树描述的lambda表达式，编译为可执行代码，并生成该lambda表达式的委托；

            Console.WriteLine(f(1, 1, 1, 1) + "");  //打印2
            Console.ReadKey();

            //5
            Func<int, string> gw5 = p => p + 10 + "--返回类型为string";
            Console.WriteLine(gw5(10) + "");   //打印‘20--返回类型为string’，z对应参数b，p对应参数a
            Console.ReadKey();
            //6
            Func<int, int, bool> gw6 = (p, j) =>
            {
                if (p + j == 10)
                {
                    return true;
                }
                return false;
            };
            Console.WriteLine(gw6(5, 5) + "");   //打印‘True’，z对应参数b，p对应参数a
            Console.ReadKey();
        }

    }
}