c#泛型委托

委托：

这个比较简单，从delegate关键字到匿名委托方法，再到Action和Func，Action是无返回值的委托，Func是有返回值的委托，现在基本上都是基于Action和Func来写委托的，有些和业务逻辑连接起来写匿名委托方法。

首先建立一个student类

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace ConsoleApp4
{
    public class Student
    {
        public int ID { get; set; }
 
        public string name { get; set; }
 
        public int age { get; set; }
 
        public string sex { get; set; }
 
        public string grade { get; set; }
    }
}

2.我们初始化数据后，分别进行查询操作

进行3次查询后，然后分别输出对应的名字，这样是我们平时的做法，一点错误都没有。可以看到都是where的功劳，现在我们可以使用泛型委托自己自定义一个和where一样的功能。

只要遇到代码有重复的逻辑，就可以使用委托

using System;
using System.Collections.Generic;
using System.Linq;
 
namespace ConsoleApp4
{
    static class Program
    {
        static void Main(string[] args)
        {
            List<Student> student = new List<Student>();
            student.Add(new Student() { age = 20, grade = "一班", ID = 1, name = "张三", sex = "男" });
            student.Add(new Student() { age = 21, grade = "一班", ID = 2, name = "李四", sex = "女" });
            student.Add(new Student() { age = 22, grade = "二班", ID = 3, name = "王五", sex = "女" });
            student.Add(new Student() { age = 23, grade = "二班", ID = 4, name = "老六", sex = "男" });
 
            var stu = student.Where(s => s.grade == "一班");
            foreach (var item in stu)
            {
                Console.WriteLine(item.name);
            }
 
            Console.WriteLine("==============分界线==============");
            var stu1 = student.Where(s => s.sex == "男");
            foreach (var item in stu1)
            {
                Console.WriteLine(item.name);
            }
 
            Console.WriteLine("==============分界线==============");
            var stu2 = student.Where(s => s.sex == "男" && s.age > 20);
            foreach (var item in stu2)
            {
                Console.WriteLine(item.name);
            }
        }
    }
}

但是我们建立一个WhereNew类，表示泛型委托类 特别要注意，Invoke是执行方法，传入参数，返回bool值

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace ConsoleApp4
{
    public static class WhereNew
    {
        public static IEnumerable<T> NewWhere<T>(this IEnumerable<T> items, Func<T, bool> func)
        {
            List<T> ls = new List<T>();
            foreach (var item in items)
            {
                if (func.Invoke(item)) //执行方法，传入参数，返回bool值
                {
                    ls.Add(item);
                }
            }
            return ls;
        }
    }
}

在使用

using System;
using System.Collections.Generic;
using System.Linq;
 
namespace ConsoleApp4
{
    static class Program
    {
        static void Main(string[] args)
        {
            List<Student> student = new List<Student>();
            student.Add(new Student() { age = 20, grade = "一班", ID = 1, name = "张三", sex = "男" });
            student.Add(new Student() { age = 21, grade = "一班", ID = 2, name = "李四", sex = "女" });
            student.Add(new Student() { age = 22, grade = "二班", ID = 3, name = "王五", sex = "女" });
            student.Add(new Student() { age = 23, grade = "二班", ID = 4, name = "老六", sex = "男" });
 
            //var stu = student.Where(s => s.grade == "一班");
            IEnumerable<Student> stu = student.NewWhere(s => s.grade == "一班");
            foreach (var item in stu)
            {
                Console.WriteLine(item.name);
            }
 
            Console.WriteLine("==============分界线==============");
            //var stu1 = student.Where(s => s.sex == "男");
            IEnumerable<Student> stu1 = student.NewWhere(s => s.sex == "男");
            foreach (var item in stu1)
            {
                Console.WriteLine(item.name);
            }
 
            Console.WriteLine("==============分界线==============");
            //var stu2 = student.Where(s => s.sex == "男" && s.age > 20);
            IEnumerable<Student> stu2 = student.NewWhere(s => s.sex == "男" && s.age > 20);
            foreach (var item in stu2)
            {
                Console.WriteLine(item.name);
            }
        }
    }
}

从中我们可以看到where和自定义的NewWhere，功能是一样的，也就是说where也是用了泛型委托。泛型就是T，委托就是func。所以，我们可以传递一个类型的数据进去，然后再传递一个方法进去，自动就加工返回值了。