(精华)2020年8月11日 C#基础知识点 Lambda表达式

public class LambdaShow
{
    public delegate void NoReturnNoPara();//无参数无返回值委托
    public delegate void NoReturnWithPara(string x, string y);//有参数无返回值委托
    public delegate int WithReturnNoPara();
    public delegate string WithReturnWithPara(out int x, ref int y);
    public void Show()
    {
        //lambda演变历史
        {
            Console.WriteLine("***********************.NetFramework1.0****************************");
            //.NetFramework1.0/1.1时代 
            NoReturnNoPara method = new NoReturnNoPara(this.DoNothing);
            NoReturnWithPara noReturnWithPara = new NoReturnWithPara(this.Study);
            noReturnWithPara.Invoke("参数一", "参数二");
        }
        int i = 10;
        {
            Console.WriteLine("***********************.NetFramework2.0****************************");
            //.NetFramework2.0  匿名方法，增加了一个delgate关键字 可以访问局部变量
            NoReturnWithPara noReturnWithPara = new NoReturnWithPara(delegate (string x, string y)
            {
                Console.WriteLine(i);
                Console.WriteLine("我是匿名委托");
            });
            noReturnWithPara.Invoke("参数一", "参数二");
        }
        { 
            Console.WriteLine("***********************.NetFramework3.0****************************");
            //.NetFramework3.0  去掉delegate关键字，添加了一个=>  goes to
            NoReturnWithPara noReturnWithPara = new NoReturnWithPara((string x, string y) =>
            {
                Console.WriteLine(i);
                Console.WriteLine("This is Study");
            });
            noReturnWithPara.Invoke("元让", "阳光下的微笑");
        }
        {
            NoReturnWithPara noReturnWithPara = new NoReturnWithPara((x, y) => // 通过委托自动推断来的（语法糖）
            {
                Console.WriteLine(i);
                Console.WriteLine("This is Study");
            });
            noReturnWithPara.Invoke("参数一", "参数二");
        }
        {
            NoReturnWithPara noReturnWithPara = new NoReturnWithPara((x, y) => Console.WriteLine("This is Study")
            );
            noReturnWithPara.Invoke("参数一", "参数二");
        }
        {
            NoReturnWithPara noReturnWithPara = (x, y) => Console.WriteLine("This is Study");
            noReturnWithPara.Invoke("参数一", "参数二");
        }
        {
            Action<string, string> action = (x, y) => Console.WriteLine($"{x}_{y}");
            Action<string> action1 = x => Console.WriteLine($"{x}"); // 如果只有一个参数的时候，可以省略小括号 
            Func<string, string> func = x => "代码男孩";
            Func<string> func1 = () => "代码男孩";  //如果有返回值，而且方法体只有一行，可以省略return;
        }
        // 通过反编译之后，发现Lambda表达式生成在一个名称为<> 密封类中的一个方法； 
        {
            //lambda在多播委托 
            NoReturnWithPara noReturnWithPara = new NoReturnWithPara(this.Study);
            noReturnWithPara += this.Study;
            noReturnWithPara += (x, y) =>
            {
                Console.WriteLine($"{x}_{y}");
            };
            noReturnWithPara -= this.Study;
            noReturnWithPara -= (x, y) => { Console.WriteLine($"{x}_{y}"); };  //lambda表达式在多播委托中不能被移除
            noReturnWithPara("哗哗", "便便");
        }
    }
    private void DoNothing()
    {
        Console.WriteLine("无参数无返回值委托");
    }
    private void Study(string x, string y)
    {
        Console.WriteLine("有参数无返回值委托");
    }
}