这里的运算符重载，为什么老是提示：必须包含类型？？

Question

类苹果C#运算符重载

Answer 1

+运算符重载必须定义在Apple或者Water两个类之一中才行。

Answer 2

第一点，所谓+，无论重载与否，都是，1+1=2，类型都是数字，那么水果+水果=2个水果，类型一样
第二点，从你的类型来看，你program类型里，又包含了2个类型，那么，对于强类型的C#,楼主是没有类型做到一致
第三点，可封箱到program里，再进行加减

Answer 3

在   苏小喵  和  极品老土豆  的指导下，我把代码做了修改，初步实现题目要求。
代码如下：
namespace 书本例题
{
    class Program
    {
        public class AppleWater  //苹果汁
        {
            public string name;
            public string colour;
            public string form;
            public string taste;
            public void show()   //显示
            {
                Console.WriteLine("名称：{0}", name);
                Console.WriteLine("颜色：{0}", colour);
                Console.WriteLine("形状：{0}", form);
                Console.WriteLine("味道：{0}", taste);
                Console.WriteLine("********************");
            }
        }
        public class Apple    //苹果
        {
            public string name;
            public string colour;
            public string form;
            public string taste;
            public Apple(string name, string colour, string form, string taste)
            {
                this.name = name;
                this.colour = colour;
                this.form = form;
                this.taste = taste;
            }
            public Apple()
            {
                this.name = "苹果";
                this.colour = "红色";
                this.form = "圆形";
                this.taste = "酸甜";
            }
            public void show()   //显示
            {
                Console.WriteLine("名称：{0}", name);
                Console.WriteLine("颜色：{0}", colour);
                Console.WriteLine("形状：{0}", form);
                Console.WriteLine("味道：{0}", taste);
                Console.WriteLine("********************");
            }
        }
        public class Water   //水
        {
            public string name;
            public string colour;
            public string form;
            public string taste;
            public Water(string name, string colour, string form, string taste)
            {
                this.name = name;
                this.colour = colour;
                this.form = form;
                this.taste = taste;
            }
            public Water()
            {
                this.name = "水";
                this.colour = "无色";
                this.form = "无形";
                this.taste = "无味";
            }
            public void show()   //显示
            {
                Console.WriteLine("名称：{0}", name);
                Console.WriteLine("颜色：{0}", colour);
                Console.WriteLine("形状：{0}", form);
                Console.WriteLine("味道：{0}", taste);
                Console.WriteLine("********************");
            }
            public static AppleWater operator +(Apple app, Water wat)   //+运算符重载
            {                AppleWater appwat = new AppleWater();
                if (app.name == "苹果" && wat.name == "水")
                {
                    appwat.name = "苹果汁";
                }
                else
                {
                    app.name = "匿名";
                }
                if (app.colour == "红色" || app.name == "青色" && wat.colour == "无色")
                {
                    appwat.colour = "橙色";
                }
                else
                {
                    appwat.colour = "匿色";
                }
                if (app.form == "圆形" && wat.form == "无形")
                {
                    appwat.form = "无形";
                }
                else
                {
                    appwat.form = "匿形";
                }
                if (app.taste == "酸甜" && wat.taste == "无味")
                {
                    appwat.taste = "酸甜可口";
                }
                else
                {
                    appwat.taste = "匿味";
                }
                return appwat;
            }        }        static void Main(string[] args)
        {
            Apple app = new Apple();
            app.show();
            Water wat = new Water();
            wat.show();
            AppleWater appwat = new AppleWater();
            appwat = app + wat;  //调用+运算符重载
            appwat.show();
          
            Console.ReadKey();
        }
    }
}
运行结果：
名称：苹果
颜色：红色
形状：圆形
味道：酸甜
"********************"
运行结果：
名称：水
颜色：无色
形状：无形
味道：无味
"********************"
运行结果：
名称：苹果汁
颜色：橙色
形状：无形
味道：酸甜可口
"********************"

Answer 4

感谢信
---------------
感谢 苏小喵  和  极品老土豆 两位大侠！另：1、这个程序代码一定还可以再简化，不知如何优化？
        2、封箱在一个program里，进行加减。这句话，我不懂，该怎么理解？