利用现有运算符，针对自定义类或结构，简化地自定义某运算操作，一般选用有相关性的既有运算符

操作数至少有一个是类或结构

不能改变运算符优先级

不能重载的运算符： =，&&，||，[],()等

class Program    {        static void Main(string[] args)        {            Pet P = new Dog("pp");            P++;//++有了重载定义的计算方式            P.ShowAge();        }    }    abstract public class Pet    {        protected string Name;        public Pet(string name)        {            Name = name;        }        public void PrintName()        {            Console.WriteLine("Print" + Name);        }               abstract public void Speak();    }   abstract public class Pet    {        protected string Name;        protected int Age;        public Pet(string name)        {            Name = name;            Age = 0;        }        public void PrintName()        {            Console.WriteLine("Print" + Name);        }        public static Pet operator ++(Pet pet)//重载运算符++//针对类Pet        {            ++pet.Age;            return pet;        }        public void ShowAge()        {            Console.WriteLine(Name+ " Age = " + Age);        }             abstract public void Speak();    }    public class Dog : Pet    {        static int Num;        static Dog()        {            Num = 0;        }        public Dog(string name) : base(name)        {            ++Num;        }        new public void PrintName()        {            Console.WriteLine("子print名字是 " + base.Name);        }               {            Console.WriteLine("子说,我是狗狗 " + Name);        }        static public void ShowNum()        {            Console.WriteLine("Dog's number = " + Num);        }        public void Lost()        {            --Num;            Console.WriteLine(Name + " is Lost!");        }         }