类型转换、类型安全以及is和as的使用

class Program
    {
        static void Main(string[] args)
        {
            //1、类型转换
            {
                //隐式转换：不需要转型，因为new返回一个Employee对象，而Object是Employee的基类型
                object o = new Employee();

                //显示转换：需要转型，因为Employee派生自Object
                Employee employee = (Employee)o;
            }

            //2、类型安全
            {
                //由于Manager是派生自Employee类型，运行正常
                Manager m = new Manager();
                PromoteEmployee(m);

                //由于DateTime不是派生自Employee类型，运行时会抛出异常
                //DateTime newYears = new DateTime(2019, 8, 25);
                //PromoteEmployee(newYears);
            }

            //3、类型检查 is ，用is判断类型相比比较低
            /*
             * 这是由于is操作符首先核实o是否兼容于Employee类型，再次核实o是否引用了一个Employee
             * 
             * is类型检查，永远不会抛出异常，结果只会返回 true  或 false
             * 
             * **/
            {
                object o = new object();
                if ((o is object))
                {

                }
                if ((o is Employee))
                {

                }
            }

            //4、类型转换 as ，用as判断类型可提升其性能
            /*
             * as 类型转换，如果类型转换成功，则返回对象，否则返回null，此转换方式也永远不会抛出异常
             * 
             * **/
            {
                object o = new object();
                Employee e = o as Employee;
                if (e != null)
                {

                }
            }

            //5、类型转行 as ，测试
            {
                object o = new object();
                Employee e = o as Employee;
            }

            //6、类型转换小测试
            {
                //编译错误，由于是显示转换，不一定成功
                //B b1 = new object();
                //D d1 = new object();

                //运行错误，由于是显示转换，不一定成功
                B b1 = new B();
                D d1 = (D)b1;

                //运行错误，由于是显示转换，不一定成功
                object o = new object();
                B b2 = (B)o;
            }
            Console.ReadKey();
        }
        //应该将Object参数类型改为Employee类型，这样在编译时就能报错
        static void PromoteEmployee(object o)
        {
            Employee e = (Employee)o;
        }
    }
    class D : B
    {

    }
    class B
    {

    }
    public class Manager : Employee
    {
    }
    public class Employee
    {
    }