[From 4.2]as,is,和强制转换

例子1：

if(o is Employee)

    Employee e = (Employee)o;

例子2：

Employee e = o as Employee;

例子3

Employee e = (Employee)o;

 

例子1和例子3相比：例子1永远不会报错，例子3一旦无法转换便会报错

例子2和例子1，例子3相比：比例子1效率更快，比例子3更快。