[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。