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is检查一个对象是否兼容于指定的类型，返回Boolean值。注意is操作符永远不会抛异常。is操作符通常这样使用：
if（o is Employee） { Employee e=(Employee) 0; //其他处理... }
在这段代码中，CLR实际会检查两次对象类型。is操作符首先核实o是否兼容于Employee类型。如果是，将在if语句内部再次核实o是否引用一个Employee。CLR的类型检查增强了安全性，但无疑也对性能造成一定影响。这是因为CLR首先必须判断变量(o)引用的对象的实际类型。然后，CLR必须遍历继承层次结构用每一个基类型去核对指定的类型。这是一个相当常用的编程模式，所以C#专门提供了as操作符，目的简化这种代码的写法，同时提升其性能。
Employee e=o as Employee; if（e!=null） { //后续其他处理 }
在这段代码中，CLR核实o是否兼容于Employee类型；如果是，as会返回对同一个对象的一个非null引用。如果不兼容，返回null。as操作符造成CLR只校验一次对象的类型。if语句只是检查e是否为null。这个检查速度比校验对象的类型快得多。
实际中其实可以这么写：
public class Program { static void Main(string[] args) { Object o = new
Test(); if (o is Test test) { var a = test.Age; } Console.ReadKey(); } } public
class Test { public int Age }
反编译后发现：原本写的是is，反编译后代码是as。并且会自动判断能否装换成功，能成功，则返回转换后的对象赋给test。
