as与is用法小结

as与is用法总结

1 is关键字

is关键字检查对象是否与给定类型兼容。

例如：

Object obj=new Object();if (obj is string){}

代码中检查obj 对象是否与类型string兼容。 

示例代码：

public class Student{    public string Name { get; set; }    public string Class { get; set; }    public float Score { get; set; }}public class Teacher{    public string Name { get; set; }    public string Course { get; set; }}static void Main(string[] args){    Student student = new Student();    Teacher teacher = new Teacher();    Test(student);    Test(teacher);    Test("Wolf");}static void Test(object obj){    if (obj is Student)    {        Student student = (Student)obj;        Console.WriteLine("I'm a Student");    }    else if (obj is Teacher)    {        Teacher teacher = (Teacher)obj;        Console.WriteLine("I'm a Teacher");    }    else    {        Console.WriteLine("I'm not a Teacher,not a Student,and I'm not a Person");    }}

Test方法中对传入对象使用is关键字判断其是否与指定类型兼容，兼容则进行指定操作。

运行结果如下：

 对象与给定类型兼容又该怎样理解呢？

“兼容”表示对象是该类型，或者派生于该类型。

关于这一点，下面做个简单实验。

在前面示例代码的基础上，增加两个类Person及LittleStudent，且Student类继承自Person类。

public class Person{}public class LittleStudent:Student{}public class Student:Person{    //其他代码不变}Object obj = new Object();Person person = new Person();Student student = new Student();Teacher teacher = new Teacher();LittleStudent lstudent = new LittleStudent();if (obj is Student) //falseif (person is Student) //falseif (student is Student)//trueif (teacher is Student)//falseif (lstudent is Student)//true

2 as关键字

as关键字用于在兼容的引用类型之间执行转换。类似于强制转换操作；如果转换不可行，as 会返回 null 而不会引发异常。

等效于：

expression is type ? (type)expression : (type)null

例如：

代码片段1：

Object obj = new Object();Student s = obj as Student ;if(s != null){    //做你想做的}

代码片段1等价于：

代码片段2：

Object obj = new Object();if (obj is Student){    Student student = (Student)obj;}

在上面两种方式中，后一个版本进行了两次类型检测，效率较低。

3 写在后面：

is 运算符只考虑引用转换、装箱转换和取消装箱转换。不考虑其他转换，如用户定义的转换。