大二下学期

　　if(a as Dog) //1

　　{

　　　　Dog d = (Dog)a; //2

　　　　...

　　}

　　Dog d = a as Dog; //3

　　d.Shout(); //4

　　if(a as Dog) //1

　　{

　　　　Dog d = (Dog)a; //2

　　　　...

　　}

　　Dog d = a as Dog; //3

　　d.Shout(); //4

var d1 = (Car)a; //1

var d2 = a as Car; //2

var d3 = a is Car; //3

var d1 = a as Student; //1

var d2 = a is Student; //2

var d3 = (Student)a; //3

var d1 = a as Student; //1

var d2 = a is Student; //2

var d3 = (Student)a; //3

有1个委托代码如下

class Extents //1

{

    public static DateTime ToDateTime(String pid) //2

    {

        DateTime date = null; //3

class Extents //1

{

    public static DateTime ToDateTime() //2

    {

        DateTime date = null; //3

static void Main(string[] args)

{

//Tools类中定义