Mammal jean=new Mammal();//Vertebrata是抽象类
Vertebrata someone=jean;等价于Vertebra someone=new Mamanl
if(someone is Mamal) Console.Writeline("someone is Mammal")
id(someone is Vertebrata)Console.Writeline（”Someone is Mammal“）能输出someone既是哺乳动物也是脊椎动物；
class A
{
public void wordA()
{
console.WriteLine("A");
}
}
class B:A
{
public void wordB()
{
console.Writeline("B");
}
}
static void Main(string [] args)
{
B b=new B();
A a=b;//向下类型转换到这里可以实现派生类的虚方法
If(a is B)//为真
{
B b1=(B)a;//彻底转弯换B类（派生类）
b1.wordB();
}
原文：http://www.cnblogs.com/jaen-home/p/6457870.html