数组的协变

eg:

class A{......};
class B:A{.......};

class Progam
{
 static void Main()
 {
  A[] AArray1 =new A[3];
  A[] AArray2 = new A[3];
  
  AArray1[0]=new A();AArray1[1]=new A();AArray1[2]=new A();
  //协变,
  AArray2[0] = new B();AArray2[1] = new B();AArray2[2] = new B();
 }
}