Abstract class in C #. Net (keyword Abstract)

/*
* Created by sharpdevelop.
* User: noo
* Date: 2009-8-16
* Time:
*
* Abstract class
*/

Using system;
Abstract Class AA // equivalent to internal abstract class A, the class can only be accessed in the current project, cannot be instantiated (no constructor), can only inherit
{
Public abstract void output (); // It can have abstract members, but like the members in the interface, it cannot have any implementation code.
Public void input ()
{
Console. writeline ("this is an abstract class method ");
}
}
Class BB: AA
{
Public override void output () // implementation
{
Console. writeline ("this is an abstract method inherited from an abstract class ");
}
}
Class Test
{
Static void main ()
{
Bb BB = new BB ();
BB. Output ();
BB. Input ();

Aa aa = BB;
AA. Output ();
AA. Input ();
}
}