Polymorphism
is the ability of an object oriented language (C# in this context) to
allow a base class to define a set of members (formally termed
the polymorphic interface) that are available to all
descendents. A class’s polymorphic interface is constructed using
any number of virtual or abstract members.
A
virtual member is a member in a base class that
defines a default implementation that may be changed (overridden)
by a derived class.
In
contrast, an abstract method is a member in a
base class that does not provide a default implementation, but
does provide a signature. When a class derives from a base
class defining an abstract method, it must be overridden by a
derived type.
In
either case (virtual or abstract), when derived types override the
members defined by a base class, they are essentially redefining how
they respond to the same request.
Method
Overriding – Method
Overriding is a process in which a derived class can define its own
version of a method, which was originally defined by its base class.
Virtual
Keyword – The virtual
keyword indicates that the base class wants to define a method which
may be
(but does not have to be) overridden
by a derived class. A virtual method provides a default
implementation that all derived types automatically inherit.
If a child class so chooses, it may override the method but
does not have to.
Note:
Subclasses are never required to override virtual methods.
Override
Keyword – The override keyword is used by a
derived class to to change the implementation details of a virtual
method (defined in the base class). Each overridden method is free to
leverage the default behavior of the parent class using the base
keyword.
Sealed
Keyword
– The sealed
keyword is basically applied to classes to prevent other types from
extending its behavior via inheritance.
It
can also be applied to virtual methods to prevent derived types from
overriding those methods. Ex.
public
override
sealed
void GiveBonus(float amount)
{
...
}
Any
effort to override the above sealed method in the derived class will
generate compile time error.
Abstract
Classes – The abstract
keyword can be used in the class definition to prevent direct
creation of an instance of the class. Although abstract classes
cannot be created directly,
it is still assembled in memory when derived classes are
created. Thus, it is perfectly fine for abstract classes to define
any number of constructors that are called indirectly when
derived classes are allocated.
An
abstract class can define any number of abstract members
(members that does not supply
a default implementation, but must
be accounted for by each
derived class).
The
polymorphic interface of an abstract base class simply refers to its
set of virtual and abstract methods. Methods marked with abstract are
pure protocol. They simply define the name, return type (if any), and
parameter set (if required).
If
you do not override an abstract method (of the base abstract class)
in the derived class, the derived class will also be considered
abstract, and would have to be marked abstract with the abstract
keyword.
Although
it is not possible to directly create an instance of an
abstract base class, you can freely store references to any subclass
with an abstract base variable.
Q:
What is an efficient way to force each child class to
override a virtual method?
A:
To force each child class to override a virtual method, we can better
define an abstract method (which by definition means you
provide no default implementation whatsoever), in an abstract class.
