Virtual Methods

Virtual Methods

Virtual methods allow a reference to the base class to access "up into" the derived class.
You can use a reference to a base class to call a method in the derived class if the following are true:

• The method in the derived class and the method in the base class each have the same signature and return type.
• The method in the base class is labeled virtual.
• The method in the derived class is labeled override.

See following example:

class person
{
 public string firstName;
 public string lastName;
 public ushort age;
  
 public virtual void greet()
 {
  Console.WriteLine("Person say: Hello "+ firstName + " " + lastName + ". I'm " + age);
 }
}

class employee : person
{
 public string company;

 public override void greet()
 {
  Console.WriteLine("Employee say: Hello " + firstName + " " + lastName + ". I'm " + age);
  Console.WriteLine("I work for " + company);
 }
}

class Program
{
 static void Main(string[] args)
 {
  employee me = new employee() { firstName="Yang",lastName="Sopiana",age=29,company="Company.inc"};
  Console.WriteLine("Calling from derived class");
  me.greet();

  person personMe = (person)me;
  Console.WriteLine("Calling from base class");
  personMe.greet();
 }
}

The code result following display:

Calling from derived class
Employee say: Hello Yang Sopiana. I'm 29
I work for Company.inc
Calling from base class
Employee say: Hello Yang Sopiana. I'm 29
I work for Company.inc

Important things to know about the virtual and override modifiers are the following:

• The overriding and overridden methods must have the same accessibility. In other words, the overridden method cannot be, for example, private, and the overriding method public.
• Can not override a method that is static or is not declared as virtual.
• Methods, properties, and indexers and event can all be declared virtual and override.

Abstract Members

An abstract member is a function member that is designed to be overridden. An abstract member has the following characteristics:

• It must be a function member. That is, fields and constants cannot be abstract members.
• It must be marked with the abstract modifier.
• It must not have an implementation code block. The code of an abstract member is represented by a semicolon.
• Four types of members can be declared as abstract: Methods, Properties, Events, Indexers

See following example:

abstract class person
{
 public string firstName;
 public string lastName;
 public ushort age;
 
 public abstract void greet();  //abstract doesn't have implementation
}

class employee : person
{
 public string company;
 
 public override void greet()  //abstract method should be overridden
 {
  Console.WriteLine("Employee say: Hello " + firstName + " " + lastName + ". I'm " + age);
  Console.WriteLine("I work for " + company);
 }
}

class Program
{
 static void Main(string[] args)
 {
  employee me = new employee() { firstName="Yang",lastName="Sopiana",age=29,company="Company.inc"};
  Console.WriteLine("Calling from derived class");
  me.greet();

  person personMe = (person)me;
  Console.WriteLine("Calling from base class");
  personMe.greet();
 }
}

The code will display:

Calling from derived class
Employee say: Hello Yang Sopiana. I'm 29
I work for Company.inc
Calling from base class
Employee say: Hello Yang Sopiana. I'm 29
I work for Company.inc

Virtual Vs Abstract

Following table shows you the differences between virtual and abstract:

	Virtual Member	Abstract Member
Keyword	virtual	abstract
Implementation body	Has an implementation body	No implementation body—semicolon instead
Overridden in a derived class	Can be overridden	Must be overridden
Types of members	Methods, Properties, Events, Indexers	Methods, Properties, Events, Indexers

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