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Overriding (Cross Ref > OOP Details)

Overriding

General Info: Method Overriding

Where you define or implement a virtual method in a parent class and then replace it in a descendant class.

When you decide to declare a method as virtual, you are giving permission to derived classes to extend and override the method with their own implementation. You can have the extended method call the parent method's code too.

In most OO languages you can also choose to hide a parent method. When you introduce a new implementation of the same named method with the same signature without overriding, you are hiding the parent method.

ASP Classic:   Not Supported

Since ASP Classic does not support inheritance, there is no concept of a descendant class nor overriding.

C#:   virtual, override

In C#, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword.

Use the base keyword in the descendant method to execute the code in the parent method, i.e. base.SomeMethod().

Syntax Example:
class Robot
{
  public virtual void Speak()
  {
  }
}
 
class Cyborg:Robot
{
  public override void Speak()
  {
  }
}

Override Details

  • You cannot override a regular non-virtual method, nor a static method.
  • The first version of the parent method must be virtual or abstract.
  • You can override any parent method marked virtual, abstract, or override (already overridden).
  • The methods must have the same signature.
  • The methods must have the same visibility (the same access level).
  • Use the base keyword to refer to the parent class as in base.SomeMethod().

C# Override Example

The following code snippet demonstrates using virtual and override to override a parent method in a descendant class.

using System;
 
class Dog
{
    public virtual void Bark()
    {
          Console.WriteLine("RUFF!");
    }
}
 
class GermanShepard:Dog
{
    public override void Bark()
    {
          Console.WriteLine("Rrrrooouuff!!");
    }
}
 
class Chiuaua:Dog
{
    public override void Bark()
    {
          Console.WriteLine("ruff");
    }
}
 
class InclusionExample
{
    public static void Main()
    {
         Dog MyDog=new Dog();
 
         MyDog=new GermanShepard();
         MyDog.Bark();  // prints Rrrrooouuff!!

         MyDog=new Chiuaua();
         MyDog.Bark();  // prints ruff;
    }
}

Hiding a Method with New

Use the new keyword to introduce a new implementation of a parent method (this hides the parent method). You can hide a method without using new but you will get a compiler warning. Using new will suppress the warning.

The new and override modifiers have different meanings. The new modifier creates a new member with the same name, signature, and visibility and hides the original member. The override modifier extends the implementation for an inherited member and allows you to implement inheritance-based polymorphism.

Avoid Introducing New Members: Sometimes there are clear reasons to introduce a new method with the same name, signature, and visibility of a parent method. In those clear cases, introducing a new member is a powerful feature. However, if you do not have a clear reason, then avoid introducing a new version of a method by naming the new method something unique and appropriate.

class Robot : System.Object
{
  public void Speak()
  {
    MessageBox.Show("Robot says hi");
  }
}
  
class Cyborg : Robot
{
  new public void Speak()
  {
    MessageBox.Show("hi");
  }
}

Calling the Base Class Version

A common task In OO is to extend a method by first executing the parent method code and then adding code. Use the base keyword to refer to the parent class as in base.SomeMethod().

class Robot : System.Object
{
  public virtual void Speak()
  {
    MessageBox.Show("Robot says hi");
  }
}
  
class Cyborg : Robot
{
  public override void Speak()
  {
  base.Speak(); 
  MessageBox.Show("hi");
  }
}

 

More Info

Code:  C# Overriding (virtual, override)
Definition:  Method Overriding

Corel Paradox:   Not Supported

Delphi:   virtual, override

In Delphi, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword. Call Inherited in the descendant method to execute the code in the parent method.

Syntax Example:
TRobot = class(TObject)
public
  procedure Speak; virtual;
end;
  
TCyborg = class(TRobot)
  procedure Speak; Override;
end;

Working Delphi 2009 Override Example

The following code assumes a Windows application with a single form�with a button. It�demonstrates using virtual and override to override a parent method in a descendant class.

unit Unit1;
  
interface  
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
  
type
  TForm1 = class(TForm)
     Button1: TButton;
     procedure Button1Click(Sender: TObject);
  end;
  
  TRobot = class(TObject)
  public
    procedure Speak; virtual;
  end;
  
  TCyborg = class(TRobot)
    procedure Speak; override;
  end;
  
var
  Form1: TForm1;
  
implementation
{$R *.dfm}
  
procedure TRobot.Speak;
begin
  ShowMessage('Robot says hi');
end;
  
procedure TCyborg.Speak;
begin
  ShowMessage('Cyborg says hi');
end;
  
procedure TForm1.Button1Click(Sender: TObject);
var
  Robot: TRobot;
  Cyborg: TCyborg;
begin
  Robot := TRobot.Create;
  Cyborg := TCyborg.Create;
  
  Robot.Speak;
  Cyborg.Speak;
  
  FreeAndNil(Robot);
  FreeAndNil(Cyborg);
end;  
end.

Hiding a Method with 'reintroduce'

Use the reintroduce keyword to introduce a new implementation of a parent method (this hides the parent method). You can hide a method without using�reintroduce but you will get a compiler warning. Using�reintroduce will suppress the warning.

The�reintroduce and override modifiers have different meanings. The�reintroduce modifier creates a new member with the same name, signature, and visibility and hides the original member. The override modifier extends the implementation for an inherited member and allows you to implement inheritance-based polymorphism.

Avoid Introducing New Members: Sometimes there are clear reasons to introduce a new method with the same name, signature, and visibility of a parent method. In those clear cases, introducing a new member is a powerful feature. However, if you do not have a clear reason, then avoid introducing a new version of a method by naming the new method something unique and appropriate.

//interface section:
TRobot = class(TObject)
public
  procedure Speak;
end;
  
TCyborg = class(TRobot)
  procedure Speak; reintroduce;
end;
  
//implementation section:
procedure TRobot.Speak;
begin
  ShowMessage('Robot says hi');
end;
  
procedure TCyborg.Speak;
begin
  ShowMessage('Cyborg says hi');
end;

Invoking the Parent Method with 'inherited'

A common task In OO is to extend a method by first executing the parent method code and then adding code. Use the Inherited keyword to invoke the parent class method.

method Robot.Speak;
begin
  MessageBox.Show("Robot says hi");
end;
  
method Cyborg.Speak;
begin
  inherited;
  MessageBox.Show("Cyborg says hi");
end;

More Info

Code:  Delphi Overriding (virtual, override)
Definition:  Method Overriding

Delphi Prism:   virtual, override

Same as Delphi. In Prism, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword. Call Inherited in the descendant method to execute the code in the parent method.

Use final to prevent further extending of a member and Sealed to prevent all members of a class from further extension.

Syntax Example:
Robot = class(System.Object)
public
  method Speak; virtual;
end;
  
Cyborg = class(Robot)
public
  method Speak; override;
end;

Override Details

  • You cannot override a regular non-virtual method, nor a static method.
  • The first version of the parent method must be virtual or abstract.
  • You can override any parent method marked virtual, abstract, or override (already overridden).
  • The methods must have the same signature.
  • The methods must have the same visibility (the same access level).
  • Use the Inherited keyword to refer to cal the parent class method.

Working Prism Override Example

The following code assumes a Windows application with a single form with a button. It demonstrates using virtual and override to override a parent method in a descendant class.

namespace CR_Override;
  
interface
uses
  System.Drawing,
  System.Collections,
  System.Collections.Generic,
  System.Linq,
  System.Windows.Forms,
  System.ComponentModel;
  
type
  /// <summary>
  /// Summary description for MainForm.
  /// </summary>
  MainForm = partial class(System.Windows.Forms.Form)
  private
    method button1_Click(sender: System.Object; e: System.EventArgs);
  protected
    method Dispose(disposing: Boolean); override;
  public
    constructor;
  end;
  
  Robot = class(System.Object)
  public
    method Speak; virtual;
  end;
  
  Cyborg = class(Robot)
  public
    method Speak; override;
  end;
  
implementation
{$REGION Construction and Disposition}
constructor MainForm;
begin
  InitializeComponent();
end;
  
method MainForm.Dispose(disposing: Boolean);
begin
  if disposing then begin
    if assigned(components) then
      components.Dispose();
  end;
  inherited Dispose(disposing);
end;
{$ENDREGION}

method Robot.Speak;
begin
  MessageBox.Show("Robot says hi");
end;
  
method Cyborg.Speak;
begin
  MessageBox.Show("Cyborg says hi");
end;
  
method MainForm.button1_Click(sender: System.Object; e: System.EventArgs);
begin
  var MyRobot: Robot;
  var MyCyborg: Cyborg;
  
  MyRobot := new Robot;
  MyCyborg := new Cyborg;
  
  MyRobot.Speak;
  MyCyborg.Speak;
end;
end.

Hiding a Method with 'reintroduce'

Use the reintroduce keyword to introduce a new implementation of a parent method (this hides the parent method). You can hide a method without using reintroduce but you will get a compiler warning. Using reintroduce will suppress the warning.

The reintroduce and override modifiers have different meanings. The reintroduce modifier creates a new member with the same name, signature, and visibility and hides the original member. The override modifier extends the implementation for an inherited member and allows you to implement inheritance-based polymorphism.

Avoid Introducing New Members: Sometimes there are clear reasons to introduce a new method with the same name, signature, and visibility of a parent method. In those clear cases, introducing a new member is a powerful feature. However, if you do not have a clear reason, then avoid introducing a new version of a method by naming the new method something unique and appropriate.

//interface section:
Robot = class(System.Object)
public
  method Speak;
end;
  
Cyborg = class(Robot)
public
  method Speak; reintroduce;
end;
  
//implementation section:
method Robot.Speak;
begin
  MessageBox.Show("Robot says hi");
end;
  
method Cyborg.Speak;
begin
  MessageBox.Show("Cyborg says hi");
end;

Calling the Base Class Version

A common task In OO is to extend a method by first executing the parent method code and then adding code. Use the Inherited keyword to invoke the parent class method.

method Robot.Speak;
begin
  MessageBox.Show("Robot says hi");
end;
  
method Cyborg.Speak;
begin
  inherited;
  MessageBox.Show("Cyborg says hi");
end;

 

More Info

Code:  Delphi Prism Overriding (virtual, override)
Definition:  Method Overriding

VB.Net:   Overridable, Overrides

In VB.Net, you specify a virtual method with the Overridable keyword in a parent class and extend (or replace) it in a descendant class using the Overrides keyword.

Use the base keyword in the descendant method to execute the code in the parent method, i.e. base.SomeMethod().

Syntax Example:
Public Class Robot
  Public Overridable Sub Speak()
    MessageBox.Show("Robot says hi")
  End Sub
End Class
  
Public Class Cyborg
  Inherits Robot
  
  Public Overrides Sub Speak()
    MessageBox.Show("hi")
  End Sub
End Class

Override Details

  • You cannot override a regular non-virtual method, nor a static method.
  • The first version of the parent method must be Overridable or MustOverride.
  • You can override any parent method marked Overridable, MustOverride, or Overrides (already overridden).
  • The methods must have the same signature.
  • The methods must have the same visibility (the same access level).
  • Use the base keyword to refer to the parent class as in MyBase.SomeMethod().

Working VB.Net 2008 Override Example

The following code assumes a Windows application with a single form with a button. It demonstrates using Overridable and Overrides to override a parent method in a descendant class.

Public Class Form1
  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim MyRobot As Robot
    Dim MyCyborg As Cyborg
  
    MyRobot = New Robot
    MyCyborg = New Cyborg
  
    MyRobot.Speak()
    MyCyborg.Speak()
  End Sub
End Class
  
Public Class Robot
  Public Overridable Sub Speak()
    MessageBox.Show("Robot says hi")
  End Sub
End Class
  
Public Class Cyborg
  Inherits Robot
  
  Public Overrides Sub Speak()
    MessageBox.Show("hi")
  End Sub
End Class

Hiding a Method with Shadows

Use the Shadows keyword to introduce a new implementation of a parent method (this hides the parent method). You can hide a method without using Shadows but you will get a compiler warning. Using Shadows will suppress the warning.

The Shodaws and Overrides modifiers have different meanings. The Shadows modifier creates a new member with the same name, signature, and visibility and hides the original member. The Overrides modifier extends the implementation for an inherited member and allows you to implement inheritance-based polymorphism.

Avoid Introducing New Members: Sometimes there are clear reasons to use Shadows to introduce a new method with with the same name, signature, and visibility of a parent method. In those clear cases, introducing a new member is a powerful feature. However, if you do not have a clear reason, then avoid introducing a new version of a method by naming the new method something unique and appropriate.

Public Class Robot
  Public Sub Speak()
    MessageBox.Show("Robot says hi")
  End Sub
End Class
  
Public Class Cyborg
  Inherits Robot
  
  Public Shadows Sub Speak()
    MessageBox.Show("hi")
  End Sub
End Class

Calling the Parent Class Version with MyBase

A common task In OO is to extend a method by first executing the parent method code and then adding code. Use the MyBase keyword to refer to the parent class as in MyBase.SomeMethod().

Public Class Robot
  Public Overridable Sub Speak()
    MessageBox.Show("Robot says hi")
  End Sub
End Class
  
Public Class Cyborg
  Inherits Robot
  
  Public Overrides Sub Speak()
    MyBase.Speak()
    MessageBox.Show("hi")
  End Sub
End Class

 

More Info

Definition:  Method Overriding
Code:  VB.Net Overriding (Overridable, Overrides)




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