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==Class hierarchy: override and virtual==

<source lang="csharp">
/*
* C# Programmers Pocket Consultant
* Author: Gregory S. MacBeth
* Email: gmacbeth@comporium.net
* Create Date: June 27, 2003
* Last Modified Date:
*/
using System;
namespace Client.Chapter_5___Building_Your_Own_Classes
{
public class MyMainClass13
{
static void Main(string[] args)
{
//The function called is based
//upon the type called by new.
B MyB = new C();
MyB.Display(); //Calls C"s Display
}
}
abstract class A
{
public abstract void Display();
}
class B: A
{
public override void Display()
{
Console.WriteLine("Class B"s Display Method");
}
}
class C: B
{
public override void Display()
{
Console.WriteLine("Class C"s Display Method");
}
}
class D: C
{
public override void Display()
{
Console.WriteLine("Class D"s Display Method");
}
}
}

</source>

==Demonstrate a virtual method==

<source lang="csharp">
/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/

// Demonstrate a virtual method.

using System;

class Base {
// Create virtual method in the base class.
public virtual void who() {
Console.WriteLine("who() in Base");
}
}

class Derived1 : Base {
// Override who() in a derived class.
public override void who() {
Console.WriteLine("who() in Derived1");
}
}

class Derived2 : Base {
// Override who() again in another derived class.
public override void who() {
Console.WriteLine("who() in Derived2");
}
}

public class OverrideDemo {
public static void Main() {
Base baseOb = new Base();
Derived1 dOb1 = new Derived1();
Derived2 dOb2 = new Derived2();

Base baseRef; // a base-class reference

baseRef = baseOb;
baseRef.who();

baseRef = dOb1;
baseRef.who();

baseRef = dOb2;
baseRef.who();
}
}

</source>

==Demonstrates the use of a virtual method to override a base class method==

<source lang="csharp">
/*
C# Programming Tips & Techniques
by Charles Wright, Kris Jamsa
Publisher: Osborne/McGraw-Hill (December 28, 2001)
ISBN: 0072193794
*/
//
// Virtual.cs -- Demonstrates the use of a virtual method to override
// a base class method.
//
// Compile this program with the following command line:
// C:>csc Virtual.cs
namespace nsVirtual
{
using System;

public class VirtualclsMain
{
static public void Main ()
{
clsBase Base = new clsBase();
clsFirst First = new clsFirst();
clsSecond Second = new clsSecond();
Base.Show();
First.Show();
Second.Show ();
}
}
class clsBase
{
public void Show ()
{
Describe ();
}
virtual protected void Describe ()
{
Console.WriteLine ("Called the base class Describe() method");
}
}
class clsFirst : clsBase
{
override protected void Describe ()
{
Console.WriteLine ("Called the derived class Describe() method");
}
}
class clsSecond : clsBase
{
}
}

</source>

==Demonstrates the use of a virtual property to override a base class property==

<source lang="csharp">
/*
C# Programming Tips & Techniques
by Charles Wright, Kris Jamsa
Publisher: Osborne/McGraw-Hill (December 28, 2001)
ISBN: 0072193794
*/
//
// VProp.cs -- Demonstrates the use of a virtual method to override
// a base class method.
//
// Compile this program with the following command line:
// C:>csc VProp.cs
namespace nsVirtual
{
using System;

public class VPropclsMain
{
static public void Main ()
{
clsBase Base = new clsBase();
clsFirst First = new clsFirst();
Base.SetString ("This should set the base class property");
First.SetString ("This should set the derived class property");
Console.WriteLine();
Console.WriteLine (Base.GetString());
Console.WriteLine (First.GetString());
}
}
class clsBase
{
public void SetString (string str)
{
StrProp = str;
}
public string GetString ()
{
return (StrProp);
}
virtual protected string StrProp
{
get
{
Console.WriteLine ("Getting Base string");
return (m_BaseString);
}
set
{
Console.WriteLine ("Setting Base string");
m_BaseString = value;
}
}
private string m_BaseString = "";
}
class clsFirst : clsBase
{
override protected string StrProp
{
get
{
Console.WriteLine ("Getting derived string");
return (m_DerivedString);
}
set
{
Console.WriteLine ("Setting derived string");
m_DerivedString = value;
}
}
private string m_DerivedString = "";
}
}

</source>

==illustrates polymorphism==

<source lang="csharp">
/*
Mastering Visual C# .NET
by Jason Price, Mike Gunderloy
Publisher: Sybex;
ISBN: 0782129110
*/
/*
Example7_2.cs illustrates polymorphism
*/
using System;

// declare the MotorVehicle class
class MotorVehicle
{
// declare the fields
public string make;
public string model;
// define a constructor
public MotorVehicle(string make, string model)
{
this.make = make;
this.model = model;
}
// define the Accelerate() method (may be overridden in a
// derived class)
public virtual void Accelerate()
{
Console.WriteLine(model + " accelerating");
}
}

// declare the Car class (derived from MotorVehicle)
class Car : MotorVehicle
{
// define a constructor
public Car(string make, string model) :
base(make, model)
{
// do nothing
}
// override the base class Accelerate() method
public override void Accelerate()
{
Console.WriteLine("Pushing gas pedal of " + model);
base.Accelerate(); // calls the Accelerate() method in the base class
}
}

// declare the Motorcycle class (derived from MotorVehicle)
class Motorcycle : MotorVehicle
{
// define a constructor
public Motorcycle(string make, string model) :
base(make, model)
{
// do nothing
}
// override the base class Accelerate() method
public override void Accelerate()
{
Console.WriteLine("Twisting throttle of " + model);
base.Accelerate(); // calls the Accelerate() method in the base class
}
}

public class Example7_2
{
public static void Main()
{
// create a Car object and call the object"s Accelerate() method
Car myCar = new Car("Toyota", "MR2");
myCar.Accelerate();
// create a Motorcycle object and call the object"s Accelerate() method
Motorcycle myMotorcycle = new Motorcycle("Harley-Davidson", "V-Rod");
myMotorcycle.Accelerate();
}
}

</source>

==Method override 3==

<source lang="csharp">
/*
Learning C#
by Jesse Liberty
Publisher: O"Reilly
ISBN: 0596003765
*/
using System;
class Dog
{
private int weight;
// constructor
public Dog(int weight)
{
this.weight = weight;
}
// override Object.ToString
public override string ToString()
{
return weight.ToString();
}
}

public class TesterOverride
{
static void Main()
{
int i = 5;
Console.WriteLine("The value of i is: {0}", i.ToString());
Dog milo = new Dog(62);
Console.WriteLine("My dog Milo weighs {0} pounds", milo.ToString());
}
}

</source>

==Polymorphism==

<source lang="csharp">

using System;
public class MotorVehicle {
public string make;
public string model;
public MotorVehicle(string make, string model) {
this.make = make;
this.model = model;
}
public virtual void Accelerate() {
Console.WriteLine(model + " accelerating");
}
}
public class Product : MotorVehicle {
public Product(string make, string model) :
base(make, model) {
}
public override void Accelerate() {
Console.WriteLine("Pushing gas pedal of " + model);
base.Accelerate();
}
}
public class Motorcycle : MotorVehicle {
public Motorcycle(string make, string model) :
base(make, model) {
// do nothing
}
public override void Accelerate() {
Console.WriteLine("Twisting throttle of " + model);
base.Accelerate();
}
}

class MainClass {
public static void Main() {
Product myProduct = new Product("Toyota", "MR2");
myProduct.Accelerate();
Motorcycle myMotorcycle =
new Motorcycle("Harley-Davidson", "V-Rod");
myMotorcycle.Accelerate();
}
}
</source>

==Test Polymorphism Virtual Functions==

<source lang="csharp">
/*
A Programmer"s Introduction to C# (Second Edition)
by Eric Gunnerson
Publisher: Apress L.P.
ISBN: 1-893115-62-3
*/
// 01 - Object-Oriented Basics\Polymorphism and Virtual Functions
// copyright 2000 Eric Gunnerson
using System;
public PolymorphismVirtualFunctions
{
public static void CallPlay(MusicServer ms)
{
ms.Play();
}
public static void Main()
{
MusicServer ms = new WinAmpServer();
CallPlay(ms);
ms = new MediaServer();
CallPlay(ms);
}
}
public abstract class MusicServer
{
public abstract void Play();
}
public class WinAmpServer: MusicServer
{
public override void Play()
{
Console.WriteLine("WinAmpServer.Play()");
}
}
public class MediaServer: MusicServer
{
public override void Play()
{
Console.WriteLine("MediaServer.Play()");
}
}

</source>

==Use virtual methods and polymorphism==

<source lang="csharp">
/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/

// Use virtual methods and polymorphism.

using System;

class TwoDShape {
double pri_width; // private
double pri_height; // private
string pri_name; // private

// A default constructor.
public TwoDShape() {
width = height = 0.0;
name = "null";
}

// Parameterized constructor.
public TwoDShape(double w, double h, string n) {
width = w;
height = h;
name = n;
}

// Construct object with equal width and height.
public TwoDShape(double x, string n) {
width = height = x;
name = n;
}

// Construct an object from an object.
public TwoDShape(TwoDShape ob) {
width = ob.width;
height = ob.height;
name = ob.name;
}

// Properties for width, height, and name
public double width {
get { return pri_width; }
set { pri_width = value; }
}

public double height {
get { return pri_height; }
set { pri_height = value; }
}

public string name {
get { return pri_name; }
set { pri_name = value; }
}

public void showDim() {
Console.WriteLine("Width and height are " +
width + " and " + height);
}

public virtual double area() {
Console.WriteLine("area() must be overridden");
return 0.0;
}
}

// A derived class of TwoDShape for triangles.
class Triangle : TwoDShape {
string style; // private

// A default constructor.
public Triangle() {
style = "null";
}

// Constructor for Triangle.
public Triangle(string s, double w, double h) :
base(w, h, "triangle") {
style = s;
}

// Construct an isosceles triangle.
public Triangle(double x) : base(x, "triangle") {
style = "isosceles";
}

// Construct an object from an object.
public Triangle(Triangle ob) : base(ob) {
style = ob.style;
}

// Override area() for Triangle.
public override double area() {
return width * height / 2;
}

// Display a triangle"s style.
public void showStyle() {
Console.WriteLine("Triangle is " + style);
}
}

// A derived class of TwoDShape for rectangles.
class Rectangle : TwoDShape {
// Constructor for Rectangle.
public Rectangle(double w, double h) :
base(w, h, "rectangle"){ }

// Construct a square.
public Rectangle(double x) :
base(x, "rectangle") { }

// Construct an object from an object.
public Rectangle(Rectangle ob) : base(ob) { }

// Return true if the rectangle is square.
public bool isSquare() {
if(width == height) return true;
return false;
}

// Override area() for Rectangle.
public override double area() {
return width * height;
}
}

public class DynShapes {
public static void Main() {
TwoDShape[] shapes = new TwoDShape[5];

shapes[0] = new Triangle("right", 8.0, 12.0);
shapes[1] = new Rectangle(10);
shapes[2] = new Rectangle(10, 4);
shapes[3] = new Triangle(7.0);
shapes[4] = new TwoDShape(10, 20, "generic");

for(int i=0; i < shapes.Length; i++) {
Console.WriteLine("object is " + shapes[i].name);
Console.WriteLine("Area is " + shapes[i].area());

Console.WriteLine();
}
}
}

</source>

==Virtual and overload==

<source lang="csharp">
/*
* C# Programmers Pocket Consultant
* Author: Gregory S. MacBeth
* Email: gmacbeth@comporium.net
* Create Date: June 27, 2003
* Last Modified Date:
*/
using System;
namespace Client.Chapter_5___Building_Your_Own_Classes
{
public class MyMainClass12 {
static void Main(string[] args)
{
//The function called is based
//upon the type called by new.
B MyB = new C();
D MyD = new D();
MyB.Display(); //Calls C Display
MyD.Display(); //Calls D Display
}
}
public interface A
{
void Display();
}
class B: A
{
public virtual void Display()
{
Console.WriteLine("Class B"s Display Method");
}
}
class C: B
{
public override void Display()
{
Console.WriteLine("Class C"s Display Method");
}
}
class D: C
{
public override void Display()
{
Console.WriteLine("Class D"s Display Method");
}
}
}

</source>

==Virtual keyword can be used to start a new inheritance ladder==

<source lang="csharp">
using System;
public class Class1 {
public static void Main(string[] strings) {
BankAccount ba = new BankAccount();
Test1(ba);
SavingsAccount sa = new SavingsAccount();
Test1(sa);
Test2(sa);
SpecialSaleAccount ssa = new SpecialSaleAccount();
Test1(ssa);
Test2(ssa);
Test3(ssa);
SaleSpecialCustomer ssc = new SaleSpecialCustomer();
Test1(ssc);
Test2(ssc);
Test3(ssc);
Test4(ssc);
}
public static void Test1(BankAccount account) {
Console.Write("to Test(BankAccount)");
account.Withdrawal(100);
}
public static void Test2(SavingsAccount account) {
Console.Write("to Test(SavingsAccount)");
account.Withdrawal(100);
}
public static void Test3(SpecialSaleAccount account) {
Console.Write("to Test(SpecialSaleAccount)");
account.Withdrawal(100);
}
public static void Test4(SaleSpecialCustomer account) {
Console.Write("to Test(SaleSpecialCustomer)");
account.Withdrawal(100);
}
}
public class BankAccount {
virtual public void Withdrawal(double dWithdrawal) {
Console.WriteLine(" invokes BankAccount.Withdrawal()");
}
}
public class SavingsAccount : BankAccount {
override public void Withdrawal(double dWithdrawal) {
Console.WriteLine(" invokes SavingsAccount.Withdrawal()");
}
}
public class SpecialSaleAccount : SavingsAccount {
new virtual public void Withdrawal(double dWithdrawal) {
Console.WriteLine(" invokes SpecialSaleAccount.Withdrawal()");
}
}
public class SaleSpecialCustomer : SpecialSaleAccount {
override public void Withdrawal(double dWithdrawal) {
Console.WriteLine
(" invokes SaleSpecialCustomer.Withdrawal()");
}
}
</source>

==When a virtual method is not overridden, the base class method is used==

<source lang="csharp">
/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/

/* When a virtual method is not overridden,
the base class method is used. */

using System;

class Base {
// Create virtual method in the base class.
public virtual void who() {
Console.WriteLine("who() in Base");
}
}

class Derived1 : Base {
// Override who() in a derived class.
public override void who() {
Console.WriteLine("who() in Derived1");
}
}

class Derived2 : Base {
// This class does not override who().
}

public class NoOverrideDemo {
public static void Main() {
Base baseOb = new Base();
Derived1 dOb1 = new Derived1();
Derived2 dOb2 = new Derived2();

Base baseRef; // a base-class reference

baseRef = baseOb;
baseRef.who();

baseRef = dOb1;
baseRef.who();

baseRef = dOb2;
baseRef.who(); // calls Base"s who()
}
}

</source>

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