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Inheritance
 

Introduction to Inheritance
 

Definition
 
Volley Ball Football Basketball Handball Golf
Volley Ball Football Basketball Handball Golf

The primary characteristic of the objects on the above pictures is that they are balls used in different sports. Another characteristic they share is that they are round. On the other hand, although these balls are used in sport, one made for one sport cannot (or should not) be used in another sport (of course, it is not unusual for a footballer to mess with a basketball on a lawn but it is not appropriate). The common characteristics of these objects can be listed in a group like a C# class. The class would appear as:

 
class Ball
{
    TypeOfSport;
    Size;
}

If you were asked to create a class to represent these balls, you may be tempted to implement a general class that defines each ball. This may be a bad idea because, despite their round resemblance, there are many internal differences among these balls. Programming languages like C# provide an alternate solution to this type of situation.

Inheritance consists of creating a class whose primary definition or behavior is based on another class. In other words, inheritance starts by having a class that can provide behavior that other classes can improve on.
 

Practical LearningPractical Learning: Introducing Inheritance

Single Family

  1. Start Microsoft Visual C# 2005 Express Edition and create a new Console Application named RealEstate2
  2. To create a new class, in the Class View, right-click the name of the project, position the mouse on Add and click Class...
  3. Set the Name to Property and press Enter
  4. Change the file as follows:
      
    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RealEstate2
{
    public enum PropertyCondition
    {
        Unknown,
        Excellent,
        Good,
        NeedsRepair,
        BadShape
    }

    public class Property
    {

        private string propNbr;
        private PropertyCondition cond;
        private int beds;
        private float baths;
        private int yr;
        private decimal val;

        public Property()
        {
        }

        public string PropertyNumber
        {
            get { return propNbr; }
            set
            {
                if (propNbr == "")
                    propNbr = "N/A";
                else
                    propNbr = value;
            }
        }

        public PropertyCondition Condition
        {
            get { return cond; }
            set { cond = value; }
        }

        public int Bedrooms
        {
            get
            {
                if (beds <= 1)
                    return 1;
                else
                    return beds;
            }
            set { beds = value; }
        }

        public float Bathrooms
        {
            get { return baths; }
            set { baths = value; }
        }

        public int YearBuilt
        {
            get { return yr; }
            set { yr = value; }
        }

        public decimal Value
        {
            get { return val; }
            set { val = value; }
        }
    }
}
  5. To create a new class, in the Solution Explorer, right-click the name of the project, position the mouse on Add and click Class...
  6. Set the Name to PropertyListing and press Enter
  7. Change the file as follows:
      
    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RealEstate2
{
    public enum PropertyType
    {
        Unknown,
        SingleFamily,
        Townhouse,
        Condominium
    }

    public class PropertyListing
    {
        private Property prop;
        private PropertyType tp;

        public Property ListProperty
        {
            get { return prop; }
            set { prop = value; }
        }

        public PropertyType Type
        {
            get { return tp; }
            set { tp = value; }
        }

        public PropertyListing()
        {
            prop = new Property();
        }

        public void CreateListing()
        {
            Console.WriteLine(" =//= Altair Realty =//=");
            Console.WriteLine("-=- Property Creation -=-");

            Console.WriteLine("\nTypes of Properties");
            Console.WriteLine("1. Single Family");
            Console.WriteLine("2. Townhouse");
            Console.WriteLine("3. Condonium");
            Console.WriteLine("4. Don't Know");
            Console.Write("Enter Type of Property: ");
            int propType = int.Parse(Console.ReadLine());

            Console.Write("\nEnter Property #: ");
            ListProperty.PropertyNumber = Console.ReadLine();
            Console.WriteLine("\nProperties Conditions");
            Console.WriteLine("1. Excellent");
            Console.WriteLine("2. Good (may need minor repair)");
            Console.WriteLine("3. Needs Repair");
            Console.Write("4. In Bad Shape (property needs ");
            Console.WriteLine("major repair or rebuild)");
            Console.Write("Enter Property Condition: ");
            int condition = int.Parse(Console.ReadLine());
            if (condition == 1)
                ListProperty.Condition = PropertyCondition.Excellent;
            else if (condition == 2)
                ListProperty.Condition = PropertyCondition.Good;
            else if (condition == 3)
                ListProperty.Condition = PropertyCondition.NeedsRepair;
            else if (condition == 4)
                ListProperty.Condition = PropertyCondition.BadShape;
            else
                ListProperty.Condition = PropertyCondition.Unknown;

            switch ((PropertyType)propType)
            {
                case PropertyType.SingleFamily:
                    Type = PropertyType.SingleFamily;
                    break;

                case PropertyType.Townhouse:
                    Type = PropertyType.Townhouse;
                    break;

                case PropertyType.Condominium:
                    Type = PropertyType.Condominium;
                    break;

                default:
                    Type = PropertyType.Unknown;
                    break;
            }

            Console.Write("\nHow many bedrooms?  ");
            ListProperty.Bedrooms = int.Parse(Console.ReadLine());
            Console.Write("How many bathrooms? ");
            ListProperty.Bathrooms = float.Parse(Console.ReadLine());
            Console.Write("Year built:         ");
            ListProperty.YearBuilt = int.Parse(Console.ReadLine());
            Console.Write("Property Value:     ");
            ListProperty.Value = decimal.Parse(Console.ReadLine());
        }

        public void ShowProperty()
        {
            Console.WriteLine("==================================");
            Console.WriteLine(" =//=//= Altair Realty =//=//=");
            Console.WriteLine("-=-=-=- Properties Listing -=-=-=-");
            Console.WriteLine("----------------------------------");
            Console.WriteLine("Property #:            {0}",
                ListProperty.PropertyNumber);
            Console.WriteLine("Property Type:         {0}", Type);

            switch (Type)
            {
                case PropertyType.SingleFamily:
                case PropertyType.Townhouse:
                    Type = PropertyType.SingleFamily;
                    break;

                case PropertyType.Condominium:
                    break;
            }

            Console.WriteLine("Condition:             {0}",
                ListProperty.Condition);
            Console.WriteLine("Bedrooms:              {0}",
                ListProperty.Bedrooms);
            Console.WriteLine("Bathrooms:             {0}",
                ListProperty.Bathrooms);
            Console.WriteLine("Year Built:            {0}",
                ListProperty.YearBuilt);
            Console.WriteLine("Market Value:          {0:C}",
                ListProperty.Value);
            Console.WriteLine("==================================");
        }
    }
}
  8. Access the Program.cs file and change it as follows:
      
    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RealEstate2
{
    class Program
    {
        static void Main()
        {
            PropertyListing listing = new PropertyListing();

            listing.CreateListing();
            Console.WriteLine("\n");
            listing.ShowProperty();
            Console.WriteLine();
        }
    }
}
  9. Execute the application and test it. Here is an example:
      
    =//= Altair Realty =//=
-=- Property Creation -=-

Types of Properties
1. Single Family
2. Townhouse
3. Condominium
4. Don't Know
Enter Type of Property: 2

Enter Property #: 66DG8P

Properties Conditions
1. Excellent
2. Good (may need minor repair)
3. Needs Repair
4. In Bad Shape (property needs major repair or rebuild)
Enter Property Condition: 2

How many bedrooms?  2
How many bathrooms? 1.0
Year built:         1994
Property Value:     325880.75

==================================
 =//=//= Altair Realty =//=//=
-=-=-=- Properties Listing -=-=-=-
----------------------------------
Property #:            66DG8P
Property Type:         Townhouse
Condition:             Good
Bedrooms:              2
Bathrooms:             1
Year Built:            1994
Market Value:          $325,880.75
==================================

Press any key to continue . . .
  10. Close the DOS window

Class Derivation

As you may have guessed, in order to implement inheritance, you must first have a class that provides the fundamental definition or behavior you need. There is nothing magical about such a class. It could appear exactly like any of the classes we have used so far. Here is an example:

using System;

public class Circle
{
    private double _radius;

    public double Radius
    {
	get
	{
	    return _radius;
	}
	set
	{
	    if( _radius < 0 )
		_radius = 0.00;
	    else
		_radius = value;
	}
    }

    public double Diameter
    {
	get
	{
	    return Radius * 2;
	}
    }

    public double Circumference
    {
	get
	{
	    return Diameter * 3.14159;
	}
    }
	
    public double Area
    {
	get
	{
	    return Radius * Radius * 3.14159;
	}
    }
}
 Circle 
class Exercise
{
    public static int Main()
    {
	var Round = new Circle();
	Round.Radius = 25.55;

	Console.WriteLine("Circle Characteristics");
	Console.WriteLine("Side:     {0}", Round.Radius);
	Console.WriteLine("Diameter: {0}", Round.Diameter);
	Console.WriteLine("Circumference: {0}", Round.Circumference);
	Console.WriteLine("Area:     {0}", Round.Area);

	return 0;
    }
}

This would produce:

Circle Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     2050.837805975
Press any key to continue

The above class is used to process a circle. It can request or provide a radius. It can also calculate the circumference and the area of a circle. Now, suppose you want to create a class for a sphere. You could start from scratch as we have done so far. On the other hand, since a sphere is primarily a 3-dimensional circle, and if you have a class for a circle already, you can simply create your sphere class that uses the already implemented behavior of a circle class.

Creating a class that is based on another class is also referred to as deriving a class from another. The first class serves as parent or base. The class that is based on another class is also referred to as child or derived. To create a class based on another, you use the following formula:

class NewChild : BaseClass
{
      // Body of the new class
}

In this formula, you start with the class keyword followed by a name from your class. On the right side of the name of your class, you must type the : operator, followed by the name of the class that will serve as parent. Of course, the BaseClass class must have been defined; that is, the compiler must be able to find its definition. Based on the above formula, you can create a sphere class based on the earlier mentioned Circle class as follows:

class Sphere : Circle
{
      // The class is ready
}

After deriving a class, it becomes available and you can use it just as you would any other class. Here is an example: 

using System;

class Circle
{
    private double _radius;

    public double Radius
    {
	get
	{
	    if( _radius < 0 )
		return 0.00;
	    else
		return _radius;
	}
	set
	{
	    _radius = value;
	}
    }

    public double Diameter
    {
	get
	{
	    return Radius * 2;
	}
    }

    public double Circumference
    {
	get
	{
	    return Diameter * 3.14159;
	}
    }

    public double Area
    {
	get
	{
	    return Radius * Radius * 3.14159;
	}
    }
}

class Sphere : Circle
{
}

class Exercise
{
    public static int Main()
    {
	var Round = new Circle();
	Round.Radius = 25.55;

	Console.WriteLine("Circle Characteristics");
	Console.WriteLine("Side:     {0}", Round.Radius);
	Console.WriteLine("Diameter: {0}", Round.Diameter);
	Console.WriteLine("Circumference: {0}", Round.Circumference);
	Console.WriteLine("Area:     {0}", Round.Area);

	var Ball = new Sphere();
	Ball.Radius = 25.55;

	Console.WriteLine("\nSphere Characteristics");
	Console.WriteLine("Side:     {0}", Ball.Radius);
	Console.WriteLine("Diameter: {0}", Ball.Diameter);
	Console.WriteLine("Circumference: {0}", Ball.Circumference);
	Console.WriteLine("Area:     {0}", Ball.Area);

	return 0;
    }
}

This would produce: 

Circle Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     2050.837805975

Sphere Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     2050.837805975
Press any key to continue

When a class is based on another class, all public (we will also introduce another inheritance-oriented keyword for this issue) members of the parent class are made available to the derived class that can use them as easily. While other methods and classes can also use the public members of a class, the difference is that the derived class can call the public members of the parent as if they belonged to the derived class. That is, the child class doesn't have to "qualify" the public members of the parent class when these public members are used in the body of the derived class. This is illustrated in the following program: 

using System;

class Circle
{
    private double _radius;

    public double Radius
    {
	get
	{
	    if( _radius < 0 )
		return 0.00;
	    else
		return _radius;
	}
	set
	{
	    _radius = value;
	}
    }

    public double Diameter
    {
	get
	{
	    return Radius * 2;
	}
    }

    public double Circumference
    {
	get
	{
	    return Diameter * 3.14159;
	}
    }

    public double Area
    {
	get
	{
	    return Radius * Radius * 3.14159;
	}
    }

    public void ShowCharacteristics()
    {
	Console.WriteLine("Circle Characteristics");
	Console.WriteLine("Side:     {0}", Radius);
	Console.WriteLine("Diameter: {0}", Diameter);
	Console.WriteLine("Circumference: {0}", Circumference);
	Console.WriteLine("Area:     {0}", Area);
    }
}

class Sphere : Circle
{
    public void ShowCharacteristics()
    {
	// Because Sphere is based on Circle, you can access
	// any public member(s) of Circle without qualifying it(them)
	Console.WriteLine("\nSphere Characteristics");
	Console.WriteLine("Side:     {0}", Radius);
	Console.WriteLine("Diameter: {0}", Diameter);
	Console.WriteLine("Circumference: {0}", Circumference);
	Console.WriteLine("Area:     {0}\n", Area);
    }
}

class Exercise
{
    public static int Main()
    {
	var Round = new Circle();

	Round.Radius = 25.55;
	Round.ShowCharacteristics();

	var Ball = new Sphere();

	Ball.Radius = 25.55;
	Ball.ShowCharacteristics();

	return 0;
    }
}

This would produce the same result.

 

Practical Learning Practical Learning: Inheriting
  1. On the main menu, click Project -> Add -> Class...
  2. Set the Name to HouseType and press Enter
  3. To derive a class, change the file as follows:
      
    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RealEstate2
{
    public class HouseType : Property
    {
        private int nbrOfStories;
        private bool basement;
        private bool garage;

        public int Stories
        {
            get { return nbrOfStories; }
            set { nbrOfStories = value; }
        }

        public bool FinishedBasement
        {
            get { return basement; }
            set { basement = value; }
        }

        public bool IndoorGarage
        {
            get { return garage; }
            set { garage = value; }
        }
    }
}
  4. On the main menu, click Project -> Add -> Class...
  5. Set the Name to Condominium and press Enter
  6. To create another class based on the Property class, change the file as follows:
      
    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RealEstate2
{
    public class Condominium : Property
    {
        private bool handicap;

        public bool HandicapAccessible
        {
            get { return handicap; }
            set { handicap = value; }
        }
    }
}
  7. Access the PropertyListing.cs file file and change it as follows:
      
    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace RealEstate2
{
    public enum PropertyType
    {
        Unknown,
        SingleFamily,
        Townhouse,
        Condominium
    }

    class PropertyListing
    {
        private Property prop;
        private HouseType hse;
        private Condominium cond;
        private PropertyType tp;

        public Property ListProperty
        {
            get { return prop; }
            set { prop = value; }
        }

        public HouseType House
        {
            get { return hse; }
            set { hse = value; }
        }

        public Condominium Condo
        {
            get { return cond; }
            set { cond = value; }
        }
        
        public PropertyType Type
        {
            get { return tp; }
            set { tp = value; }
        }

        public PropertyListing()
        {
            prop = new Property();
            hse = new HouseType();
            cond = new Condominium();
        }

        public void CreateListing()
        {
            char answer;

            Console.WriteLine(" =//= Altair Realty =//=");
            Console.WriteLine("-=- Property Creation -=-");
            Console.WriteLine("\nTypes of Properties");
            Console.WriteLine("1. Single Family");
            Console.WriteLine("2. Townhouse");
            Console.WriteLine("3. Condonium");
            Console.WriteLine("4. Don't Know");
            Console.Write("Enter Type of Property: ");
            int propType = int.Parse(Console.ReadLine());
            
            Console.Write("\nEnter Property #: ");
            ListProperty.PropertyNumber = Console.ReadLine();
            Console.WriteLine("\nProperties Conditions");
            Console.WriteLine("1. Excellent");
            Console.WriteLine("2. Good (may need minor repair)");
            Console.WriteLine("3. Needs Repair");
            Console.Write("4. In Bad Shape (property needs ");
            Console.WriteLine("major repair or rebuild)");
            Console.Write("Enter Property Condition: ");
            int condition = int.Parse(Console.ReadLine());
            if (condition == 1)
                ListProperty.Condition = PropertyCondition.Excellent;
            else if (condition == 2)
                ListProperty.Condition = PropertyCondition.Good;
            else if (condition == 3)
                ListProperty.Condition = PropertyCondition.NeedsRepair;
            else if (condition == 4)
                ListProperty.Condition = PropertyCondition.BadShape;
            else
                ListProperty.Condition = PropertyCondition.Unknown;

            switch ((PropertyType)propType)
            {
                case PropertyType.SingleFamily:
                    Type = PropertyType.SingleFamily;
                    Console.Write("\nHow many stories (levels)? ");
                    House.Stories = int.Parse(Console.ReadLine());
                Console.Write("Does it have an indoor car garage (y/n): ");
                    answer = char.Parse(Console.ReadLine());
                    if ((answer == 'y') || (answer == 'Y'))
                        House.IndoorGarage = true;
                    else
                        House.IndoorGarage = false;
                    Console.Write("Is the basement finished(y/n): ");
                    answer = char.Parse(Console.ReadLine());
                    if ((answer == 'y') || (answer == 'Y'))
                        House.FinishedBasement = true;
                    else
                        House.FinishedBasement = false;
                    break;

                case PropertyType.Townhouse:
                    Type = PropertyType.Townhouse;
                    Console.Write("\nHow many stories (levels)? ");
                    House.Stories = int.Parse(Console.ReadLine());
                Console.Write("Does it have an indoor car garage (y/n): ");
                    answer = char.Parse(Console.ReadLine());
                    if ((answer == 'y') || (answer == 'Y'))
                        House.IndoorGarage = true;
                    else
                        House.IndoorGarage = false;
                    Console.Write("Is the basement finished(y/n): ");
                    answer = char.Parse(Console.ReadLine());
                    if ((answer == 'y') || (answer == 'Y'))
                        House.FinishedBasement = true;
                    else
                        House.FinishedBasement = false;
                    break;

                case PropertyType.Condominium:
                    Type = PropertyType.Condominium;
      Console.Write("\nIs the building accessible to handicapped (y/n): ");
                    answer = char.Parse(Console.ReadLine());
                    if ((answer == 'y') || (answer == 'Y'))
                        Condo.HandicapAccessible = true;
                    else
                        Condo.HandicapAccessible = false;
                    break;

                default:
                    Type = PropertyType.Unknown;
                    break;
            }

            Console.Write("\nHow many bedrooms?  ");
            ListProperty.Bedrooms = int.Parse(Console.ReadLine());
            Console.Write("How many bathrooms? ");
            ListProperty.Bathrooms = float.Parse(Console.ReadLine());
            Console.Write("Year built:         ");
            ListProperty.YearBuilt = int.Parse(Console.ReadLine());
            Console.Write("Property Value:     ");
            ListProperty.Value = decimal.Parse(Console.ReadLine());
        }

        public void ShowProperty()
        {
            Console.WriteLine("==================================");
            Console.WriteLine(" =//=//= Altair Realty =//=//=");
            Console.WriteLine("-=-=-=- Properties Listing -=-=-=-");
            Console.WriteLine("----------------------------------");
            Console.WriteLine("Property #:            {0}",
                ListProperty.PropertyNumber);
            Console.WriteLine("Property Type:         {0}", Type);

            switch(Type)
            {
                case PropertyType.SingleFamily:
                case PropertyType.Townhouse:
                    Type = PropertyType.SingleFamily;
                    Console.WriteLine("Stories:               {0}",
                        House.Stories);
                    Console.WriteLine("Has Indoor Car Garage: {0}",
                        House.IndoorGarage);
                    Console.WriteLine("Finished Basement:     {0}",
                        House.FinishedBasement);
                    break;

                case PropertyType.Condominium:
                    Console.WriteLine("Handicapped Accessible Building: {0}",
                        Condo.HandicapAccessible);
                    break;
            }

    Console.WriteLine("Condition:             {0}", ListProperty.Condition);
    Console.WriteLine("Bedrooms:              {0}", ListProperty.Bedrooms);
    Console.WriteLine("Bathrooms:             {0}", ListProperty.Bathrooms);
    Console.WriteLine("Year Built:            {0}", ListProperty.YearBuilt);
    Console.WriteLine("Market Value:          {0:C}", ListProperty.Value);
        }
    }
}
  8. Execute the application and test it. Here is an example:
      
    =//= Altair Realty =//=
-=- Property Creation -=-

Types of Properties
1. Single Family
2. Townhouse
3. Condonium
4. Don't Know
Enter Type of Property: 1

Enter Property #: 4LP804

Properties Conditions
1. Excellent
2. Good (may need minor repair)
3. Needs Repair
4. In Bad Shape (property needs major repair or rebuild)
Enter Property Condition: 3

How many stories (levels)? 3
Does it have an indoor car garage (y/n): y
Is the basement finished(y/n): n

How many bedrooms?  4
How many bathrooms? 2.5
Year built:         1996
Property Value:     640885.80


==================================
 =//=//= Altair Realty =//=//=
-=-=-=- Properties Listing -=-=-=-
----------------------------------
Property #:            4LP804
Property Type:         SingleFamily
Stories:               3
Has Indoor Car Garage: True
Finished Basement:     False
Condition:             NeedsRepair
Bedrooms:              4
Bathrooms:             2.5
Year Built:            1996
Market Value:          $640,885.80

Press any key to continue . . .
  9. Close the DOS window and return to your programming environment

Single Family

Implementation of Derived Members 

You can notice in the above example that the derived class produces the same results as the base class. In reality, inheritance is used to solve various Object-Oriented Programming (OOP) problems. One of them consists of customizing, adapting, or improving the behavior of a feature (property or method, etc) of the parent class. For example, although both the circle and the sphere have an area, their areas are not the same. A circle is a flat surface but a sphere is a volume, which makes its area very much higher. Since they use different formulas for their respective area, you should implement a new version of the area in the sphere. Based on this, when deriving your class from another class, you should be aware of the properties and methods of the base class so that, if you know that the parent class has a certain behavior or a characteristic that is not conform to the new derived class, you can do something about that. A new version of the area in the sphere can be calculated as follows:

using System;

class Circle
{
    private double _radius;

    public double Radius
    {
	get
	{
	    if( _radius < 0 )
		return 0.00;
	    else
		return _radius;
	}
	set
	{
	    _radius = value;
	}
    }

    public double Diameter
    {
	get
	{
	    return Radius * 2;
	}
    }

    public double Circumference
    {
	get
	{
	    return Diameter * 3.14159;
	}
    }

    public double Area
    {
	get
	{
	    return Radius * Radius * 3.14159;
	}
    }
}

class Sphere : Circle
{
    public double  Area
    {
	get
	{
	    return 4 * Radius * Radius * 3.14159;
	}
    }
}

class Exercise
{
    public static int Main()
    {
	var Round = new Circle();
	Round.Radius = 25.55;

	Console.WriteLine("Circle Characteristics");
	Console.WriteLine("Side:     {0}", Round.Radius);
	Console.WriteLine("Diameter: {0}", Round.Diameter);
	Console.WriteLine("Circumference: {0}", Round.Circumference);
	Console.WriteLine("Area:     {0}", Round.Area);

	var Ball = new Sphere();
	Ball.Radius = 25.55;

	Console.WriteLine("\nSphere Characteristics");
	Console.WriteLine("Side:     {0}", Ball.Radius);
	Console.WriteLine("Diameter: {0}", Ball.Diameter);
	Console.WriteLine("Circumference: {0}", Ball.Circumference);
	Console.WriteLine("Area:     {0}", Ball.Area);

   	return 0;
    }
}

This would produce:

Circle Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     2050.837805975

Sphere Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     8203.3512239

Press any key to continue

Notice that, this time, the areas of both figures are not the same even though their radii are similar.

Besides customizing member variables and methods of a parent class, you can add new members as you wish. This is another valuable feature of inheritance. In our example, while the is a flat shape, a sphere has a volume. In this case, you may need to calculate the volume of a sphere as a new method or property of the derived class. Here is an example:

using System;

class Circle
{
    private double _radius;

    public double Radius
    {
        get
        {
            if (_radius < 0)
                return 0.00;
            else
                return _radius;
        }
        set
        {
            _radius = value;
        }
    }
    public double Diameter
    {
        get
        {
            return Radius * 2;
        }
    }
    public double Circumference
    {
        get
        {
            return Diameter * 3.14159;
        }
    }
    public double Area
    {
        get
        {
            return Radius * Radius * 3.14159;
        }
    }
}

class Sphere : Circle
{
    public double Area
    {
        get
        {
            return 4 * Radius * Radius * 3.14159;
        }
    }

    public double Volume
    {
        get
        {
            return 4 * 3.14159 * Radius * Radius * Radius / 3;
        }
    }
}

class Exercise
{
    public static int Main()
    {
        var Round = new Circle();
        Round.Radius = 25.55;

        Console.WriteLine("Circle Characteristics");
        Console.WriteLine("Side:     {0}", Round.Radius);
        Console.WriteLine("Diameter: {0}", Round.Diameter);
        Console.WriteLine("Circumference: {0}", Round.Circumference);
        Console.WriteLine("Area:     {0}", Round.Area);

        var Ball = new Sphere();
        Ball.Radius = 25.55;

        Console.WriteLine("\nSphere Characteristics");
        Console.WriteLine("Side:     {0}", Ball.Radius);
        Console.WriteLine("Diameter: {0}", Ball.Diameter);
        Console.WriteLine("Circumference: {0}", Ball.Circumference);
        Console.WriteLine("Area:     {0}", Ball.Area);
        Console.WriteLine("Volume: {0}\n", Ball.Volume);

        return 0;
    }
}

This would produce:

Circle Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     2050.837805975

Sphere Characteristics
Side:     25.55
Diameter: 51.1
Circumference: 160.535249
Area:     8203.3512239
Volume:   209595.623770645

Press any key to continue

If you create a property or method in a derived class and that property or method already exists in the parent class, when you access the property or method in the derived class, you must make sure you indicate what member you are accessing. To make this possible, the C# language provides the base keyword. To access a property or method of a parent class from the derived class, type the base keyword, followed by the period operator, followed by the name of the property or method of the base class.

The new Modifier

Imagine you create a class used to process a circle as we saw earlier. You can use this as the base class for a sphere. Both the circle and the sphere have an area but their values are different. This means that, as mentioned in our introduction to inheritance, when deriving the sphere class, you would have to calculate a new area for the cube.

If you create or declare a new member in a derived class and that member has the same name as a member of the base class, when creating the new member, you may want to indicate to the compiler that, instead of overriding that same method that was defined in the base class, you want to create a brand new and independent version of that method. When doing this, you would be asking the compiler to hide the member of the base class that has the same name, when the member of the current class is invoked. To do this, type the new keyword to its left. Here is an example:

using System;

class Circle
{
    private double _radius;

    public double Radius
    {
	get { return (_radius < 0) ? 0.00 : _radius; }
	set	{ _radius = value; }
    }

    public double Diameter
    {
	get	{ return Radius * 2; }
    }

    public double Circumference
    {
	get	{ return Diameter * 3.14159; }
    }

    public double Area
    {
	get	{ return Radius * Radius * 3.14159; }
    }
}

class Sphere : Circle
{
    new public double Area
    {
	get	{ return 4 * Radius * Radius * 3.14159; }
    }
	
    public double Volume
    {
	get	{ return 4 * 3.14159 * Radius * Radius * Radius / 3; }
    }
}

class Exercise
{
    public static int Main()
    {
	var Round = new Circle();
	Round.Radius = 25.55;

	Console.WriteLine("Circle Characteristics");
	Console.WriteLine("Side:     {0}", Round.Radius);
	Console.WriteLine("Diameter: {0}", Round.Diameter);
	Console.WriteLine("Circumference: {0}", Round.Circumference);
	Console.WriteLine("Area:     {0}", Round.Area);

	var Ball = new Sphere();
	Ball.Radius = 25.55;

	Console.WriteLine("\nSphere Characteristics");
	Console.WriteLine("Side:     {0}", Ball.Radius);
	Console.WriteLine("Diameter: {0}", Ball.Diameter);
	Console.WriteLine("Circumference: {0}", Ball.Circumference);
	Console.WriteLine("Area:     {0}", Ball.Area);
	Console.WriteLine("Volume: {0}\n", Ball.Volume);

	return 0;
    }
}

 
 
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