VB.NET Language - Lesson 11: Inheritance

	Inheritance and the .NET Framework

Object: The Ancestor to all Classes

The .NET Framework is the main library used by the Microsoft Visual Basic .NET programming language. When VBasic was developed, inheritance was kept in mind art all levels so that the .NET Framework library provides as much functionality as possible to enhance all types of applications necessary. To provide this functionality, the .NET Framework provides a rich set of classes (and namespaces as we will see).

At the highest level, the library provides the Object class that serves as the common ancestor to all classes used in the .NET Framework. In fact, any time you create a class to use in your VBasic application, the class is automatically derived from Object. Consider the following Square class:

Imports System

Imports System

 Public Class Square
    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Shared Sub Main()

        Dim sqr As Square = New Square

        Console.Write("Enter Side: ")
        sqr.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine()
        Console.WriteLine("Square Characteristics")
        Console.WriteLine("Side:      {0:F}", sqr.Side)
        Console.WriteLine("Perimeter: {0:F}", sqr.CalculatePerimeter())
        Console.WriteLine("Area:      {0:F}", sqr.CalculateArea())
        Console.WriteLine()
    End Sub

End Class

Here is an example of running the program:

Enter Side: 42.48

Square Characteristics
Side:      42.48
Perimeter: 169.92
Area:      1804.55

Although the Square class doesn't indicate that it is inheriting from any class, by virtue of belonging to a VBasic application, it inherits from Object. For this reason, the above code could also have been written as follows:

Imports System

Public Class Square
    Inherits Object

    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Shared Sub Main()

        Dim sqr As Square = New Square

        Console.Write("Enter Side: ")
        sqr.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine()
        Console.WriteLine("Square Characteristics")
        Console.WriteLine("Side:          {0:F}", sqr.Side)
        Console.WriteLine("Perimeter: {0:F}", sqr.CalculatePerimeter())
        Console.WriteLine("Area:         {0:F}", sqr.CalculateArea())
        Console.WriteLine()
    End Sub

End Class

This would produce the same results. Most of the time, if not always, you don't need to derive a class from Object: this inheritance is automatic and it is implied. The obvious question would be, "What does Object offer to its descendant?". By itself, the Object class provides little but useful functionality to its children. In fact it is equipped with only half a dozen methods. Still, this little functionality allows the various child class to interact easily or perform some operations that would require casting.

All of the methods of the Object class are public, making them directly available to the descendant classes. Most are "Overridable"s, meaning if you want to use them, you should implement your own version in your class.

String Conversion

One of the functionalities provided by the Object class consists of converting a class to a string. Because this can mean different things to different classes. The Object class provides the method named ToString. Its syntax is:

Public Overridable Function ToString() As String

In some cases, you can directly call this method as it is available to your class already. Here is an example:

Imports System

Public Class Square

    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Shared Sub Main()

        Dim sqr As Square = New Square

        sqr.ToString()
        
    End Sub

End Class

Otherwise, most of the time, you will need to indicate to the compiler how this method should be interpreted by your class, which is done by overriding it. To override this method, follow the rules of overriding a method by associating the Overrides keyword with the syntax of the method. In the body of the method, implement it as you see fit. Here is an example:

Imports System

Public Class Square

    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Public Overrides Function ToString() As String
        Console.WriteLine("Square Characteristics")
        Console.WriteLine("Side:      {0:F}", Side)
        Console.WriteLine("Perimeter: {0:F}", CalculatePerimeter())
        Console.WriteLine("Area:      {0:F}", CalculateArea())
    End Function

    Shared Sub Main()

        Dim sqr As Square = New Square

        Console.Write("Enter Side: ")
        sqr.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine()
        sqr.ToString()
        Console.WriteLine()
    End Sub

End Class

Because the Object.ToString() method returns a String object, you can assign its result to a string or pass it to a function or method that takes a string as argument. Here is an example:

Imports System

Public Class Square

    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Public Overrides Function ToString() As String
        Console.WriteLine("Square Characteristics")
        Console.WriteLine("Side:      {0:F}", Side)
        Console.WriteLine("Perimeter: {0:F}", CalculatePerimeter())
        Console.WriteLine("Area:      {0:F}", CalculateArea())
    End Function

    Shared Sub Main()

        Dim sqr As Square = New Square

        Console.Write("Enter Side: ")
        sqr.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine("{0}", sqr.ToString())

    End Sub

End Class

Object and Classes Comparisons

Another valuable method of the Object class is called Equals. This method is used to compare two instances of a class for equality. This method is overloaded with two versions and each returns a Boolean value.

One of the versions of the Object.Equals() method has the following syntax:

Overloads Public Overridable Function Equals(ByVal obj As Object) As Boolean

This method can be called by any class of a .NET Framework application and it takes as argument an instance of the class that the called needs to be compared to. Here is an example:

Imports System

Class Exercise

    Public Shared Sub Main()
        Dim number1 As Integer, number2 As Integer

        number1 = 248
        number2 = 2480
        Console.WriteLine("{0} = {1}: {2}", number1, number2, _
                                      number1.Equals(number2))
    End Sub

End Module

This would produce:

248 = 2480: False

The second version of the Object.Equals() method has the following syntax:

Overloads Public Shared Function Equals(ByVal objA As Object,_
					ByVal objB As Object) As Boolean

This version is declared as Shared. This means that it is not called by a specific instance of a class. Instead, it takes two arguments that each represents an instance of the classes that need to be compared. Here is an example of calling it:

Imports System

Class Exercise

    Public Shared Sub Main()
        Dim Country As String, Pais As String

        Country = "Senegal"
        Pais = "Senegal"
        Console.WriteLine("{0} = {1}: {2}", Country, Pais, _
                                      Equals(Country, Pais))
    End Sub

End Class

This would produce:

Senegal = Senegal: True

Although this method is made available to all .NET classes by through inheritance from the Object class, in most cases, to make sure it rightly behaves, you should customize its implementation in most of your classes where you intend to call it. Consider the following program:

Imports System

Public Class Square

    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Public Overrides Function ToString() As String
        Console.WriteLine("Square Characteristics")
        Console.WriteLine("Side:      {0:F}", Side)
        Console.WriteLine("Perimeter: {0:F}", CalculatePerimeter())
        Console.WriteLine("Area:      {0:F}", CalculateArea())
    End Function

    Shared Sub Main()

        Dim sqr1 As Square = New Square
        Dim sqr2 As Square = New Square

        Console.WriteLine(" =+= First Square =+=")
        Console.Write("Enter Side: ")
        sqr1.Side = Double.Parse(Console.ReadLine())
        Console.WriteLine(" =+= Second Square =+=")
        Console.Write("Enter Side: ")
        sqr2.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine()
        Console.WriteLine("{0}", sqr1.ToString())
        Console.WriteLine("{0}", sqr2.ToString())

        Console.WriteLine("Squares Equality: {0}", sqr1.Equals(sqr2))
    End Sub

End Class

Here is an example of executing it:

=+= First Square =+=
Enter Side: 125.84
 =+= Second Square =+=
Enter Side: 125.84

Square Characteristics
Side:      125.84
Perimeter: 503.36
Area:      15835.71

Square Characteristics
Side:      125.84
Perimeter: 503.36
Area:      15835.71

Squares Equality: False

Notice that, although both square instances have the same Side value and produce the same area, the compiler renders them not equal. This is an indication that the compiler doesn't know how to compare two instances of the Square class. The solution to this type of problem is to override the Equals() method in your class instead of relying on the default implementation from the Object class. Here are two overrides of the Equals() methods as overridden for the above Square class:

Imports System

Public Class Square

    Public Side As Double

    Function CalculatePerimeter() As Double
        Return Side * 4
    End Function

    Function CalculateArea() As Double
        Return Side * Side
    End Function

    Public Overrides Function ToString() As String
        Console.WriteLine("Square Characteristics")
        Console.WriteLine("Side:      {0:F}", Side)
        Console.WriteLine("Perimeter: {0:F}", CalculatePerimeter())
        Console.WriteLine("Area:      {0:F}", CalculateArea())
    End Function

    Public Overridable Overloads Function Equals(ByVal sqr As Square) As Boolean
        ' We will only compare the side of the square
        ' because the calculations of the perimeter and the area
        ' directly depend on the side
        ' If the side of the square passed as argument is equal
        ' to the side of this object, both objects are equal
        If sqr.Side = Me.Side Then Return True
        ' If the sides are not equal, then the objects are not equal
        Return False
    End Function

    Public Overloads Shared Function Equals(ByVal first As Square, _
                                            ByVal second As Square) As Boolean
        ' We will only compare the side of the square
        ' If the side of the first square is equal
        ' to the side of the second one, then both squares are equal
        If first.Side = second.Side Then Return True
        ' If the sides are not equal, then the objects are not equal
        Return False
    End Function

    Shared Sub Main()

        Dim sqr1 As Square = New Square
        Dim sqr2 As Square = New Square

        Console.WriteLine(" =+= First Square =+=")
        Console.Write("Enter Side: ")
        sqr1.Side = Double.Parse(Console.ReadLine())
        Console.WriteLine(" =+= Second Square =+=")
        Console.Write("Enter Side: ")
        sqr2.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine()
        Console.WriteLine("{0}", sqr1.ToString())
        Console.WriteLine("{0}", sqr2.ToString())

        Console.WriteLine("Squares Equality: {0}", sqr1.Equals(sqr2))

        Console.WriteLine()
        Console.WriteLine(" =+= First Square =+=")
        Console.Write("Enter Side: ")
        sqr1.Side = Double.Parse(Console.ReadLine())
        Console.WriteLine(" =+= Second Square =+=")
        Console.Write("Enter Side: ")
        sqr2.Side = Double.Parse(Console.ReadLine())

        Console.WriteLine()
        Console.WriteLine("{0}", sqr1.ToString())
        Console.WriteLine("{0}", sqr2.ToString())

        Console.WriteLine("Squares Equality: {0}", Equals(sqr1, sqr2))
    End Sub

End Class

Here is an example of testing the program:

=+= First Square =+=
Enter Side: 125.84
 =+= Second Square =+=
Enter Side: 125.84

Square Characteristics
Side:      125.84
Perimeter: 503.36
Area:      15835.71

Square Characteristics
Side:      125.84
Perimeter: 503.36
Area:      15835.71

Squares Equality: True

 =+= First Square =+=
Enter Side: 38.45
 =+= Second Square =+=
Enter Side: 16.82

Square Characteristics
Side:      38.45
Perimeter: 153.80
Area:      1478.40

Square Characteristics
Side:      16.82
Perimeter: 67.28
Area:      282.91

Squares Equality: False

Here is another run of the same program:

=+= First Square =+=
Enter Side: 70.68
 =+= Second Square =+=
Enter Side: 42.04

Square Characteristics
Side:      70.68
Perimeter: 282.72
Area:      4995.66

Square Characteristics
Side:      42.04
Perimeter: 168.16
Area:      1767.36

Squares Equality: False

 =+= First Square =+=
Enter Side: 58.26
 =+= Second Square =+=
Enter Side: 58.26

Square Characteristics
Side:      58.26
Perimeter: 233.04
Area:      3394.23

Square Characteristics
Side:      58.26
Perimeter: 233.04
Area:      3394.23

Squares Equality: True

Notice that, this time, the compiler knows how to perform the comparison of two Square objects using either version of the Equals() method.


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