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modifier for the class’s members. So, Method4 must be either removed or implemented. Let’s delete the declaration of the Method4 member. Since Method4 is no longer a member of the ParentClass, you must also remove the entry in the DerivedClass that overrides it.

MyBase and MyClass

The MyBase and MyClass keywords let you access the members of the base class and the derived class explicitly. To see why they’re useful, edit the ParentClass as shown here:

Public Class ParentClass

Public Overridable Function Method1() As String

Return (Method4())

End Function

Public Overridable Function Method4() As String

Return (“I’m the original Method4”)

End Function

Then override Method4 in the derived class, as shown here:

Public Class DerivedClass

Inherits ParentClass

Overrides Function Method4() As String

Return(“Derived Method4”)

End Function

Switch to the test form, add a button, declare a variable of the derived class, and call its Method4:

Dim objDerived As New DerivedClass()

Console.WriteLine(objDerived.Method4)

What will you see if you execute these statements? Obviously, the string “Derived Method4.” So far, all looks reasonable, and the class behaves intuitively. But what if we add the following method in the derived class?

Public Function newMethod() As String

Return (Method1())

End Function

This method calls Method1 in the ParentClass class, because Method1 is not overridden in the derived class. Method1 in the base class calls Method4. But which Method4 gets invoked? Surprised? It’s the derived Method4! To fix this behavior (assuming you want to call the Method4 of the base class) change the implementation of Method1 to the following:

Public Overridable Function Method1() As String

Return (MyClass.Method4())

End Function

If you run the application again, the statement:

Console.WriteLine(objDerived.newMethod)

will print the string:

I’m the original Method4

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WHO CAN INHERIT WHAT? 387

Is it reasonable for a method of the base class to call the overridden method? It is, because the overridden class is newer than the base class and the compiler tries to use the newest members. If you had other classes inheriting from the DerivedClass class, their members would take precedence.

Use the MyClass keyword to make sure you’re calling a member in the same class, and not an overriding member in an inheriting class. Likewise, you can use the keyword MyBase to call the implementation of a member in the base class, rather than the equivalent member in a derived class.

VB.NET at Work: The Matrix Class

The Matrix project on the CD demonstrates many of the topics discussed in this chapter, plus a few rather advanced techniques, like complicated constructors. The Matrix class exposes the functionality you need to process two-dimensional matrices and can be your starting point for a class that implements advanced matrix operations, as opposed to the simple ones implemented in this example. I realize most readers aren’t interested in math; you can skip this section if that’s you. I will quickly describe the class, its methods, and how to use it, and you can explore the code on your own.

The Matrix class maintains a two-dimensional table of Doubles and the table’s dimensions. The table’s dimensions are exposed as properties—Rows and Cols—and they’re stored in the _rows and _cols local variables. New tables are instantiated through the New constructor. If New is called without arguments, it creates a matrix with a single element. You can also pass the desired dimensions in the New constructor. The implementations of the two overloaded forms of the constructor are shown in Listing 8.43.

Listing 8.43: The Overloaded New() Constructor of a Matrix

Sub New(ByVal R As Integer, ByVal C As Integer) MyBase.new()

_rows = R

_cols = C

ReDim _table(_rows, _cols) End Sub

Sub New() MyBase.new() _rows = 1 _cols = 1

ReDim _table(_rows, _cols) End Sub

If you don’t know the dimensions of a table when you declare it, you can set them later with the Rows and Cols properties, which are implemented as seen in Listing 8.44. Actually, you will see shortly when you may have to declare a matrix without specific dimensions.

Listing 8.44: The Rows and Cols Properties of a Matrix

Public Property Rows() As Integer

Get

Rows = _rows

End Get

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388 Chapter 8 BUILDING CUSTOM CLASSES

Set(ByVal Value As Integer)

_rows = Value

End Set

End Property

Public Property Cols() As Integer

Get

Cols = _cols

End Get

Set(ByVal Value As Integer)

_cols = Value

End Set

End Property

To populate a matrix, call the Cell method (Listing 8.45) to assign a value to a specified cell. The Cell method accepts two arguments, which are the row and column number of the cell that will be set to the specified value.

Listing 8.45: The Cell Method of a Matrix

Public Property Cell(ByVal row As Integer, ByVal col As Integer) As Double Get

Cell = _table(row, col) End Get

Set(ByVal Value As Double) _table(row, col) = Value

End Set

End Property

The following statements create a new matrix and populate it with random integer values in the range 0 to 300:

Dim a As Matrix = New Matrix(3, 4) Dim i, j As Integer

Dim rnd As System.Random = New System.Random() For i = 0 To a.Rows - 1

For j = 0 To a.Cols - 1 a.Cell(i, j) = rnd.Next(300)

Next Next

The most useful methods of the Matrix class are those that perform matrix operations. I’ve implemented only a few matrix operations, but you can easily extend the class by adding new methods. These methods are the Add, Subtract, and Multiply methods, which perform the simpler matrix operations. All three methods accept either one or two Matrix objects as arguments, and they return the result of the operation. If you pass a single argument, the second matrix is the current instance of the class. If you pass two arguments, the methods add, subtract, or multiply the two matrices.

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WHO CAN INHERIT WHAT? 389

The result of a matrix operation is another matrix, unless the two matrices are incompatible for the operation. For example, you can’t add two matrices of different dimensions. You can modify the code so that it makes the smaller matrix equal to the larger one by appending zeros for the missing elements. However, you can’t multiply two matrices, unless the number of columns in the first matrix is equal to the number of rows in the second matrix. If the arguments passed to the Add method (or any other method of the Matrix class) are incompatible, the method returns an empty matrix. You must examine the size of the matrix returned by the method and act accordingly. Alternatively, you can throw an exception from within the method’s code (the statement Throw

New System.ArgumentException() is all you need).

Listing 8.46 shows the implementation of the Add method. The first overloaded form of the method acts on the two matrices passed as arguments; this code is rather trivial. The second overloaded form adds the matrix passed as argument to the matrix represented by the current instance of the class. To access the elements of the current matrix, the code uses the object MyClass. The first form of the Add method is a shared method, while the second one is an instance method (it requires an instance of the class).

Listing 8.46: The Overloaded Matrix Add Method

Public Overloads Function Add(ByVal A As Matrix, ByVal B As Matrix) As Matrix Dim Row, Col As Integer

If Not (A.Rows = B.Rows And A.Cols = B.Cols) Then Add = New Matrix()

Exit Function End If

Dim newMatrix As New Matrix(A.Rows, A.Cols) For Row = 0 To A.Rows - 1

For Col = 0 To A.Cols - 1

newMatrix.Cell(Row, Col) = A.Cell(Row, Col) + B.Cell(Row, Col) Next

Next

Add = newMatrix End Function

Public Overloads Function Add(ByVal A As Matrix) As Matrix Dim Row, Col As Integer

If Not (A.Rows = MyClass.Rows And A.Cols = MyClass.Cols) Then Add = New Matrix()

Exit Function End If

Dim newMatrix As New Matrix(MyClass.Rows, MyClass.Cols) For Row = 0 To MyClass.Rows - 1

For Col = 0 To MyClass.Cols - 1

newMatrix.Cell(Row, Col) = A.Cell(Row, Col) + MyClass.Cell(Row, Col) Next

Next

Add = newMatrix End Function

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