Programming Microsoft Visual Basic .NET/Programming Microsoft Visual Basic .NET (Microsoft Press)(X08-78517)(2002).bin/05 inheritance/inheritancedemo/classes.vb

Text File | 2002-03-16 | 21.8 KB | 776 lines

' The first version of the Person class demonstrates properties, methods, indexers,
' read-only, write-only, and default properties

Class Person
    ' Visual Basic.NET classes can expose Public constants.
    Public Const DefaultPassword As String = "mypwd"

    ' Fields visible from outside the class
    Public FirstName As String
    Public LastName As String

    ' You can define up to four addresses for this person,
    ' from Address(0) to Address(3).
    Public Address(3) As String

    ' an overridable method

    Overridable Function CompleteName(Optional ByVal title As String = "") As String
        ' Use the title, if provided.
        If title <> "" Then CompleteName = title & " "
        ' Append first and last name.
        CompleteName &= FirstName & " " & LastName
    End Function

    ' the BirthDate property

    Dim m_BirthDate As Date

    Property BirthDate() As Date
        Get
            Return m_BirthDate
        End Get
        Set(ByVal Value As Date)
            m_BirthDate = Value
        End Set
    End Property

    ' the Spouse property

    Private m_Spouse As Person

    Property Spouse() As Person
        Get
            Return m_Spouse
        End Get
        Set(ByVal Value As Person)
            m_Spouse = Value
        End Set
    End Property

    ' The Age property is read-only.

    ReadOnly Property Age() As Integer
        Get
            Return Year(Now) - Year(m_BirthDate)
        End Get
    End Property

    ' LoginDate is a write-only property.

    Dim m_LoginDate As Date

    WriteOnly Property LoginDate() As Date
        Set(ByVal Value As Date)
            m_LoginDate = Value
        End Set
    End Property

    ' an example of indexer

    Dim m_Notes(10) As String

    ' The Attachment property takes an Integer argument.
    ' Note that this is also the default property.

    Default Property Notes(ByVal index As Integer) As String
        Get
            If index < 0 Or index > UBound(m_Notes) Then
                Throw New IndexOutOfRangeException("Invalid note index")
            End If
            Return m_Notes(index)
        End Get
        Set(ByVal Value As String)
            If index < 0 Or index > UBound(m_Notes) Then
                Throw New IndexOutOfRangeException("Invalid note index")
            End If
            m_Notes(index) = Value
        End Set
    End Property

    ' The GotMail event is used to demonstrate event inheritance

    Event GotMail(ByVal msgText As String)

    Sub NotifyNewMail(ByVal msgText As String)
        ' Let all listeners know that we got mail.
        RaiseEvent GotMail(msgText)
    End Sub

    ' The Father, Mother and AreBrothers method are used to demonstrate shared member inheritance

    Public Father As Person
    Public Mother As Person

    Shared Function AreBrothers(ByVal p1 As Person, ByVal p2 As Person) As Boolean
        Return (p1.Father Is p2.Father) Or (p1.Mother Is p2.Mother)
    End Function
End Class

' This second version of the Person class demonstrates overloaded constructors,
' read-only fields, and the Finalize method.

Class Person2
    ' a public read-only field that can be assigned only from inside the constructor.
    Public ReadOnly CreateTime As Date

    ' First version takes first and last name.
    Sub New(ByVal firstName As String, ByVal lastName As String)
        Me.FirstName = firstName
        Me.LastName = lastName
        ' Remember when this instance was created.
        CreateTime = Now()
    End Sub

    ' Second version takes complete name (e.g. "Joe Doe").
    Sub New(ByVal completeName As String)
        Dim i As Integer
        i = InStr(completeName, " ")
        ' Error if there are fewer than two words.
        If i = 0 Then Throw New ArgumentException("Invalid Name")
        ' Initialize main properties.
        Me.FirstName = RTrim(Left(completeName, i - 1))
        Me.LastName = LTrim(Mid(completeName, i + 1))
        ' Remember when this instance was created.
        CreateTime = Now()
    End Sub


    Private m_FirstName As String
    Private m_LastName As String

    Property FirstName() As String
        Get
            Return m_FirstName
        End Get
        Set(ByVal Value As String)
            If Value = "" Then
                Throw New ArgumentException("Invalid FirstName property")
            End If
            m_FirstName = Value
        End Set
    End Property

    Property LastName() As String
        Get
            Return m_LastName
        End Get
        Set(ByVal Value As String)
            If Value = "" Then
                Throw New ArgumentException("Invalid LastName property")
            End If
            m_LastName = Value
        End Set
    End Property

    ' the BirthDate property

    Dim m_BirthDate As Date

    Property BirthDate() As Date
        Get
            Return m_BirthDate
        End Get
        Set(ByVal Value As Date)
            m_BirthDate = Value
        End Set
    End Property

    Overridable Function CompleteName() As String
        ' Append first and last name.
        Return FirstName & " " & LastName
    End Function

    Protected Overrides Sub Finalize()
        ' NOTE: we output this message to the debug window (instead of the console)
        ' because if the Finalize method is called by the very last GC that fires
        ' when the application completes, then the Console object isn't available any longer.
        Debug.WriteLine("Person " & Me.CompleteName() & " is being destroyed")
    End Sub

End Class


' The Person3 class demonstrates how to use the MyClass keyword and the problems
' you have when you omit it

Enum Gender
    NotSpecified
    Male
    Female
End Enum

Class Person3
    ' (In a real-world class, these would be properties.)
    Public FirstName As String
    Public LastName As String
    Public Gender As Gender = Gender.NotSpecified

    ' the usual constructor
    Sub New(ByVal firstName As String, ByVal lastName As String)
        Me.FirstName = firstName
        Me.LastName = lastName
    End Sub


    Dim m_Title As String
    Overridable Property Title() As String
        Get
            Return m_Title
        End Get
        Set(ByVal Value As String)
            m_Title = Value
        End Set
    End Property

    ' Prefix the name with a title if one has been specified.
    Function TitledName() As String
        If Title <> "" Then
            Return Title & " " & FirstName & " " & LastName
        Else
            Return FirstName & " " & LastName
        End If
    End Function

    Public BirthDate As Date

    ' Age is defined as the number of whole years passed from BirthDate.
    Overridable ReadOnly Property Age() As Integer
        Get
            Age = CInt(DateDiff(DateInterval.Year, BirthDate, Now()))
            If Month(Now) < Month(BirthDate) Or _
                (Month(Now) = Month(BirthDate) And _
                Day(Now) < Day(BirthDate)) Then
                ' Correct if this year's birthday hasn't occurred yet.
                Age = Age - 1
            End If
        End Get
    End Property

    ' an incorrect version of the CanVote property
    ReadOnly Property CanVote() As Boolean
        Get
            ' This statement isn't correct.
            Return (Age >= 18)
        End Get
    End Property

    ' the correct version of the CanVote property
    ReadOnly Property CanVote2() As Boolean
        Get
            ' Ensure that it always uses the non-overridden
            ' version of the Age property.
            Return (MyClass.Age >= 18)
        End Get
    End Property

    ' an non-overridable Address property
    Dim m_Address As String

    Property Address() As String
        Get
            Return m_Address
        End Get
        Set(ByVal Value As String)
            m_Address = Value
        End Set
    End Property

End Class


' The Widget class demonstrates the IDisposable interface.
' and performance of Overridable and NotOverridable methods.

Class Widget
    Implements IDisposable

    Function GetInteger() As Integer
        Return 1
    End Function

    Overridable Function GetInteger2() As Integer
        Return 1
    End Function

    Sub Dispose() Implements IDisposable.Dispose
        ' Close files and release other resources here.
        ' ...
    End Sub
End Class


' This version of the DataFile class uses the GC.SuppressFinalize method
' for optimized clean-up code.

Class DataFile2
    Implements IDisposable

    ' The file handle
    Private handle As Integer

    ' Open a file, store its handle.
    Sub Open(ByVal inputFile As String)
        ' Throw an exception if the object has been already disposed.
        If disposed Then Throw New ObjectDisposedException("DataFile2")
        ' Continue with regular operations.
        handle = FreeFile()
        FileOpen(handle, inputFile, OpenMode.Input)
    End Sub

    ' Close the file, don't throw an exception if already closed.
    Sub Close()
        ' Throw an exception if the object has been already disposed.
        If disposed Then Throw New ObjectDisposedException("DataFile2")
        ' Continue with regular operations.
        If handle <> 0 Then
            FileClose(handle)
            handle = 0
        End If
    End Sub

    ' This private variable is True if the object has been disposed.
    Protected disposed As Boolean

    Sub Dispose() Implements IDisposable.Dispose
        Debug.WriteLine("Dispose method")
        ' Execute the code that does the clean up.
        Dispose(True)
        ' Let the CLR know that Finalize doesn't have to be called.
        GC.SuppressFinalize(Me)
    End Sub

    Protected Overrides Sub Finalize()
        Debug.WriteLine("Finalize method")
        ' Execute the code that does the clean up.
        Dispose(False)
    End Sub

    ' This procedure is where the actual cleanup occurs.
    Protected Overridable Sub Dispose(ByVal disposing As Boolean)
        ' Exit now if the object has been already disposed.
        If disposed Then Exit Sub

        If disposing Then
            ' The object is being disposed, not finalized.
            ' It is safe to access other objects (other than the base
            ' object) only from inside this block.
        End If

        ' Perform clean up chores that have to be executed in either case.
        Close()

        ' Remember that the object has been disposed.
        disposed = True
    End Sub
End Class

' This class inherits from DataFile2 and overrides the Dispose and FInalize methods.

Class BetterDataFile2
    Inherits DataFile2

    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        ' Exit now if the object has been already disposed.
        If disposed Then Exit Sub

        If disposing Then
            ' The object is being disposed, not finalized.
            ' It is safe to access other objects (other than the base
            ' object) only from inside this block.
        End If

        ' Perform clean up chores that have to be executed in either case.
        ' ...

        ' Call the base class's Dispose method
        MyBase.Dispose(disposing)
    End Sub

End Class

' The Employee class demonstrate inheritance

Class Employee
    Inherits Person

    ' Two new public fields
    Public BaseSalary As Single
    Public HoursWorked As Integer
    ' A new private field
    Private m_HourlySalary As Single

    ' A new property
    Property HourlySalary() As Single
        Get
            Return m_HourlySalary
        End Get
        Set(ByVal Value As Single)
            m_HourlySalary = Value
        End Set
    End Property

    ' A new method
    Function Salary() As Single
        Return BaseSalary + m_HourlySalary * HoursWorked
    End Function

    ' redefine the AreBrothers shared method

    Shared Shadows Function AreBrothers(ByVal e1 As Employee, _
        ByVal e2 As Employee) As Boolean
        Return Person.AreBrothers(e1, e2) And (e1.LastName = e2.LastName)
    End Function

End Class

' The second version of the Employee class demonstrates constructors with arguments

Class Employee2
    Inherits Person2

    Public Title As String                 ' A new field

    ' two overloaded constructors

    Sub New(ByVal firstName As String, ByVal lastName As String)
        ' The first executable statement *must* be a call
        ' to the constructor in the base class.
        MyBase.New(firstName, lastName)
        ' You can continue with the initialization step here.
        ' ...
    End Sub

    Sub New(ByVal firstName As String, ByVal lastName As String, ByVal title As String)
        ' The first executable statement *must* be a call
        ' to the constructor in the base class.
        MyBase.New(firstName, lastName)
        ' set the new field
        Me.Title = title
    End Sub

    Overrides Function CompleteName() As String
        If Title <> "" Then CompleteName = Title & " "
        CompleteName &= MyBase.CompleteName()
    End Function
End Class

Class Employee3
    Inherits Person3

    ' the usual constructor
    Sub New(ByVal firstName As String, ByVal lastName As String)
        MyBase.New(firstName, lastName)
    End Sub

    ' Always provide a title if one hasn't been assigned.
    Overrides Property Title() As String
        Get
            If MyBase.Title <> "" Then
                Return MyBase.Title
            ElseIf Gender = Gender.Male Then
                Return "Mr."
            ElseIf Gender = Gender.Female Then
                Return "Mrs."
            End If
        End Get
        Set(ByVal Value As String)
            MyBase.Title = Value
        End Set
    End Property

    ' Age is defined as difference between the current year
    ' and the year when the employee was born.
    Overrides ReadOnly Property Age() As Integer
        Get
            Age = CInt(DateDiff(DateInterval.Year, BirthDate, Now()))
        End Get
    End Property

    ' the Address property demonstrates that you can always "manually" override
    ' a procedure that is marked as non-overridable in the base class.
    Shadows Property Address() As String
        Get
            Return MyBase.Address
        End Get
        Set(ByVal Value As String)
            If Value = "" Then Throw New ArgumentException()
            MyBase.Address = Value
        End Set
    End Property

End Class

' Shape virtual class defines a behavior for other classes

MustInherit Class Shape
    ' Position on the X-Y plane
    Public X, Y As Single

    ' Move the object on the X-Y plane.
    Sub Offset(ByVal deltaX As Single, ByVal deltaY As Single)
        X = X + deltaX
        Y = Y + deltaY
        ' Redraw the shape at the new position.
        Display()
    End Sub

    ' A virtual method
    MustOverride Sub Display()
End Class

' The Square class derives from Shape and also demonstrates how you can use
' private constructors

Class Square
    Inherits Shape

    Public Side As Single

    Overrides Sub Display()
        ' add here the statements that draw a square
    End Sub

    ' This private constructor prevents clients from
    ' instancing this class directly.
    Private Sub New(ByVal side As Single)
        Me.Side = side
    End Sub

    ' Clients can create a square only through this shared method.
    Shared Function CreateSquare(ByVal side As Single) As Square
        Return New Square(side)
    End Function

End Class

' The Animal and Peripheral classes demonstrate nested classes

Class Animal
    ' ...
    ' This class can be referred to as Animal.Mouse.
    Class Mouse
        ' ...
    End Class
End Class

Class Peripheral
    Dim m As Mouse
    Dim kb As Keyboard
    Dim k As Keyboard.Key

    ' ...
    ' This class can be referred to as Peripheral.Mouse.
    Class Mouse
        Dim kb As Keyboard
        Dim k As Keyboard.Key
        ' ...
    End Class

    ' This class can be referred to as Peripheral.Keyboard.
    Class Keyboard

        ' A private member
        Dim m_Brand As String

        ' This shared member is True if this class supports
        ' non-Latin keyboards.
        Public Shared SupportsNonLatinKeyboards As Boolean

        ReadOnly Property Brand() As String
            Get
                ' Code inside the outer class can access a private
                ' member without any reference (Me is implicit).
                Return m_Brand
            End Get
        End Property

        ' This class can be referred to as Peripheral.Keyboard.Key.
        Class Key

            ' This public field is meant to be assigned when creating
            ' an instance of the Key class.
            Public ParentKeyboard As Keyboard

            ReadOnly Property Brand() As String
                Get
                    ' Code inside the inner class can access a private member
                    ' in the outer class, but requires an object reference.
                    Return ParentKeyboard.m_Brand
                End Get
            End Property

            ReadOnly Property SupportsNonLatin() As Boolean
                Get
                    ' You can access a shared member in the outer class
                    ' without an object reference.
                    Return SupportsNonLatinKeyboards
                End Get
            End Property

        End Class
    End Class
End Class

' The Customer, GoodCustomer, and ForeignCustomer classes demonstrate scope concepts

Class Customer
    ' This member is visible to this class and
    ' classes derived from this class.
    Protected AlwaysPaysOnTime As Boolean

    ' Compute the discount percentage on products.
    Protected Overridable Function ProductDiscount() As Single
        ' Offer an additional discount if the customer always pays on time.
        If AlwaysPaysOnTime Then
            ProductDiscount = 15
        Else
            ProductDiscount = 10
        End If
    End Function

    ' By default make no discount on shipment.
    Protected Overridable Function ShipmentDiscount() As Single
        Return 0
    End Function

    ' Compute the actual discount on an order, given the
    ' amount of products purchased and the amount of shipment.
    Function TotalOrderAmount(ByVal ProductAmount As Single, _
        ByVal ShipmentAmount As Single) As Single
        Return ProductAmount * (1 - ProductDiscount() / 100) _
            + ShipmentAmount * (1 - ShipmentDiscount() / 100)
    End Function

    ' a protected nested class
    Protected Class OrderHistory
        Public Count As Integer
        Public TotalAmount As Single
        ' ...
    End Class

End Class

Class GoodCustomer
    Inherits Customer

    Sub New()
        ' Note that no MyBase.New is needed, because the base
        ' class has no constructor with parameters.
        AlwaysPaysOnTime = True
    End Sub

    ' A derived class sees protected nested classes.
    Dim oh As Customer.OrderHistory
    ' Note that you don't even need the dot syntax.
    Dim oh2 As OrderHistory

End Class

Class ForeignCustomer
    Inherits Customer

    ' A convenient constructor that lets us test well- and
    ' ill-behaved foreign customers.
    Sub New(ByVal alwaysPaysOnTime As Boolean)
        Me.AlwaysPaysOnTime = alwaysPaysOnTime
    End Sub

    ' We don't charge shipment to well-behaved foreign customers.
    Protected Overrides Function ShipmentDiscount() As Single
        If AlwaysPaysOnTime Then ShipmentDiscount = 100
    End Function
End Class

' the DataReader and FileDateReader classes demonstrate the simplest technique
' for a derived class to trap events from a base class

Class DataReader
    Event DataAvailable()

    Sub GetNewData()
        RaiseEvent DataAvailable()
    End Sub
End Class

Class FileDataReader
    Inherits DataReader

    ' This variable will point to the object itself (Me).
    Dim WithEvents EventSink As FileDataReader
    ' This counter must be incremented after each event.
    Public EventCounter As Integer

    Sub New()
        MyBase.New()
        EventSink = Me
    End Sub

    Private Sub NotifyDataAvailable() Handles EventSink.DataAvailable
        ' Increment the counter.
        EventCounter += 1
    End Sub
End Class

' The DataReader2 and FileDataReader2 classes demonstrate a more sophisticated technique
' for a derived class to control events fired in its base class

Class DataReader2
    Event DataAvailable()

    Sub GetNewData()
        OnDataAvailable()
    End Sub

    Protected Overridable Sub OnDataAvailable()
        RaiseEvent DataAvailable()
    End Sub
End Class

Class FileDataReader2
    Inherits DataReader2

    ' This counter must be incremented after each event.
    Public EventCounter As Integer

    Protected Overrides Sub OnDataAvailable()
        ' Increment the counter.
        EventCounter += 1
        ' Raise only up to 10 events.
        If EventCounter <= 10 Then MyBase.OnDataAvailable()
    End Sub
End Class

' these classes are used to test member shadowing

Class AAA
    Sub DoSomething()
        Console.WriteLine("AAA.DoSomething")
    End Sub
    Sub DoSomething(ByVal msg As String)
        Console.WriteLine("AAA.DoSomething({0})", msg)
    End Sub

    Sub DoSomething2()
        Console.WriteLine("AAA.DoSomething2")
    End Sub
    Sub DoSomething2(ByVal msg As String)
        Console.WriteLine("AAA.DoSomething2({0})", msg)
    End Sub

End Class

Class BBB
    Inherits AAA

    Overloads Sub DoSomething()
        Console.WriteLine("BBB.DoSomething")
    End Sub

    Private Shadows Sub DoSomething2()
        Console.WriteLine("BBB.DoSomething2")
    End Sub

End Class