Distance Learning Center

1
Distance Learning Center

• Lecture 13
• Introduction to Visual Basic
• Programming
• Melissa Lin, IT Consultant
• HenEm, Inc. Parkville, Missouri
• linm_at_ipfw.edu
• http//www.etcs.ipfw.edu/linm

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Lecture 13

• Review
• Classes, objects, properties, and methods
• OOP terminology
• Constructor
• Destructor
• Overloading
• Inheritance
• Overriding
• Object Browser
• Problem 6.7

3
Review - Class

• Creating a new object based on a class
• Create an instance of the class by using the New
  keyword
• General Form

New className( )
Dim arialFont As Font New Font ("Arial",
12) messageLabel.Font arialFont OR messageLabel
.Font New Font ("Arial", 12)
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Specifying a Namespace

• In your projects, you have noticed the Inherits
  clause when VB creates a new form class

• Entire namespace is not needed for any classes in
  the namespaces that are automatically included in
  a Windows Forms project which include
• System
• System.Windows.Forms
• System.Drawing
• When referring to a class in a different
  namespace
• Write out the entire namespace or
• Add an imports statement at the top of the code
  to specify the namespace

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Design a Class Define Properties

• Analyze characteristics needed by new objects
• Characteristics or properties are defined as
  variables
• Define the properties as variables in the class
  module
• Analyze behaviors needed by new objects
• Behaviors are methods
• Define the methods as sub procedures or functions
• Define variables inside the Class module by
  declaring them as Private
• Do not make Public, that would violate
  encapsulation (each object should be in charge of
  its own data)

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Property Procedure General Form
Private ClassVariable As DataType Public
Property PropertyName( ) As DataType Get Return
ClassVariable PropertyName ClassVariable End
Get Set (ByVal Value As DataType) statements
, such As validation ClassVariable
Value End Set End Property
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Read-Only Write-Only Properties

• In some instances a value for a property should
  only be retrieved by an object and not changed
• Create a read-only property by using the ReadOnly
  modifier
• Write only the Get portion of the property
  procedure

Public ReadOnly Property PropertyName( ) As
DataType

• At times a property can be assigned by an object
  but not retrieved
• Create a property block that contains only a Set
  to create a write-only property

Public WriteOnly Property PropertyName( ) As
DataType
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Class Methods

• Create methods by coding public procedures within
  a class
• Methods declared with the Private keyword are
  available only within the class
• Methods declared with the Public keyword are
  available to external objects

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Constructors and Destructors

• Constructor
• Method that automatically executes when an object
  is instantiated
• Constructor must be public and is named New
• Destructor
• Method that automatically executes when an
  object is destroyed

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Constructor (continued)

• Overloading Constructor
• It means that two methods have the same name but
  a different list of arguments (the signature)
• It creates by giving the same name to multiple
  procedures in your class module, each with a
  different argument list
• Ex 1. Sub New() Ex 2. Sub New(ByVal
  Title As String)
• titleString Title
• End End
• Parameterized Constructor
• Constructor that requires arguments
• Allows arguments to be passed when creating an
  object
• Ex. aBookSale New BookSale(titleTextBox)

Sub New(ByVal Title As String, ByVal Quantity As
Integer, ByVal Price As Decimal) Me.Title
Title Me.Quantity Quantity
Me.Price Price End Sub
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Creating a New Object Using a Class

• It similar to creating a new tool for the toolbox
  but not yet creating an instance of the class
• The two steps to create a new object
• Declare a variable for the new object
• Instantiate the object using the New keyword

Private aBookSale As BookSale aBookSale New
BookSale( ) Or Dim aBookSale As New Booksale( )
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Creating a New Object Using a Class (continued)

• If object variable is needed in multiple
  procedures, declare the object at class level
• Instantiate the object
• Only when(if) it is needed
• Inside a Try/Catch block for error handling
  (Try/Catch block must be inside a procedure)
• Pass values for the arguments at instantiation
  when using a parameterized constructor

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Instance Variables versus Shared Variables

• Instance variables or properties
• Separate memory location for each instance of the
  object
• Shared variables or properties
• Single variable that is available for ALL objects
  of a class
• Can be accessed without instantiating an object
  of the class
• Use the Shared keyword to create

Shared Methods can also be created
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Garbage Collection

• Feature of .NET Common Language Runtime (CLR)
  that cleans up unused components
• Periodically checks for unreferenced objects and
  releases all memory and system resources used by
  the objects
• Microsoft recommends depending on Garbage
  Collection rather than Finalize procedures

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Inheritance

• New class can
• Be based on another class (base class)
• Inherit the properties and methods (but not
  constructors) of the base class, which can be
• One of the VB existing classes
• Your own class
• Cannot inherit constructors from its base class
• Use the Inherits statement following the class
  header and prior to any comments

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Overriding Methods

• Methods with the same name and the same argument
  list as the base class
• Derived class (subclass) will use the new method
  rather than the method in the base class
• To override a method
• Declare the original method with the Overridable
  keyword
• Declare the new method with the Overrides keyword

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Overriding
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Creating a Base Class Strictly for Inheritance

• Classes can be created strictly for inheritance
  by two or more similar classes and are never
  instantiated
• For a base class that you intend to inherit from,
  include the MustInherit modifier on the class
  declaration
• In each base class method that must be
  overridden, include the MustOverride modifier and
  no code in the base class method

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Inheriting Form Classes

• Creating an Inherited Form Class in two ways
• Select Project -gt Add Windows Form and type in a
  name for the new Windows form
• Select Project -gt Add Inherited Form and type the
  name of the new form
• Ex. Public Class baseForm
• inherits System.Windows.Forms.Form
• .
• End Class
• Public Class aboutForm
• inherits ProjectName.aboutForm
• .
• End Class

Multiforms
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Managing Multiclass Projects

• VB projects are automatically assigned a
  namespace which defaults to the name of the
  project
• Add an existing class to a project by copying the
  file into the project folder and then adding the
  file to the project
• Project/Add Existing Item

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Form Inheritance Example (pp. 255-258)

• Project/Add Inherited Form
• aboutForm enter this name, and click on open
• Inheritance Picker Window
• baseForm select this form

aboutForm
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Form Inheritance Example (pp. 255-258)

• Solution Explorer
• baseForm.vb
• aboutForm.vb
• Double click aboutForm.vb
• Properties Text About Form
• Method Me.Close()
• Public Class aboutForm
• Inherits Multiforms.baseForm
• Public Overrides Sub okButton_click(ByVal
  sender As System.Object, ByVal e As
  System.EventArgs) Handles okButton.Click
• End Sub
• End Class

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Form Inheritance Example (pp. 255-258)

• Solution Explorer
• baseForm.vb
• aboutForm.vb
• summaryForm
• mainForm
• aboutForm
• Public Class summaryForm
• Inherits System.Windows.Forms.Form
• Public Overridable Sub okButton_Click(ByVal
  sender As System.Object, _
• ByVal e As System.EventArgs) Handles
  okButton.Click
• ' Allow inherited classes to override this
  method.
• Me.Close()
• End Sub

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Using the Object Browser

• Use it to view the names, properties, methods,
  events and constants of VB objects, your own
  objects, and objects available from other
  applications
• To Display
• Click on Tab in Editor Window
• Object Browser toolbar button

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Object Browser
Find Symbol
Browse list
Objects list
Members list
Namespace icons
Method icon
Property icon
Constants icon
Description Pane
Class icon
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Examine Classes - class view

• Shows all classes of your project
• Classes include
• Form Objects
• Objects on Forms

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Hands-On Programming Example (pp. 262-274)

• Create a project with multiple Forms that have a
  shared design element, including a main Form, an
  About Form, and a Summary Form that displays the
  sales summary information
• Design a Base Form to use for inheritance and
  make the other three forms inherit from the base
  form
• The About Form and Summary Form must have an OK
  button, which closes the form
• The main form will have menus without OK button.
• Main Form File (Calculate Sale, Clear, Summary,
  Exit) Help (About)

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Planning GUI Design
File
Help
About
Calculate Sale
Clear
Summary
Exit
Title
Quantity
Price
Student
Extended Price
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Define GUI
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Define GUI

• Start up
• the Project
• Base Form Class
• baseForm

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Main Form - GUI and Code

• mainForm- Inheriting from
• Base Form
• Add GUIs, change properties
• Menus
• Text File, Edit, Help
• Complete event procedures (pages 270-271)

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Place Menu on the Form
Create Menu and SubMenu
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Summary Form GUI and Code

• summaryForm
• Inheriting
• From Base From
• Add GUIs, change roperties
• Complete obButton_click event procedure (page
  271)

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Output

• Click
• Calculate
• Selection,
• After
• Entering
• all inputs

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Output (continued)

• Click Summary

Click About
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Problem 6.7 in Text Book

• Modify Programming Exercise (5.2) (the check
  transactions) to separate business logic from the
  user interface
• Create a Transaction Class and derived classes
  for deposit, Check, and Service Charges
• Display summary information on a separate form
  rather than a message box

Note that the 1st edition of the text mistakenly
has 5.2 here.
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Authors structure for 5.2
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Code StructureAuthors solution of Problem 5.2

• Private balanceDecimal As Decimal
• Private totalChecksDecimal, totalDepositsDecimal,
  totalChargesDecimal As Decimal
• Private depositsInteger, checksInteger,
  chargesInteger As Integer
• Private Sub fileTransactionMenuItem_Click
• Private Sub fileSummaryMenuItem_Click
• Private Sub editClearMenuItem_Click
• Private Sub fileExitMenuItem_Click
• Private Sub editFontMenuItem_Click
• Private Sub editColorMenuItem_Click
• Private Sub helpAboutMenuItem_Click
• Private Function Deposit
• Private Function Check
• Private Function ServiceCharge

See Ex0502
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Code for frmChecking
Create instance of the form
Get TotalChecks property
Show the form
See Ex0502
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Structure of 6.7
See Ex0607
Transaction is the BaseClass
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Code for helpAboutMenuItem
See Ex0607
See Ex0502
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Code for fileTransactionMenuItem
See Ex0502
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Code for Transaction Class
See Ex0607
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Overridable Sub in Base Form

• Public Class baseFrom
• Inherits System.Windows.Forms.Form
• Pulic Overridable Sub okButton_Click(ByVal
  sender As System.Object, ByVal e As
  System.EventArgs) Handles okButton.Click
• 'This button is inherited on other forms
• End Sub
• End Class
• Public Class summaryForm
• Inherits EX0607.baseFrom
• Public Overrides Sub okButton_Click(ByVal
  sender As System.Object, ByVal e As
  System.EventArgs)
• 'Close the summary form
• Me.Close()
• End Sub
• End Class

Overridable Sub with no code in Base Form!
Provides for this form to inherit all properties
of baseFrom
Overrides Sub in Base Form and closes this Form!
See Ex0607
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Overrides Subs in Code Classes
See Ex0607
46
Overrides Subs in Code Classes
See Ex0607
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Summary

• Review - Class
• classes, objects, properties and methods
• OOP terminology
• Constructor
• Destructor
• Overloading
• Inheritance
• Overriding
• Object Browser
• Problem 6.7

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Question?

• Answers
• linm_at_ipfw.edu