instance variables

property procedures for the mintQuantity instance variable

Accessing the Values of a Class• The Property Get procedure retrieves a property from a

class, making it available to the outside world.

• The Property Let procedure assigns a value from the outside world to a property of the class.

• Property Let and Property Get are the only way to assign and retrieve property values for a class.

• Otherwise, there is no other “doorway” through which they can pass.

Class Module Code

Form Code

Creating a New Object Using a Class

• Creating a class module defines a new class• Code that creates and uses objects is placed in

form and code modules, whereas a class must be placed in a class module

• Create an object of a class with the New keyword—called instantiating an object

• The New keyword creates a new instance of a class when the object is first used

• Dimensioning objects with the New keyword:– Dim Employee As New cEmployee– Private mInventory As New Cinventory

• Next, create a form that instantiates an object of the class CProduct

Using a For Each… Next LoopPrivate Sub cmdExit_Click()

‘this procedure will unload all forms that are currently loaded‘in the project

Dim OneForm As Form

For Each OneForm in FormsUnload OneForm

Next

End

This is a very useful piece of code for quitting a project

Disabling Controls using a For Each…Next Loop

Dim AnyControl As Control

For Each AnyControl in frmMain.ControlsAnyControl.Enabled = False

Next

Uses the controls collection to change a property of multiple controls

10

Example of Inheritance in VB.NET

• Examine 1st line of code for a form in the Editor (look at Visual Studio 2005 in the labs)

Public Class Form1Inherits System.Windows.Forms.Form

Inherited Class, Derived Class Subclass, Child Class

Base Class, Superclass, Parent Class

Hello World in VB.NET

Collections

• A Collection Class holds references for a series of objects created from the same class or from different classes

• Actually a collection holds references to the objects– You reference by Index Number or a Key– Similar to list box and the associated items in the

list box

Key for Collection Objects

• Key must be a string• Can be used to reference individual objects in

the collection• Declare the Key as String at the module level

of the Class module for the object (not the collection)

• Add Property Get and Let procedures

Create a collection by writing a new class module and then declaring an object variable (to create an object).

Each object in the collection is known as an item.Each object has a position known as its index.A collection is declared with a Dim, Public, or Static

statement just like any other object variable:Dim mProducts As New Collection

VB always updates the Count property of a collection every time you add or remove an item from a collection.

Collections

The Add method adds an item to the collection.mProducts.Add Prod1

The Remove method removes an item according to its index in the collection.mProducts.Remove 2

It is best to provide a unique key to each object in a collection, rather than rely upon an index (see remove example above). It is difficult to associate an index with a particular product. Using a key value makes locating an object much easier.

Add a string in the CProduct class: Private mstrProductCode As String

Use the string to add an object to a collection or remove it from a collection

Collections

Using a Collection in a Form