Vb net xp_03

Implementing Object-oriented Programming in
Visual Basic .NET
Objectives
In this lesson, you will learn to:
Identify the advantages of using classes and objects
Identify the use of structures
Identify the use of abstract classes in Visual Basic .NET
Identify the use of interfaces
Identify the differences between interfaces and abstract
classes
Identify the use of assemblies
Identify the application hierarchy in Visual Basic .NET
Create and instantiate a class
©NIIT Implementing Object-oriented Programming in VB .NET/Lesson 3/Slide 1 of 48

Visual Basic .NET
Objectives (Contd.)
Declare and import namespaces
Create an inherited form in Visual Basic .NET


Visual Basic .NET
Object-orientation in Visual Basic .NET
Visual Basic .NET:
Supports the four pillars of object-oriented programming:
® Encapsulation

® Abstraction

® Inheritance

® Polymorphism

Uses predefined classes provided by the .NET
Framework.
Allows you to create your own classes.
Supports structures that enable you to create your own
data type Data Adapter.
Also has full support for interfaces.

Visual Basic .NET
Classes and Objects
Class
Is a conceptual representation of all the entities that
share common attributes and behaviors.
Defines the attributes and behaviors of all the instances
of the class.
Object
Is an instance of a class.
Has individual copy of the common attributes and share
a common set of behaviors.


Visual Basic .NET
Advantages of Using Classes and Objects
Maintenance of code by introducing modularity.
Encapsulation of internal complexities in code from
end-users.
Reuse of code across applications.
Support for a single interface to implement multiple
methods.


Visual Basic .NET
Structure
Is a generalization of a user-defined data type (UDT).
Is created when you want a single variable to hold multiple
types of related data.
Is declared by using the Structure and End Structure
statements.
Can also include procedures as its members.
Supports event handling.


Visual Basic .NET
Structure (Contd.)
You can specify the accessibility of the data members within
a structure by using one of the following access modifiers:
Public
Protected
Friend
Protected Friend
Private


Visual Basic .NET
Structure (Contd.)
Storing and accessing data within/from a structure:
Is done by using the .(dot) notation.
Example
Dim ord1 As order_details
ord1.Inv_no = I0001“
ord1.Ord_dt = #5/31/2001#
If ord1.Inv_no = Then
MsgBox(Please enter the Invoice number)
End If


Visual Basic .NET
Classes Vs. Structure
A few similarities between a class and a structure are:
Both can have members, including constructors,
properties, constants, and events.
Both can implement interfaces.
Both can have shared constructors, with or without
parameters.


Visual Basic .NET
Classes Vs. Structure (Contd.)
A few dissimilarities between a class and a structure are
given in the following table:
Class Structure

A class is inheritable from other existing A structure is not inheritable.
classes.

A class can have instance constructors A structure can have instance
with or without parameters. constructors only if they take parameters.

A class is a reference type. A structure is a value type.

The members of a class can be The members of a structure cannot be
initialized within the class declaration. initialized within the structure declaration.


Visual Basic .NET
Abstract Class
Contains the skeleton of the methods that derived
classes can implement.
Example
Public MustInherit Class EmployeeDetails
Public MustOverride Sub Annual_Salary(ByVal m As
Integer)
End Class
Public Class Emp_details
Inherits EmployeeDetails
Public Overrides Sub Annual_Salary(ByVal m As Integer)
' Write the implementation code here
End Sub End Class


Visual Basic .NET
Abstract Class (Contd.)
You can create an abstract class by using the
MustInherit keyword in the class definition.
You can define a method in an abstract class using the
MustOverride keyword.
You must implement the method in derived classes using
the Overrides keyword.
If a derived class of an abstract class does not implement
one of the abstract methods of the abstract base class, it
also must be declared with MustInherit keyword as it
also becomes an abstract class.


Visual Basic .NET
Interface
Can contain only the declaration of members such as
properties, methods, and events.
Enables you to separate the definition of objects from their
implementation so that the objects can evolve without the
risk of introducing incompatibility in existing applications.
Is declared by using the Interface and End Interface
statements.
Example
Interface Iorderdetails
Property CustName() As String
Sub UpdateCustStatus()
Event Update_Complete()
End Interface


Visual Basic .NET
Interface (Contd.)
Statements are Public by default.
Can inherit members from an existing interface by using the
Inherits statement.
Example
Interface Validate_Cust
Sub Validate_Custname()
End Interface
Interface Iorderdetails Inherits
Validate_Cust
Property CustName() As String
Event Update_Complete()
End Interface

Visual Basic .NET
Differences Between Interfaces and Abstract Classes
Interfaces represent the highest level of abstraction in
object-oriented programming because all the methods in an
interface do not have any implementation.
In contrast, the abstract classes that are created by using
the MustInherit keyword might contain a method
that has a body.


Visual Basic .NET
Assemblies
Consist of a single or multiple executable and resource files
that contain information necessary for deploying and
maintaining the versions of an application.
Enable users to use an application even when the
application is not registered in the System Registry, since:
Every assembly is self-describing through metadata.
Every reference to a type is scoped by an assembly
reference.
.NET can automatically locate referenced assemblies.


Visual Basic .NET
Assemblies (Contd.)
The application hierarchy in Visual Basic .NET is illustrated
in the following figure:


Visual Basic .NET
Assemblies (Contd.)
Consist of manifest, module, and type.
Manifest is the descriptor of the assembly. It contains
information on:
The name and version number of the assembly.
Its interaction with other assemblies.
The types exposed by the assembly.
Security permissions required by the assembly.


Visual Basic .NET
Assemblies (Contd.)
Module is either a DLL or EXE. It contains:
Compiled code in intermediate languages.
Metadata associated with the module.
Optionally, the manifest for the assembly.
Type in Visual Basic .NET can be a class or a structure that
contain data and logic affecting the data.


Visual Basic .NET
Just a Minute…
2. Why is a structure called a composite data type?
3. Identify the syntactical error in the following declaration of
an interface:
Interface Iproductdetails
Property ProductName() As String
Label1.Text=“This is a Sub Procedure to check the status of
the product”
End Sub
Event Update_complete()
End Interface

3. How are interfaces different from abstract classes?


Visual Basic .NET
Problem Statement 3.D.1
The call centers at Diaz Telecommunications need to maintain
customer information to respond to customer queries. The
details of the customers need to be accepted through a
graphical interface. The customer information also needs to be
stored in the relevant memory variables. The details of the
customers essentially include customer id, customer’s first
name, customer’s last name, address, telephone number, and
customer’s e-mail id.


Visual Basic .NET
Task List
Identify the data that needs to be captured.
Identify the type of user interface to be used.
Identify the user interface components to accept data.
Identify the mechanism to store data in the relevant
memory variables.
Identify the memory variables to store customer
information.
Perform appropriate steps to create the user interface
screen, as designed.
Add a class to the project.
Write the code to add the relevant members to a class.


Visual Basic .NET
Task List (Contd.)
Write the code to instantiate the class.
Write the code to store and retrieve data from the class.
Save the application.
Run the application.


Visual Basic .NET
Task 1: Identify the data that needs to be captured.
Result:
As per the problem statement, the data that needs to be
captured is:
Customer ID
First Name
Last Name
Address
Telephone number
E-mail ID


Visual Basic .NET
Task 2: Identify the type of user interface to be used.
There are three types of user interfaces:
Windows Forms
Web Forms
Console
Result:
Since the requirement stated in the problem statement is for
a graphical user interface, you will use Windows Forms
as the user interface.


Visual Basic .NET
Task 3: Identify the user interface components to
accept data.
Guidelines for designing a user interface:
Identify the name and title of the form.
Identify the controls required.
Recommend suitable prefixes that you can use for
various controls:
Object Prefix Example

Form frm frmCustomerdetail
s
Label lbl lblCustomerName

TextBox txt txtCustomerName

Button cmd cmdSave


Visual Basic .NET
accept data. (Contd.)
Result:
Based on the guidelines, set the form name as
frmCustomerdetails and the text, which is the title of the
form, as Customer Details.
To make a user-friendly interface, add label controls to
display static text for the customer details: Customer ID,
First Name, Last Name, Address, Telephone number, and
Email ID. Next, add text box controls to accept information
for each customer detail. In addition, add two buttons
to save and retrieve the data stored for the customer.


Visual Basic .NET
Task 4: Identify the mechanism to store data in
relevant memory variables.
In Visual Basic .NET, you can store data in relevant
memory variables by implementing the object‑oriented
features. The object-oriented features enable you to create
a class that has member variables to store data during
data processing.
Result:
Since the requirement stated in the problem statement is to
store customer information in the relevant memory
variables, a class called Customer needs to be created
that will have member variables to store customer
information.


Visual Basic .NET
Task 5: Identify the memory variables to store
customer information.
Result:
Since the customer information is to be stored in the
member variables of the Customer class, you need to add
variables to the Customer class. Each member
variable of the Customer class should store a particular
type of customer detail. You must ensure that the data
type of a member variable matches the data type of the
customer detail stored by the variables. Six member
variables need to be declared in the Customer class.


Visual Basic .NET
Task 6: Perform appropriate steps to create the user
interface screen, as designed.
Task 7: Add a class to the project.
Namespace
Is a naming scheme that helps you organize the classes
available in an application so that they can be easily
found.
Enables you to avoid name collisions.
Is created using the Namespace keyword.
Every project in Visual Basic .NET has a root
namespace, which is set in the Property page of the
project.
You can use namespaces explicitly through direct
addressing or implicitly through the Imports statement.

Visual Basic .NET
Task 8: Write the code to add the relevant members
to a class.
Task 9: Write the code to instantiate the class.
Object Lifetime
The life of an object begins when an instance of a class
is created using the New keyword.
The life of an object ends after it goes out of scope or is
set to Nothing and is released by the .NET
Framework.
Constructors
Are of two types, shared constructors and instance
constructors.


Visual Basic .NET
Shared Constructors
Used to initialize the variables that are declared with the
Shared keyword.
Have an implicit Public access.
Will not run more than once during a single execution of a
program.
Example
Public Class class1
Shared x As Integer
Shared Sub New()
x = 10
End Sub
End Class


Visual Basic .NET
Instance Constructors
Used to initialize variables that are declared with Dim,
Public, Private, Friend, Protected, and Protected
Friend keywords.
Allow access to shared variables.
Example
Public Class class1
Dim x As Integer
Public Sub New()
x = 10
End Sub
End Class


Visual Basic .NET
Destructors
Are special methods that are used to release an instance of
a class from memory. There are two types of destructors in
Visual Basic .NET:
Finalize( )
Dispose( )


Visual Basic .NET
Finalize ( ) Destructor
Is called from the class to which it belongs or from the
derived classes.
Is called after the last reference to an object is released
from the memory.
Is automatically invoked by the .NET Framework to destroy
objects in the memory.


Visual Basic .NET
Dispose ( ) Destructor
Is called to release a resource, such as a database
connection, as soon as the object using such a resource is
no longer in use.
Is not called automatically, and you must explicitly call it
from a client application.


Visual Basic .NET
Task 10: Write the code to store and retrieve data
from the class.
Task 11: Save the application.
Task 12: Run the application.


Visual Basic .NET
Problem Statement 3.D.2
The call centers at Diaz Telecommunications need data entry
forms to store information in the Customers, Orders, Query
Handling, Products, and Employees databases. Every data
entry form should have a similar user interface with the Reset
and Exit buttons. Incorporate the interface for the Order details
form.


Visual Basic .NET
Task List
Identify the mechanism to provide a similar user interface.
Identify the user interface components to accept data.
Identify the additional user interface controls.
Create the user interface screen, as designed.
Add code for the controls.
Perform the prerequisite task to implement the planned
mechanism.
Implement the mechanism, as planned.


Visual Basic .NET
Task List (Contd.)
Add the additional user interface control, as planned.
Perform the tasks to display a similar user interface.
Add code for the inherited controls.
Save the application.
Run the application.


Visual Basic .NET
Task 1: Identify the mechanism to provide a similar
user interface.
In Visual Basic .NET, you can create a base form in a
project and then create a form that inherits from the base
form.
Result:
Since the requirement stated in the problem statement is to
create a similar user interface for all the data entry forms,
you can create a base form that has the Reset and Exit
buttons on it. You can then inherit all the data entry forms
from the base form.


Visual Basic .NET
accept data.
Result:
Since all the data entry forms must have the Reset and Exit
buttons, you need to design the user interface of the
base form with these two buttons. You also need to specify
the Text and Name properties of the form and the buttons
on the form.


Visual Basic .NET
Task 3: Identify the additional user interface
controls.
Result:
Since the data entry forms for the Customers, Orders,
Query Handling, Products, and Employees databases must
have various controls to accept data from a user, you
need to identify additional controls that need to be included
in the derived forms. In the current scenario, you will only
create a base form with two buttons and inherit the form
Order_details.


Visual Basic .NET
Task 4: Create the user interface screen, as
designed.
Task 5: Add code for the controls.
Task 6: Perform the prerequisite task to implement
the planned mechanism.
Task 7: Implement the mechanism, as planned.
Task 8: Add the additional user interface control, as
planned.


Visual Basic .NET
Task 9: Perform the tasks to display a similar user
interface.
Task 10: Add code for the inherited controls.
Task 11: Save the application.
Task 12: Run the application.


Visual Basic .NET
Problem Statement 3.P.1
The call centers of Diaz Telecommunications need to have
data entry forms to store information in the Customers,
Orders, Query Handling, Product, and Employees databases.
Every data entry form should have a similar user interface with
the Add, Modify, Delete, Reset and Exit buttons. Incorporate
the interface for the Query Handling data entry form.


Visual Basic .NET
Summary
In this lesson, you learned that:
Visual Basic .NET is an object-oriented programming
language.
A structure is used to create user-defined data types.
Data can be stored in and retrieved from a structure.
Abstract classes are used to define the skeleton of the
methods that the derived class can implement.
Interfaces are inheritable in Visual Basic .NET.
Classes can be added to a Visual Basic .NET project.


Visual Basic .NET
Summary (Contd.)
An assembly contains information necessary for deploying
and maintaining the versions of an application.
An assembly consists of manifest, module, and type.
An important advantage of using a namespace is the
prevention of a name collision.
Classes can be inherited in a Visual Basic .NET project.


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