Chapter 5 1

Using Visual Basic .NET within an ASP.NET Page

Introduction to ASP.NET

By Kathleen Kalata

Chapter 5 2

Objectives

• In this chapter, you will:

• Define object-oriented programming

• Use Visual Basic .NET to manipulate the code behind the page

• Create classes using Visual Basic .NET

• Use variables to store data

Chapter 5 3

Objectives

• In this chapter, you will:

• Learn how data types are stored in the .NET Framework

• Use collections to store groups of data elements

• Create and use procedures in Visual Basic .NET

• Learn how to use Visual C# .NET instead of Visual Basic .NET to create ASP.NET pages

Chapter 5 4

Object-Oriented Programming

• Programming statements, or code, can be combined into logical groupings functions, event handlers, and procedures

• In Object-Oriented Programming, you create objects based upon a class, and then you can access the object across multiple Web pages

– An object is a set of related code that is compartmentalized and built upon these classes

– The creation of an object consists of two separate tasks

• The first task is to create the object definition, called the class, it is not an object that you can access directly

• You use the class as the template for creating the new object. When you create an object, you are really creating an instance of the class

Chapter 5 5

Instantiation

• Instantiation is the process of declaring and initializing an object from a class

– You can create many objects from the same object definition

– Create a class named TaraStoreClass

• The name of the class is TaraStoreClass

• Contains one variable named StoreName

Public Class TaraStoreClass

Private StoreName As String = "Tara Store"

End Class

Chapter 5 6

Protecting the Class Code

• Restrict applications access to the class

– Public - can interact with other objects outside of the base class

– Private - can only be called within the base class

– Protected - can be called from within the base class, and within subclasses

• Subclasses are used to create a class that inherits from a base class

– Friend - can be called anywhere from within the same application

Chapter 5 7

How to Instantiate an Object

• Instantiate an object based on the class definition

– Declare a variable with the keyword Dim, to store the object

– Use the keyword New to identify that this is an object based on a class definition

– The class in the sample code below is named TaraStoreClass

• The object is stored in the variable named Ch5Class

• The class is contained within the Chapter5 namespace

Dim Ch5Class As New Chapter5.TaraStoreClass()

Chapter 5 8

Commonly Used Terms

• Decision control structures - allow you to organize the order in which the code is executed

• Event handlers - execute code when the event occurs

• Procedures - named grouping of one or more programming statements, executed when the procedure has been called by another procedure or event handler

Chapter 5 9

Commonly Used Terms

• Parameters - values passed to the subprocedures and functions; multiple values are separated in a comma delimited list within a pair of parentheses

• Built-in functions - include mathematical functions, date and time, string, and formatting functions, among others

• Functions - a named grouping of one or more programming statements that can return a value to the program that called the function by using the return keyword

Chapter 5 10

Properties

• Properties are used to set the value of a variable defined within an object

• Properties are identified by the name of the object, followed by a period, followed by the name of the property

• Properties are assigned a default value within the object definition, or the value is set as “unassigned”

• All new objects inherit the same properties as the original object definition

Chapter 5 11

Object-Oriented Programming Concepts

– Inheritance allows you to derive the interface and behaviors from another class.

• The inherits keyword allows you to inherit from another .NET class.

• With the Common Language Runtime, all objects are inherited from the System namespace, so some of the properties such as ToString will apply to most objects.

– Encapsulation - the ability to assign different values to each object’s properties. Encapsulation means that the inner workings of the object, such as the procedures and properties, are maintained within the object so objects or programs are self-contained. You can create multiple objects based on the same class, as long as the names are unique

Chapter 5 12

Object-Oriented Programming Concepts

– Abstraction is the ability to create a concept using code. For example, a customer is a person who makes purchases from a company. A customer object is the programming code that represents the customer. The customer object may contain information about the customer, such as their shipping address, and purchases

– Polymorphism is the ability to create code that can be applied to objects that are derived from different classes. If you have an application that contains a student object and a faculty object, you can write an application that retrieves the name property from both objects. Polymorphism allows you to apply programs across different classes

Chapter 5 13

WinForms

• The Visual Basic .NET application forms

– These forms interact with the user and are inherited from the System.Windows.Forms namespace

– When you build the application, Visual Studio .NET compiles the code and stores the .dll file that contains the namespace in the projects bin directory

– The bin directory has execute permission which allows the code to be executed at run time

Chapter 5 14

The Page Class

• The ASP.NET page inherits the code behind the page which is the page class

– By default, when you create an ASP.NET Web page in Visual Studio .NET, a class that represents that page is created which is named after the Web page

– On the first line of the Web page, Visual Studio .NET inserts the inherits keyword and identifies the name of the class that is created in the code behind the Web page

– When the build method is called, the code behind the page is compiled into the same .dll file that contains the namespace for the application

Chapter 5 15

Creating a Class Definition

• Referring to the Class in the class definition

– Because you have only created the class definition, and have not created an instance of the object yet, you cannot refer to the object by its name within the class definition

– Therefore, the keyword Me can be used in place of the object name within the class definition (Note: In C# the keyword Me is replaced with the keyword this.)

Chapter 5 16

Creating a Class

• Create Chapter5 Web application and a class named TaraStoreClass

– The class definition creates two variables that can be called from the ClassVariables.aspx page

– Create the StoreName and StoreEmail instance variables

• These variables are private to the TaraStoreClass class

• They are not available outside of the TaraStoreClass class

Public StoreName As String = "Tara Store"

Public StoreEmail As String = "info@TaraStore.com"

Chapter 5 17

Creating a Class

• Create Chapter5 Web application and a class named TaraStoreClass

– Modify the Code View Options

– Add line numbers to the code view

– Tools | Options | Text Editor | Basic | Line Numbers

Chapter 5 18

Modifying the Code View Options

Chapter 5 19

Instantiate the Object

• Import the ClassVariables.aspx

– In the Page_Load procedure create a new object named Ch5Class based on the TaraStoreClass

Dim Ch5Class As New Chapter5.TaraStoreClass()

Chapter 5 20

Instantiate the Object

• Use the ToString method to retrieve the values from the variable in the new class

Chapter 5 21

Chapter 5 22

Viewing the ClassVariables Page

Chapter 5 23

IL Disassembler

– Use the IL Disassembler (ILDASM) tool to view the namespace of the Chapter5 assembly and locate any of the classes created within your Windows or Web application

– Go to the Visual Studio .NET command prompt

• Change to the bin directory of your project

cd Inetpub\wwwroot\Chapter5\bin

ILDASM Chapter5.dll

– View the TaraStoreClass class information and the StoreEmail field to see that it is a public variable of type string

Chapter 5 24

IL Disassembler

Chapter 5 25

Variables

• A variable declaration consists of two parts: a variable name and a data type

– Declaring a variable is the process of reserving the memory space for the variable before it is used in the program

– You must declare all variables before they are used

– Variables that are declared in the class are referred to as members, member variables, or instance variables

– The data type identifies what kind of data the variable can store

Chapter 5 26

Variables Declaration

• Where the variable is defined will determine where the variable can be used within the application

• You use the keywords private, public, friend, and protected to specify the scope of the variable

– Private variables are only available to code within the local class

– Public variables, called global or public variables, are used outside the class

– Friend variables are used only within the current application or project

Chapter 5 27

Naming Variables

• Choose a descriptive name, one that has some meaning or association with the contents or purpose of the variable

– No commands or keywords as your variable name

– Begin with a letter

– Do not use a period or space within a variable name

– Avoid using any special characters except for the underscore

– Visual Basic .NET commands and variables are not case sensitive

– The first letter of each word is usually capitalized

Chapter 5 28

Use the First Three Characters to Identify the Data Type

Data Type Prefix Sample Variable Name

Boolean bln blnMember

Byte byt bytZero

Char chr chrLetter

Date dat datBirthdate

Double dbl dblWeight

Decimal dec decProductPrice

Integer int intNumberProducts

Long lng sngSalary

Single sng sngAverage

Short sho shoYears

String str strLastName

Chapter 5 29

Assigning Values to Variables

• The assignment operator is the equal sign (=)

Constants

• Constant is a variable that does not change such as tax rates, shipping fees, and values used in mathematical equations

– The const keyword is used to declare a constant. The name of a constant is usually all uppercase

– When you declare a constant, you need to assign the value to the constant

Chapter 5 30

Concatenation

• Concatenation is the process of joining one or more strings

– You can also use built-in methods to determine the length of the string, locate characters within the string, and truncate the spaces at the beginning or end of the string

– The string can be a literal string, or the result returned from an expression, or a variable that contains a string data type

Dim lblControlContent = Ch5Class.StoreName.ToString() _

& "<br/>" & Ch5Class.StoreEmail.ToString()

lblContact.Text = lblControlContent

Chapter 5 31

Data Types

• The two categories of types:

– Reference types are strings, classes, arrays, collections, and objects.

– Value types are also referred to as primitive types or structures; they are Boolean, Integer, Decimal, Char, and DateTime

• The managed heap contains memory addresses that are used to store variables

Chapter 5 32

String

– Strings are variable in length so you don’t have to specify the number of characters in the string when you create the string object

• Ltrim, Rtrim, and Trim remove the blank spaces from the preceding, ending, and both ends of the string respectively

• Asc provides the string’s ANSI character equivalent

• Chr provides the ASCII character equivalent. Chr is a useful method when you want to create control characters such as carriage returns

• Replace replaces one string with another

Chapter 5 33

String Object Methods

• Split returns a string that has been split using a delimiter to identify where to split the string

• Concat allows you to join strings together without using the concatenation operator

• Compare - allows you to compare two strings. If they are equal, the method will return 0. If the value on the left side is less than the value on the right side, then a negative integer is returned; otherwise a positive integer is returned

• LCase and UCase - converts the case to lower and upper case respectively

Dim Password As String

Password = LCase(txtPassword.Value)

LblPassword.Text = Password

Chapter 5 34

Char

• Char data type allows you to store a single text value as a number

– Stores a number between 0 and 65, 535

– Represents a character in categories such as digit, letter, punctuation, and control character

Chapter 5 35

Numeric Values

• Numeric Data Types

– Byte - stores an integer between 0 and 255

– Short - is a 16-bit number. Therefore, a short data type can only store values from -32,768 to 32,767

– Integer data type is a 32-bit whole number

– Long - is a 64-bit number

• Real number data types

– Single - represents a single-precision floating point number

– Double - supports larger numbers than the single data type

– Decimal - can store numbers up to 28 decimal places and is often used to store currency data

Chapter 5 36

DateTime

– DateTime data type is used to store any dates and

times between 01/01/0001 and 12/31/9999 as

mm/dd/yyyy and hh:mm:ss

– The date value is enclosed within a pair of pound

signs

Dim MyBirthday As DateTime

MyBirthday = #3/22/2002#

Chapter 5 37

Boolean

• A Boolean data type only has two possible

values, True value or False value

– In binary math, one represents true and zero

represents false

– In Visual Studio .NET the True value is converted

to -1, and the False value is 0

Chapter 5 38

Using a Property to Retrieve and Set the Value of a Variable

• To use a variable outside of a class from a different class, use the property function, or declare the variable public

– Set the value of the variable directly, or use a property

• A property is a method that is used to get and set values

• Property methods are used to keep private the variables internal to your code

– Property method - to retrieve and set the variable’s value

– Set the value and retrieve them indirectly

Chapter 5 39

Using a Property

• The ReadOnly and WriteOnly keywords are placed before the Property keyword

– ReadOnly prevents you from writing to the property

– WriteOnly allows the value to be changed, but not retrieved from the property

Public ReadOnly Property NewStoreName() As String

Public WriteOnly Property NewStoreName() As String

Chapter 5 40

Collections• Data can be stored in structures called collections

• A collection is an object that can store other elements

– By storing some types of data in a collection, you would not need to use a database

– The System.Collections namespace provides access to classes that manage data

– Each item in the collection is also referred to as an element

• These five collections

• ArrayList, HashTable, and SortedList collections provide access to any element without having to rotate through the other elements

• Queue and Stack collections need to rotate through the collection sequentially in order to locate an element

Chapter 5 41

ArrayList

• The ArrayList stores each item in sequential order, and each item is indexed with a number

– Do not need to define the size of the ArrayList

– Each item in the ArrayList is identified using an index number that indicates its position

– All of the items must be of the same data type

– ArrayLists are zero-based. The first item in the ArrayList is at position 0. So, an ArrayList size of 3 means it has 4 items

Chapter 5 42

Create an ArrayList

• Create an ArrayList

Dim StateAbbrev As New ArrayList

StateAbbrev.Add = "IL"

StateAbbrev.Add = "MI"

StateAbbrev.Add = "IN"

• Retrieve the value of any element individually, or using a for-next loop

Response.write(StateAbbrev())

• Insert an element into the first position and remove it

StateAbbrev.Insert(0, "OK")

StateAbbrev.Remove("OK")

Chapter 5 43

Properties and Methods of ArrayList

• Add and Remove will add or delete a single element.

• Insert and RemoveAt methods will add and remove elements at a specific index position.

• AddRange and RemoveRange methods allow you to add or remove a group of elements.

• IndexOf property allows you to find the position of the element in the list. A value of -1 means the element was not found in the list.

• Count property identifies the number of items in the array which will be the largest index number plus 1.

• Clear method allows you to remove all of the elements.

Chapter 5 44

HashTables

• HashTable collection creates the index of elements using an alphanumeric key like an encyclopedia

– The keys is a collection of alphanumeric values, and the values is a collection of elements

• Add and Remove method

– Items are added using the key and value pair separated with a comma

– The key is the first parameter passed with quotation marks

– The second parameter is the value

Chapter 5 45

Other Collections

• SortedList Class- Indexed by both the key and the item so the index position will change frequently. You can iterate through the list with a For-Next loop

• Queue Class - Provides sequential access to the elements but stores them in First In, First Out (FIFO). This is similar to a roller coaster ride. Typically, the people in the first car are let out first

• Stack class - Provides sequential access but stores them in Last In, First Out (LIFO) order. The Stack is similar to the line in a theatre, church, or crowded elevator. The first one to enter the room is the last one to leave the room

Chapter 5 46

Procedures

• Subprocedures do not return values and cannot be used in an expression value

• Create a Subprocedure

– Use the keywords public or private

– Declared using the keyword sub

– Exit sub statement to exit the subprocedure

– End with the keywords end sub

• Call keyword can be used to call a function or subprocedure and is optional

Chapter 5 47

Event Procedure

• You can intercept the event using an event handler

– Events such as click are intercepted by event handlers such as onServerClick

– An event handler is also known as an event procedure

– An event procedure is not executed until an event triggers the event procedure

– An event procedure does not return a value

• The Page_Load event procedure is triggered when the page is loaded into the browser

Chapter 5 48

Event Procedure

• Event procedures names are based upon the name of the object and the event name

– Event handler is identified with the prefix “on” and the event name

– An underscore (_) is used to separate the object name and the event name

Sub objectName_eventHandler(sender as Object, e as EventArgs)

action and control statements

End Sub

Chapter 5 49

Functions

• A function is a block of code that is grouped into a named unit.

– Built-in functions inherit from a .NET Framework class

– User defined functions are declared

• Public functions are visible to all other functions in the application

• Private functions are only available within the context where they are declared

Public Function GetStoreName() As Integer

'This function returns an integer

Return 23422

End Function

Chapter 5 50

Passing an Argument to a Function

• A pair of parentheses follows with zero or more arguments, also known as parameters, which are passed to the function when it is called

– If no arguments are passed, you use an empty pair of parentheses

– If multiple arguments are used, you use a comma to separate each argument

Chapter 5 51

Returning a Value From a Function

• The keyword return is used to identify the value that is returned to the function call

Exiting a Function

• Exit function is a jumping control; jumping controls allow you to temporarily halt the execution of a code block, and move to another section of code

Create a Function

• You can create a function within the class of the Web page, or within the Visual Basic .NET file

Chapter 5 52

Introduction to C#

• The syntax of C# is similar to JavaScript and C++

– The program is compiled by the C# compiler; the code is compiled into the same managed Intermediate Language code that is also generated by the Visual Basic .NET compiler

• The .NET Base Classes and the Visual Studio development environment are available across programming languages

– You have access to the same Windows Form tools and ASP.NET WebForm tools

Chapter 5 53

Differences with C#

• C# is case sensitive

– When you create an array using C#, instead of using parentheses around the index position, use square brackets

– Anytime you work with one or more statements, you enclose the code in curly braces { }

– To assign a value to a variable, specify the data type first, then specify the variable name

– Sample code to declare a variable and assign it a value

String StoreName = "Tara Store";

int counter = 1;

Chapter 5 54

Differences with C#

• Comments in C# are similar to comments in JavaScript

// This is a single line comment

/*

This is a multiline comment

Always document your code

*/

– Declare the variable public in C#; you use all lower case letters for the keyword public

– When you create an object data specify the data type of the object first, then the name of the object. Use the assignment operator to assign the name to a new Object() where the keyword new is lowercase