Using Visual Basic .NET within an ASP.NET Page

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Using Visual Basic .NET within an ASP.NET Page

• Introduction to ASP.NET
• By Kathleen Kalata

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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

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

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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

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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

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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

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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()

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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

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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

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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

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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 objects 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

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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

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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

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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

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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.)

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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_at_TaraStore.com
  "

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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

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Modifying the Code View Options
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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()

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Instantiate the Object

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

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Viewing the ClassVariables Page
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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

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IL Disassembler
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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

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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

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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

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Use the First Three Characters to Identify the
Data Type
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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

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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.ToStrin
  g() _
• "ltbr/gt" Ch5Class.StoreEmail.ToString()
• lblContact.Text lblControlContent

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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

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String

• Strings are variable in length so you dont 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 strings 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

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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

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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

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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

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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 hhmmss
• The date value is enclosed within a pair of pound
  signs
• Dim MyBirthday As DateTime
• MyBirthday 3/22/2002

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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

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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
  variables value
• Set the value and retrieve them indirectly

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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

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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

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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

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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")

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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