ASP.NET 2.0

1
ASP.NET 2.0

• Chapter 2
• Introduction to Programming

2
Objectives
3
Integrate Programming and Web Forms

• The Web Form is a web page that can contain
• HTML controls (ltpgt)
• Client-side scripts including JavaScript
• ASP.NET controls these run on the web server
  and are rendered as HTML and JavaScript on the
  client
• HTML server (ltp runatserver idP1gtlt/pgt
• Web server controls (ltaspcalendar runatserver
  idCalendar1 /gt
• Controls can be written manually or user can use
  the visual tools to add them to the Web Form

4
Integrate Programming and Web Forms (continued)

• Compiled server programs
• Inline in the page using lt gt delimiters
• Embedded
• Usually at the top of the page with ltscriptgt tags
  
• Set the runatserver property
• Change the language to VB
• Code behind the page
• Filename has the extension .aspx.vb
• Separating server programming and Web Forms
  allows you to alter your presentation tier
  without interfering with how the page is
  processed by the business programming code

5
Configure the Page Directive

• Set the default page-level properties
• Default Language property set to VB indicates the
  code behind the page is written in Visual Basic
  .NET
• lt_at_ Page Language"VB" gt
• File location is identified by the CodeFile
  property
• Inherits property indicates that the code behind
  the page inherits the partial class
• lt_at_ Page Language"VB" CodeFile"Login.aspx.vb"
  Inherits"Login" title" Login Page"
  AutoEventWireup"false" gt

6
Where to Place Your Programming Statements

• Only server controls interact with the code
  behind the page (web server and HTML server
  controls)
• Must set the ID property
• Must set the runatserver property
• Line continuation character is the underscore
• Do not break strings across lines
• Concatenate strings with or

7
Using Variables and Constants in a Web Form

• Variable declaration
• Declaration keywords define what parts of the web
  application will have access to the variable
• Variable name refers to the name of the variable
  as well as a section of memory in the computer
• Data type identifies what kind of data the
  variable can store such as integer or string of
  characters
• Multiple variables declare on a separate line, or
  use a single line
• Declare variables before used
• Explicit property of the Page directive
  configures the page to require you to declare all
  variables before use

8
Declaring a Variable

• Declaring a variable is the process of reserving
  the memory space for the variable before it is
  used in the program
• Scope identifies what context the application can
  access the variable
• Where your web application defines the variable
  determines where the variable can be used within
  the application
• Local variables
• Defined within a procedure
• Used only where they were declared
• Persist in memory while the procedure is being
  executed
• More readable, easier to maintain, require less
  memory
• Choose local variables unless multiple procedures
  require them

9
Declaring a Variable (continued)

• Module-level variables
• Defined in the class but outside the procedures
• Used by any procedures in the page
• Not available to other Web Forms
• Any procedure within the page can set or use
  value
• Reserve memory space before it is used
• When page loads, memory allocated and a value
  assigned
• After page unloads, memory can be reallocated

10
Declaring a Variable (continued)

• Keywords specify the scope of the variable
• Dim With no other keyword, its use will mean the
  variable is public
• Public Defines variable global or public,
  available outside the Web Form
• Friend Defines variables used only within the
  current web application or project
• Protected Defines variables used only by the
  procedure where declared

11
Declaring a Variable (continued)

• Variable Names
• Cannot use any of the Visual Basic .NET commands
  or keywords as your variable name
• Must begin with a letter
• Cannot use a period or space within a variable
  name
• Avoid any special characters in a variable name
  except for the underscore
• Store values in variables using the assignment
  operator, which is the equal sign ()
• Dim CompanyName As String "Barkley Builders"

12
Declaring Constants

• Assign a value that does not change
• Tax rates
• Shipping fees
• Mathematical equations
• Const keyword is used to declare a constant
• Naming rules for variables also apply
• Usually all uppercase
• Must assign the value when you declare the
  constant
• Const TAXRATE As Integer 8

13
Working with Different Data Types

• Text and Strings
• Numeric
• Date
• Boolean
• True -1
• False 0

14
Working with Text Data

• Character text stored in strings and chars
• Char data type stores a single text value as a
  number between 0 and 65,535
• String data type stores one or more text
  characters
• Social Security numbers, zip codes, and phone
  numbers
• Numbers stored as text strings are explicitly
  converted to numerical data before using in an
  arithmetic expression
• Assign a string a value the value must be within
  quotation marks
• Stored as Unicode and variable in length

15
Strings

• Modify string using methods of the String object
• Replace replaces one string with another
• Concatenation joins one or more strings
• Dim ContactEmail As String CompanyEmail.ToString
  () "ltbr /gt"
• Plus sign () represents the addition operator
  for mathematical equations and is used with
  non-strings technically can be used instead of
  , but it is not recommended

16
Working with Numeric Data

• Data Types
• Byte integer between 0 and 255 stored in a
  single byte
• Short 16-bit number from 32,768 to 32,767
  uses two bytes
• Integer 32-bit whole number uses four bytes
• Long 64-bit number uses eight bytes
• Single single-precision floating point
  (decimal) uses four bytes
• Double larger decimal numbers requires eight
  bytes
• Decimal up to 28 decimal places used to store
  currency
• Mathematical methods
• Such as Floor, Ceiling, Min, and Max, Sqrt,
  Round, Truncate
• Add, Equals, Divide, Multiply, and Subtract
• Can use arithmetic operators (such as / - )

17
Working with Numeric Data (continued)

• System.String.Format method
• Predefined Formats
• Number (N) is used to format the number using
  commas
• Currency (C or c) will insert the dollar symbol
  and two decimals
• Percent (P) will format the number as a percent
• Fixed number (F or f) represents a fixed number
  with two decimals
• System.String.Format("0C", 1234.45)
• String.Format("0P", 1234.45)
• Format("Percent", 1234.45)
• User-defined format style with a mask
• 0 to represent a digit, pound () to represent a
  digit or space
• 1,231 and 24.99 and 5.54
• Format(1231.08, ",")
• Format(24.99, ",.")
• Format(0.0554, "0.00")

18
Working with Date and Time

• System.DateTime enclosed within a pair of pound
  signs
• Dim MyBirthday As DateTime
• MyBirthday 3/22/2008
• Local time
• MyBirthday.UtcNow
• UCT is Coordinated Universal Time (Greenwich Mean
  Time-GMT)
• String.Format
• d and D masks short and long date formats
• t or T masks short and long time formats
• Dim ShippingDate As DateTime 3/12/2008
• Format(ShippingDate, "d")
• User-defined format style with a mask
• Format(MyDT, "m/d/yyyy Hmm") 9/9/2007 1734

19
Converting Data Types

• System.Convert class
• CInt function to convert string
  CInt(strVariable)
• Narrowing conversions
• Convert a decimal to an integer you may lose
  some data
• Convert 23.50 to 23 you lose the .50
• Overflow exception is thrown
• Not allowed by default because data could be lost
  
• Convert 23.00 to 23 no data is lost, the .00
  digits are not significant and no exception is
  thrown
• Widening conversion
• Convert 25 to 25.00 no data is lost
• Allowed because no data loss occurs
• Set the Strict property of the Page directive to
  True to stop narrowing conversion that would
  result in data loss
• Convert data types explicitly when narrowing
  conversions occur

20
Working with Web Collections

• Each item in the collection is also referred to
  as an element
• A collection is like a movie theatre both
  containers
• Refer to the movie theatre as a whole or to an
  individual seat
• Each seat has its own number to locate the seat
• Items in the collection can be any valid data
  type such as a string or an integer, an object,
  or even another collection
• Used to decrease processing every time the web
  application had to retrieve the list
• The Systems.Collections namespace defines
  collections including the ArrayList, HashTable,
  SortedList, Queue, and Stack

21
The ArrayList

• Create an ArrayList
• Dim StateAbbrev As New ArrayList
• StateAbbrev.Add("IL")
• StateAbbrev.Add("MI")
• StateAbbrev.Add("IN")
• Modify ArrayList items
• StateAbbrev.Insert(0, "OK")
• StateAbbrev.Remove("OK")
• Iterate through the list
• Dim iPos as Integer
• iPos StateAbbrev.IndexOf("OK")
• Dim MyCount As Integer
• MyCount StateAbbrev.Count
• For I 0 to MyCount -1
• MyStates.Text StateAbbrev(I) "ltbr /gt"
• Next

22
The HashTable

• HashTable creates index of items using an
  alphanumeric key like an encyclopedia
• Modify ArrayList items
• Dim HT1 As New HashTable()
• HT1.Add("1", "Mrs. Marijean Richards")
• HT1.Add("2", "1160 Romona Rd")
• HT1.Add("3", "Wilmette")
• HT1.Add("4", "Illinois")
• HT1.Add("5", "60093")
• Iterate through the list
• Dim CustomerLabel as String
• For each Customer in HT1
• CustomerLabel Customer.Value "ltbr
  /gt"
• Next Customer

23
Working with Web Objects to Store Data

• HTTP protocols send and receive data
• Data is transmitted in the header in the data
  packet
• Date and time
• Type of request (Get or Post)
• Page requested (URL)
• HTTP version
• Default language
• Referring page
• IP address of the client
• Cookies associated with that domain
• User agent used to identify the client software
• Can add custom data fields
• Programmatically access the information that is
  transferred with the request and response using
  various programming objects

24
Using the Request Object to Retrieve Data from
the Header

• HTTP collects and stores the values as a
  collection of server variables
• Request object of the Page class retrieves data
  sent by the browser
• Server variables can be retrieved by using an
  HTTPRequest object with Request.ServerVariables("V
  ARIABLE_NAME")
• Always in uppercase
• System.Web.HttpRequest class mapped to Request
  property of Page object
• Use Page.Request.PropertyName or
  Request.PropertyName
• Request.Url, Request.UserHostAddress,
  Request.PhysicalPath, Request.UrlReferrer.Host
• User agent provides a string to detect the
  browser version
• JavaScript string parsed to locate browser
  application name and version
• System.Web.HttpBrowserCapabilities class detects
  specific features directly
• Dim MyBrowser as String Request.Browser.Browser
  "ltbr /gt" _
• Request.Browser.Type "ltbr /gt"
  Request.Browser.Version "ltbr /gt" _
  Request.Browser.Platform

25
Using the Request Object to Retrieve Data from
the Header (continued)

• Request object to retrieve form data from the
  header
• Form collection
• QueryString collection
• If Form method is Get, results are HTML encoded
  and appended with a question mark to the URL
  requested as a single string called the
  QueryString
• www.course.com/index.aspx?CompanyNameGreen20Riv
  erIndustryElectronics
• Value is retrieved and assigned to a Label
  control
• Label1.Text Request.QueryString("CompanyName")

26
Using the Request Object to Retrieve Data from
the Header (continued)

• QueryString
• Only valid URL characters, has a fixed limit
  length, and not appropriate to send sensitive or
  personal information
• URL will not support spaces and special
  characters
• Manually encode and decode a string usingR2
• Server.UrlEncode(The String)
• Server.UrlDecode(The20String)
• List of System.Web.HttpRequest variables and
  properties view Quickstart Class Browser at
  www.asp.net/QuickStart/util/classbrowser.aspx

27
Accessing From Field Data

• Access the value properties of the form field
  directly for Server controls
• Property name varies
• CompanyName.Value
• Cross-page posting sends data from one form to
  another by setting the PostBackUrl property of an
  ImageButton, LinkButton, or Button control to the
  new page
• Detect if the user came from another page by
  determining if the Page.Previous property is null
  

28
Working with the Response Object

• Response object is mapped to System.Web.HttpRespon
  se class
• All data sent is sent via the HttpResponse class
  or via the Response object
• IP address of the server and the name and version
  number of the web server software
• Cookies collection sends cookies that are written
  by the browser in a cookie file
• Status code indicates the browser request was
  successful or any error
• Access status codes with a custom error message
  page
• Methods
• WriteFile method sends entire contents of a text
  file to the web page
• Write method sends a string to the browser
  including text, HTML tags, and client script
• Dim strMessage as String "Green River
  Electronicsltbr/gt"
• Response.Write(strMessage)

29
Working with the Response Object (continued)

• Response object redirects the browser to another
  page with server-side redirection
• Visitor never knows that he or she has been
  redirected to a new page
• Browser is redirected using the HTTP headers
• Response.Redirect("http//www.course.com/")
• Keep the ViewState information, and insert true
  as the second parameter
• Server.Transfer("Page2.aspx", true)

30
Session Objects

• Session object maintains session state across a
  single users session
• Session variables stored in the servers memory
  accessed only within the session
• Cannot change the value and stored in a special
  session cookie
• Cannot track across multiple sessions
• SessionID is a unique identifier determined by
  several factors, including the current date and
  IP addresses of the client and server
• Session("Session ID") Session.SessionID
• Session("Number") Session.Count.ToString
• Session.Timeout "30"

31
Session Objects (continued)

• Application variables are released from memory
  when web application is stopped, web server is
  stopped, or when the server is stopped
• Session("Member ID") TextBox1.Text
• Session("Date of visit") DateTime.Now.ToShortDat
  eString
• Session("URL requested") Request.Url.ToString
• Session("Server") _
• Request.ServerVariables("SERVER_NAME").ToString
• Save state information in session variables or in
  __VIEWSTATE
• ViewState persisted across browser requests
  stored with HTML code only about the server
  controls
• Serialized into a base64-encoded data string and
  not encrypted and can be viewed
• ViewState("Publisher") "Course Technology"
• Retrieve the value
• Dim MyPublisher As String CStr(ViewState("Publis
  her"))

32
Saving Data Using a Session Object
33
Design .NET Web Applications

• Windows and Web Applications are different
• Web application used in a browser or an
  Internet-enabled device
• Visual Basic .NET is a programming language
• ASP.NET is technology used to develop dynamic web
  applications within the .NET Framework
• .NET Framework consists of a common language
  runtime (CLR) and a hierarchical set of base
  class libraries
• A class is a named logical grouping of code
• Class definition contains the functions, methods,
  and properties that belong to that class

34
Web Applications Architecture
35
Organization of Classes in the .NET Framework

• Base class libraries are groups of commonly used
  built-in classes stored in executable files
• Accessed from any .NET application
• Contain fundamental code structures and methods
  to communicate with system and applications
• Organized in hierarchical logical groups called
  namespaces
• System namespace is at the top of the namespace
  hierarchy, and all built-in classes inherit from
  it
• Web controls located within the
  System.Web.UI.Control.WebControl namespace

36
Programming with Visual Basic .NET

• Linear programming executed sequentially in a
  single file
• Procedural programming used decision control
  structures, functions, and procedures to control
  the execution order
• Decision control structures used conditional
  expressions to determine which block of code to
  execute
• Object-oriented programming (OOP) methods
  stored in classes can be reused throughout your
  web application

37
Creating a Class in ASP.NET

• OOP create custom objects based upon the object
  definition
• Access procedures and properties of object across
  multiple web pages
• Object is a set of related, compartmentalized
  code based upon a class
• Create a user-defined class with no visual
  component but with global variables, properties,
  and procedures
• A component is a class with a visual or graphical
  user interface
• Often used to store access to the data and
  business logic

38
Creating a Class in ASP.NET (continued)

• Create object definition, called the base class
  or class definition
• Class is not accessed directly it is the code
  that will be used to create the procedures and
  properties of the object
• Can create many objects based upon the class
  definition
• Class is a template for the new object

39
Creating a Class in ASP.NET (continued)

• Create an instance of the class
• Instantiation is the process of declaring and
  initializing an object from a class
• The source file for the class ends in .vb, placed
  in a directory named App_Code
• Public Class MyClass
• Private CompanyName As String "Green River
  Electronics
• Use the class need to import on the first line
  in the code behind the page
• Imports MyClass()

40
Creating a Class in ASP.NET (continued)

• Declare a variable to store the object, and New
  to identify that this is an object based on a
  class definition
• In the class in the sample code below, the
  MyNewClass object is based on the MyClass class
• Dim MyNewClass As New GreenRiverWeb.MyClass()
• New object can access all of the variables,
  properties, functions, and procedures defined
  within MyClass
• Label1.Text MyNewClass.CompanyName

41
Using a Procedure

• Procedures contain one or more programming
  statements and are executed when called by
  another procedure or when an event occurs (event
  procedure)
• Pass zero or more arguments, called parameters,
  in a comma delimited list and data type
• Subprocedure can be called from other locations
  and reused
• Subprocedures do not return values
• Cannot be used in an expression value
• Declared using the keyword Sub
• Exit Sub statement to stop the subprocedure

42
Creating Subprocedures

• Example Note use _ to split lines)
• Sub SubprocedureName(CompanyName As String, _
• TotalEmployees As Integer)
• Programming statements go here
• End Sub
• Call the subprocedure
• Call SubprocedureName("Green River
  Electronics", 3400)

43
Creating Event Procedures

• Event procedure is attached to an object with
  events
• Not executed until triggered by an event
• Event handler intercepts an event
• n underscore (_) is used to separate the object
  name and the event name
• Sub objectName_event(Sender As Object, _
• e As EventArgs)
• Programming statements go here
• End Sub

44
Page Events

• Page lifecycle and events occur in order
• Can intercept event handlers within event
  procedure
• Page_Init initializes the page framework
  hierarchy, creates controls, deserializes the
  ViewState, and applies the previous settings to
  the controls
• Page_Load loads Server controls into memory
• Determine if the page has previously been loaded
• Page.IsPostback property
• Page_PreRender immediately before control
  hierarchy is rendered
• Page_Unload page is removed from the memory

45
Creating Functions

• Function is a block of code that is grouped into
  a named unit
• Built-in functions .NET Framework mathematical,
  date and time, string, and formatting functions
  or create your own function with a unique name
• End Function statement identifies the end of the
  function
• Leave the function with Exit Function statement
• Returns one value using the Return statement
  (same data type)
• Public Function GetDiscount(CustomerState As
  String) As Integer
• Dim MyDiscount as Integer
• If CustomerState "MI" then
• MyDiscount 10
• Else
• MyDiscount 5
• End If
• Return MyDiscount
• End Function
• Parameter and data type passed when you call the
  function
• Dim MembershipFee As Integer
• MembershipFee 100 - GetDiscount("MI")

46
Creating a Property Method

• Property method sets value of a variable defined
  in an object
• Used to expose private variables defined within
  the class
• All new objects inherit same properties as the
  original
• Public ReadOnly Property StoreName() As String
• Get
• Return StoreName
• End Get
• End Property
• Retrieve property
• lblContact.Text ch2_classname.StoreName.ToString
  ()

47
How Web Applications Interact with the
Systems.Drawing.Graphics.Class
48
Using the Systems.Drawing.Graphics.Class
49
Dynamically Creating Server Controls
50
Dynamically Creating a Hyperlink Control
51
Programming Control Structure

• Control statements determine both whether and how
  an action statement is executed, and the order
• Decision control structures alter the execution
  order of action statements on the basis of
  conditional expressions
• Conditional expression is evaluated by the
  program as true or false
• If Then and Select Case statements
• Loop structures repeat action statements on the
  basis of conditional expressions
• While, Do While, For Next, and For Each
• Can nest any of programming control structures

52
If Then Statement

• Two options for altering the order
• If (MyBrowser.ToString "IE") Then
• Response.Write("You are running
  Internet Explorer.")
• Else
• Response.Write("You are running a
  different browser.")
• End If
• Boolean expression uses comparison operators
  (true or false)
• If (MemberDuesPaid.Checked True) Then
• If (Page.IsPostback True) Then
• If (DuesPaid.Value lt DuesOwed.Value) Then
• If (RadioButtonList1.SelectedIndex gt -1) Then
• Can include logical operators in the conditional
  expressions
• If ((a 1) And (b 2)) Then
• If ((a 1) Or (b 2)) Then
• If Not Page.IsPostBack Then

53
Select Case Statement

• Select Case used to include multiple conditions
• Select Case TxtRole.Value
• Case "Manager"
• Response.Redirect("Manager.aspx")
• Case "Admin"
• Response.Redirect("Admin.aspx")
• Case Else
• Response.Redirect("Member.aspx")
• End Select

54
While Loop

• While loop repeats a block of code while
  conditional expression is true or Nothing
• Exit While escape the loop to stop an infinite
  loop
• Dim I As Integer 0
• While i lt 10
• i 1
• Response.Write(i)
• End While

55
Do Loop

• Use Do loops to repeat statements until a
  conditional is true
• Do While will repeat the loop until the condition
  is False
• Do Until will repeat the loop until the condition
  is True
• Do loop conditional evaluation is done at the end
  of the loop so there is at least one iteration
  loop (can fix number of loops)
• Variable i (the loop index variable) is
  initialized to zero
• Increment i each time the loop is executed
• Exit Do escape the loop to stop an infinite
  loop
• Unary operator such as used to increment (or
  decrement with --) a value by one
• VB use 1 I
• Dim i As Integer 0
• Do While i lt 10
• i 1
• Response.Write(i)
• Loop

56
For Next Loop

• For Next loop repeats statements a fixed number
  of times
• Identify conditional expression, loop index start
  number, end number, and step number to increment
  in the For statement
• Exit For to exit the loop
• Dim StartNum, EndNum, StepNum, i As Integer
• StartNum 0
• EndNum 5
• StepNum 2
• For i StartNum To EndNum Step StepNum
• Response.Write(i)
• Next
• Loop

57
For Each Loop

• Each loop accesses elements of a collection,
  array, or object properties
• Exit For exit the loop
• If SelectedIndex is greater than -1, at least one
  option was selected
• Subtract one because the count always starts at 0
• Dim Result As String
• If CBL.SelectedIndex gt -1 Then
• Result "You selected "
• Dim i As Integer
• For i 0 To CBL.Items.Count 1
• If CBL.Items(i).Selected Then
• Result CBL.Items(i).Text "ltbr /gt"
• End If
• Next
• Else
• Result "You did not select a category."
• End If
• lblTopics.Text Result

58
Nested Statements

• Can nest any (Can combine with If ElseIf)
• If (Page.IsPostback True) Then
• If (MemberLoginStatus.ToString "Current")
  Then
• Label1.Text "Welcome member!"
• Else
• Label1.Text "Please login!"
• End If
• Else
• Label1.Text "Welcome."
• End If

59
Introduction to Web Configuration and Debugging

• Debugging process detects programming errors
• Error handling is a collection of techniques that
  allow you to identify programming errors in your
  code, which are often called bugs
• Incorrect syntax or spelling errors detected
  during development with IntelliSense
• Programming logic errors not detected until
  runtime
• Code interprets error messages and executes when
  an error is detected, even generating custom
  error messages and global error handlers

60
Exception Handling

• Exception object is thrown when a predefined
  runtime error occurs
• If an exception is raised and not explicitly
  handled, a general ASP.NET exception occurs,
  forcing the application to terminate resulting in
  an error page
• Use when you include database, input/output to
  the file system, e-mail, or communicating with
  other applications
• Try-Catch-Finally statement to handle exceptions

61
Using Exception Classes to Identify Exception
Errors

• The SystemException class is the base class for
  all predefined exceptions
• ExternalException class allows other classes to
  indirectly inherit from the SystemException class
• ApplicationException class provides a base class
  to create user-defined exception objects
• SqlException is thrown SQL Server DataAdapter
  when the database server does not exist
• NullReferenceException a null object is
  referenced
• IndexOutOfRangeException an Array object is
  improperly indexed

62
Common Error Status Messages

• Status message is returned with HTTP header
  request
• Status Code 200 is Success
• Codes exposed by HttpStatusCode property of
  System.Net are enumerations and correlate to the
  HTTP Status Message Codes
• Retrieve using the HttpStatusCode property

63
Creating a Custom Error Message

• ErrorPage property of the Page directive
  default page
• customErrors node in the web.config file or Web
  Site Administration Tool (WSAT)
• Web.config file and customErrors node
  (ltcustomErrorsgt) configure a generic error page
  with defaultRedirect attribute
• Mode attribute of the customErrors node set to
  RemoteOnly
• Remote clients are redirected to custom error
  page
• Viewing the web site locally (localhost) displays
  error message

64
Creating a Custom Error Message (continued)
65
Using the Web Site Administration Tool
66
Using the Web Site Administration Tool (continued)

• Web configuration file
• ltcustomErrors defaultRedirect"/ch2_genericerror.
  aspx" mode"On"gt
• lterror statusCode"403" redirect"/ch2_noacces
  s.aspx" /gt
• lterror statusCode"404" redirect"/ch2_filen
  otfound.aspx" /gt
• lt/customErrorsgt

67
Configuring the Web Application to Use the
Debugger Tool

• Set at page level or in web configuration
• lt?xml version"1.0" encoding"UTF-8" ?gt
• ltconfigurationgt
• ltsystem.webgt
• ltcompilation
• debug"true" defaultLanguage"vb"
• explicit"true" strict"true" gt
• lt/compilationgt
• lt/system.webgt
• lt/configurationgt

68
Programming Best Practices

• Best practices are commonly accepted activities
  within a given discipline
• Add comments to document the structure and
  purpose of your programs to decrease the time
  required to maintain your program
• Use appropriate data types and programming
  structures to increase the efficiency of your
  program
• Avoid narrowing conversions to avoid data
  conversion errors
• Use explicit conversion techniques to avoid data
  conversion errors
• Use error handling and exception handling
  techniques when there are likely to be runtime
  errors, such as with database connections

69
Summary

• Variables store data
• Assign a data type to a variable when the
  variable is created
• Assign a value when the variable is created
• Properties set the value of a variable defined
  within an object
• A property is a method that is used to get and
  set values you can directly manipulate variables
  within a class, or you can use a property to
  indirectly read and change the variable
• ArrayLists and HashTables are examples of
  collections used to store many data objects
• Server controls can be created or populated
  programmatically

70
Summary (continued)

• Procedures organize the order in which the code
  is executed
• Event handlers execute code when an event occurs
• Functions return values with the keyword Return
• Subprocedures do not return a value
• Values passed as parameters are separated by
  commas and include data type
• Error handling techniques allow you to identify
  programming errors called bugs
• Status message codes are exposed by the
  HTTPStatusCode property
• SystemException class is the base class for all
  predefined exceptions.
• Configure custom error pages in web page or web
  configuration