Web forms in ASP.net

WEB FORMS
ASP.NET pages (officially known as web forms) are a
vital part of an ASP.NET application.

 Provide the actual output of a web application—the
web pages that clients request and view in their browsers.

 Web forms allow you to create a web application using
the some control-based interface as a Windows
application.
1

Page Processing
 Goal of ASP.NET developers is to develop
web forms in the same way that Windows
developers can build applications.
 Web applications are very different from
traditional rich desktop/client applications:
 Web applications execute on the server.
 Web applications are stateless.
2

HTML Forms
 Simplest way to send client-side data to the server is
using a <form> tag
 Inside the <form> tag, can place other <input> tags to
represent basic user interface ingredients

3

HTML Forms cont..

 ASP.NET uses control model.

string firstName = txtFirstName.Text;

4

Dynamic User Interface
 Control model makes life easier for retrieving form information

 In classic ASP insert a script block that would write the raw HTML
string message = "<span style="color:Red">Welcome " +
FirstName + " " + LastName + "</span>";
Response.Write(message);

 On the other hand, with Label control in ASP.NET

<asp:Label id="lblWelcome" runat="server" />
Now you can simply set its properties

lblWelcome.Text = "Welcome " + FirstName + " " + LastName;
lblWelcome.ForeColor = Color.Red;
Note: Not Necessary to KNOW HTML markup syntax.
5
Hides the low-level HTML details.

The ASP.NET Event Model
 Classic ASP uses a linear processing model.

Code execution is from start to end.
More code for even simple web page.
(Example of three buttons on HTML form)
 script code must determine which button is clicked
and execute code accordingly

 ASP.NET provides event-driven model.

6

The ASP.NET Event Model cont..
Here’s a brief outline of event driven model:
1. Your page runs for the first time. ASP.NET
creates page, & control objects, the
initialization code executes, and then the page
is rendered to HTML and returned to the
client.

2. At some point, the user does something that
triggers a postback, such as clicking a button.
& page is submitted with all the form data.

3. ASP.NET intercepts the returned page and re-
7 creates the page objects.

The ASP.NET Event Model cont..
4. Next, ASP.NET checks what operation
triggered the postback, and raises the
appropriate event procedure.
5. The modified page is rendered to HTML
and returned to the client. The page
objects are released from memory.
If another postback occurs, ASP.NET
repeats the process in steps 2 through 4.

POSTBACK
1.User request web form from server

2.Web server respond back with requested web form.

3.User enters the data and submits the form to web server.

4.Web server process the form and sends the result back
to the client

Step 3 ????
Step 3 & Step 4 ????

PostBack is the name given to the process of submitting
an ASP.NET page to the server for processing.

Automatic Postbacks
 All client action cannot be handled. e.g. mouse
movement because of server side processing.
 If want to do, can use Java Script or Ajax.
 ASP.NET web controls extend this model with an
automatic postback feature.
 To use automatic postback set the AutoPostBack
property of a web control to true (the default is false)
 ASP.NET adds a JavaScript function to the rendered
HTML page named __doPostBack() (AutoPostBack
=true)

10

View State
 View state solves another problem that occurs
because of the stateless nature of HTTP—lost
changes.
 ASP.NET has its own integrated state
serialization mechanism.
 ASP.NET examines all the properties, if changed
makes a note of this information in a name/value
collection.
 ASP.NET takes all the information it has
collected and then serializes it as a Base64 string.
11

View State cont..
The next time the page is posted back, ASP.NET follows these
steps:
1. ASP.NET re-creates the page and control objects based on
its defaults (first requested state)
2. Next, ASP.NET deserializes the view state information and
updates all the controls.
3. ASP.NET adjusts the page according to the posted back
form data.
4. Now event-handling code can get involved.
e.g. code can react to change the page, move to a new
page, or perform a completely different operation.
12

View State (First Request) cont..

13

View State(Postback Request) cont..

14

View State cont..
Note: Even if you set EnableViewState to false, the
control can still hold onto a smaller amount of
view state information.
This privileged view state information is known as
control state, and it can never be disabled.

Note: It is absolutely essential to your success as an
ASP.NET programmer to remember that the web form
is re-created with every round-trip. It does not persist
or remain in memory longer than it takes to render a
single request.
15

Web Forms Processing Stages
 On the server side, processing an ASP.NET web
form takes place in stages.
 The following list shows the major processing
stages
• Page framework initialization
• User code initialization
• Validation
• Event handling
• Automatic data binding
• Cleanup
16

Web Forms Processing Stages cont..

17

Page Framework Initialization

 ASP.NET first creates the page
 Generates all the controls defined
 ASP.NET deserializes the view state information
(not being requested for the first if it’s postback),
 Next Page.Init event fires (rarely handled by the
web page, because it’s still too early to perform
page initialization)

18

User Code Initialization
 At this stage of the processing, the Page.Load event is fired.
 The Page.Load event always fires, regardless of whether the
page is being requested for the first time or whether it is being
requested as part of a postback.
 To determine the current state of the page, check the
IsPostBack property of the page, which will be false the first
time the page is requested. Here’s an example:
if (!IsPostBack)
{
// It's safe to initialize the controls for the first time.
FirstName.Text = "Enter your name here";
}
19

Validation
 ASP.NET includes validation controls
that can automatically validate other user
input controls and display error messages.
 Validation controls fire after the page is
loaded but before any other events take
Place.
 validation controls are self-sufficient

20

Event Handling
 At this point, the page is fully loaded and validated.
 ASP.NET will now fire all the events since the last postback
 Immediate response events
 Change events
 ASP.NET’s event model is still quite different from a traditional Windows
environment
 If you change text in the text box and click submit button, ASP.NET raises all
of the following events (in this order):
• Page.Init
• Page.Load
• TextBox.TextChanged
• Button.Click
• Page.PreRender
21 • Page.Unload

Automatic Data Binding
 When you use the data source controls, ASP.NET
automatically performs updates and queries against your data
source as part of the page life cycle.
 Changes are performed after all the control events have
been handled but just before the Page.PreRender event fires.
 After the Page.PreRender event fires, the data source
controls perform their queries and insert the retrieved data
into linked controls.
 This is the last stop in the page life cycle.
 Page.PreRender is last action before the page is rendered
into HTML.
22

Cleanup
 At the end of its life cycle, the page is rendered to HTML.
 Page.Unload event is fired.
 At this point, the page objects are still available, but the
final HTML is already rendered and can’t be changed.
 garbage collection service that runs periodically to release
memory tied to objects
 unmanaged resources must be to release explicitly (e.g.
Windows file handles and ODBC database connections)
 When the garbage collector collects the page, the
Page.Disposed event fires.

23

Summary of Web Forms Processing Stages

• Page framework initialization
• User code initialization
• Validation
• Event handling
• Automatic data binding
• Cleanup

24

The Page As a Control Container
 To render a page, the web form needs to collaborate with all
its constituent controls.

 When ASP.NET first creates a page, it inspects the .aspx file
for each element it finds with the runat="server" attribute, it
creates and configures a control object, and then it adds this
control as a child control of the page.

 Page.Controls collection contains all child controls on the
page
 ASP.NET models the entire page using control objects,
including elements that don’t correspond to server-side
25
content.

The Page Header
 Web form can also contain a single HtmlHead
control, which provides server-side access to the <head>
tag.

 Visual Studio default is to always make the <head> tag
into a server-side control

 Head includes other details such as the title, metadata
tags (useful for providing keywords to search engines)

 Title: This is the title of the HTML page
 StyleSheet: IStyleSheet object that represents inline
26
styles

The Page Class
 All web forms are actually instances of the ASP.NET Page class
(System.Web.UI namespace)
 Code-behind class explicitly derives from System.Web.UI.Page.
 means that every web form you create is equipped with an enormous
amount of out-of-the-box functionality
 Page class gives your code the following extremely useful properties:
• Session
• Application
• Cache
• Request
• Response
• Server
• User
27 • Trace

Session, Application, and Cache
 The Session object is an instance of the
System.Web.SessionState.HttpSessionState class.
 The Session object provides dictionary-style access to a set of
name/value pairs.
 Session state is often used to maintain user specific information.
 The Application object is an instance of the
System.Web.HttpApplicationState class
 Data is global to the entire application.
 Cache object is an instance of the System.Web.Caching.Cache
class.
 Scalable storage mechanism because ASP.NET can remove
objects if server memory becomes scarce.
28

Request Object
 An instance of the System.Web.HttpRequest
class.
 Object represents the values and properties of
the HTTP request
 contains all the URLparameters and all other
information sent by a client.
 Much of the information provided by the
Request object is wrapped by higher-level
abstractions.
 Can examine cookies
29

Request Object cont..
Property Description
AnonymousID identifies the current user
ApplicationPath ApplicationPath gets the ASP.NET
and application’s virtual directory (URL), while
PhysicalApplicat PhysicalApplicationPath gets the “real”
ionPath directory.
Browser provides a link to an
HttpBrowserCapabilities object
ClientCertificate HttpClientCertificate object that gets the
security certificate for the current request
Cookies gets the collection of cookies sent with this
30
request

Form Represents the collection of form variables
that were posted back to the page.
Headers and collection of HTTP headers and server
ServerVariables variables, indexed by name.
IsAuthenticated & These return true if the user has been
IsSecureConnectio successfully authenticated and if the user is
n connected over SSL (Secure Sockets Layer).
IsLocal This returns true if the user is requesting the
page from the local computer.
QueryString provides the parameters that were passed
along with thequery string.
31

Url and Uri object that represents the current
UrlReferrer address for
the page and the page where the user
is coming from
UserAgent a string representing the browser type.
UserHostAddre get the IP address and the DNS name
ss and of the remote client
UserHostName
UserLanguages provides a sorted string array that lists
the client’s language preferences.
32

Response object

 Response object is an instance of the
System.Web.HttpResponse class
 HttpResponse does still provide some
important functionality—namely, cookie and
Redirect() method

33

Response object cont..
Member Description
BufferOutput Can set true or false
Cache HttpCachePolicy object that allows you to configure
output caching
Cookies collection of cookies sent with the response
Expires and these properties to cache the rendered HTML
ExpiresAbsolute
sClientConnected Boolean value indicating whether the client is still
connected to the server
Redirect() transfers the user to another page in your application
or a different website
Write() write text directly to the response
stream.
BinaryWrite() and allow you to take binary content from a byte array
WriteFile()
34
or from a file and write it directly to the response
stream

Server Object
Server object is an instance of the
System.Web.HttpServerUtility class.
Member Description
MachineName Computer name of the computer on which
the page is running.
GetLastError() Retrieves the exception object for the most
recently encountered error.
HtmlEncode() Changes an ordinary string into a string
and with legal HTML characters (and back
HtmlDecode() again).
UrlEncode() and Changes an ordinary string into a string
UrlDecode() with legal URL characters (and back
35
again).

Server Object cont..
Member Description
UrlTokenEncode Performs the same work as
() and UrlEncode() and UrlDecode(), except
UrlTokenDecode they work on a byte array that
() contains Base64-encoded data.
MapPath() Returns the physical file path that
corresponds to a specified virtual file
path.
Transfer() Transfers execution to another web
page in the current application.
36

Common HTML Entities

Result Description Encoded
Entity
Non breaking space  
< Less-than symbol <
> Greater-than symbol >
& Ampersand &
" Quotation mark &
37

User Object
 User object represents information about the user
making the request of the web server

 allows you to test that user’s role membership.

 Implements System.Security.Principal.Iprincipal

 can authenticate a user based on specific class
depends Windows account information using IIS or
through cookie-based authentication with a
dedicated login page.

Web forms in ASP.net

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