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

Request Object cont.. Property Description 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

Request Object cont.. Property Description 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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