15.1 Understanding Web Forms | C# Programming: From Problem Analysis to Program Design

Web Forms implement a programming model in which web pages are dynamically generated on a web server for delivery to a browser over the Internet. They are, in some ways, the successor to ASP pages, and they marry ASP technology with traditional programming.

With Web Forms, you create an HTML page with static content, and you write C# code to generate dynamic content. The C# code runs on the server, and the data produced is integrated with your static HTML to create the web page. What is sent to the browser is standard HTML.

Web Forms are designed to run on any browser, with the server rendering the correct browser-compliant HTML. You can do the programming for the logic of the Web Form in any .NET language. I will of course use C#, which is arguably the language of choice, though some ASP developers who have used VBScript might opt for VB.NET.

Just as with Windows Forms, you can create Web Forms in Notepad (or another editor of your choice) rather than in Visual Studio. Many developers will choose to do so, but Visual Studio makes the process of designing and testing Web Forms much easier.

Web Forms divide the user interface into two parts: the visual part or user interface (UI), and the logic that lies behind it. This is very similar to developing Windows Forms as shown in Chapter 14, but with Web Forms the UI page and the code are in separate files.

The UI page is stored in a file with the extension .aspx. The logic (code) for that page can be stored in a separate code-behind C# source file. When you run the form, the code-behind class file runs and dynamically creates the HTML sent to the client browser. This code makes use of the rich Web Forms types found in the System.Web and System.Web.UI namespaces of the .NET Framework Class Library (FCL).

With Visual Studio, Web Forms programming couldn't be simpler: open a form, drag some controls onto it, and write the code to handle events. Presto! You've written a web application.

On the other hand, even with Visual Studio, writing a robust and complete web application can be a daunting task. Web Forms offer a very rich UI; the number and complexity of web controls have greatly multiplied in recent years, and user expectations about the look and feel of web applications have risen accordingly.

In addition, web applications are inherently distributed. Typically, the client will not be in the same building as the server. For most web applications, you must take network latency, bandwidth, and network server performance into account when creating the UI; a round trip from client to host might take a few seconds.

15.1.1 Web Form Events

Web Forms are event-driven. An event is an object that encapsulates the idea that "something happened." An event is generated (or raised) when the user presses a button, or selects from a list box, or otherwise interacts with the UI. Events can also be generated by the system starting or finishing work. For example, open a file for reading, and the system raises an event when the file has been read into memory.

The method that responds to the event is called the event handler. Event handlers are written in C# in the code-behind page and are associated with controls in the HTML page through control attributes.

Event handlers are delegates (see Chapter 12). By convention, ASP.NET event handlers return void and take two parameters. The first parameter represents the object raising the event. The second, called the event argument, contains information specific to the event, if any. For most events, the event argument is of type EventArgs, which does not expose any properties. For some controls, the event argument might be of a type derived from EventArgs that can expose properties specific to that event type.

In web applications, most events are typically handled on the server and, therefore, require a round trip. ASP.NET only supports a limited set of events, such as button clicks and text changes. These are events that the user might expect to cause a significant change, as opposed to Windows events (such as mouse-over) that might happen many times during a single user-driven task.

15.1.1.1 Postback versus non-postback events

Postback events are those that cause the form to be posted back to the server immediately. These include click-type events, such as the Button Click event. In contrast, many events (typically change events) are considered non-postback in that the form is not posted back to the server immediately. Instead, these events are cached by the control until the next time that a postback event occurs. You can force controls with non-postback events to behave in a postback manner by setting their AutoPostBack property to true.

15.1.1.2 State

A web application's state is the current value of all the controls and variables for the current user in the current session. The Web is inherently a "stateless" environment. This means that every post to the server loses the state from previous posts, unless the developer takes great pains to preserve this session knowledge. ASP.NET, however, provides support for maintaining the state of a user's session.

Whenever a page is posted to the server, it is recreated by the server from scratch before it is returned to the browser. ASP.NET provides a mechanism that automatically maintains state for server controls. Thus, if you provide a list and the user has made a selection, that selection is preserved after the page is posted back to the server and redrawn on the client.

15.1.2 Web Form Life Cycle

Every request for a page made from a web server causes a chain of events at the server. These events, from beginning to end, constitute the life cycle of the page and all its components. The life cycle begins with a request for the page, which causes the server to load it. When the request is complete, the page is unloaded. From one end of the life cycle to the other, the goal is to render appropriate HTML output back to the requesting browser. The life cycle of a page is marked by the following events, each of which you can handle yourself or leave to default handling by the ASP.NET server:

Initialize: Initialize is the first phase in the life cycle for any page or control. It is here that any settings needed for the duration of the incoming request are initialized.
Load ViewState: The ViewState property of the control is populated. The ViewState information comes from a hidden variable on the control, used to persist the state across round trips to the server. The input string from this hidden variable is parsed by the page framework, and the ViewState property is set. This can be modified via the LoadViewState( ) method. This allows ASP.NET to manage the state of your control across page loads so that each control is not reset to its default state each time the page is posted.
Process Postback Data: During this phase, the data sent to the server in the posting is processed. If any of this data results in a requirement to update the ViewState, that update is performed via the LoadPostData( ) method.
Load: CreateChildControls( ) is called, if necessary, to create and initialize server controls in the control tree. State is restored, and the form controls show client-side data. You can modify the load phase by handling the Load event with the OnLoad( ) method.
Send Postback Change Modifications: If there are any state changes between the current state and the previous state, change events are raised via the RaisePostDataChangedEvent( ) method.
Handle Postback Events: The client-side event that caused the postback is handled.
PreRender: This is the phase just before the output is rendered to the browser. It is essentially your last chance to modify the output prior to rendering using the OnPreRender( ) method.
Save State: Near the beginning of the life cycle, the persisted view state was loaded from the hidden variable. Now it is saved back to the hidden variable, persisting as a string object that will complete the round trip to the client. You can override this using the SaveViewState( ) method.
Render: This is where the output to be sent back to the client browser is generated. You can override it using the Render method. CreateChildControls( ) is called, if necessary, to create and initialize server controls in the control tree.
Dispose: This is the last phase of the life cycle. It gives you an opportunity to do any final cleanup and release references to any expensive resources, such as database connections. You can modify it using the Dispose( ) method.