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ASP.NET : HTML Server Controls

Article by: David Sussman, et al (7/8/2002)
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Summary: Complete Sample chapter from 'Beginning ASP.Net with C#. The chapter introduces the use of web form controls in ASP.NET
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The following article is a complete sample chapter from Wrox Press's Beginning ASP.NET with C#
Buy the complete book here.

HTML Server Controls

One of the most common tasks any web developer will perform is the collecting and storing of information from the user. This could simply be a name and e-mail address, or it might range across a whole gamut of details including address, phone number, fax, credit card, and delivery address. Whatever the information you want to gather, the processing cannot be performed within the confines of HTML on the browser alone. Therefore, you need to send the information to the web server for processing. Once the web server has extracted the requisite information, an updated version of the page, or possibly on some occasions, a separate second page, is returned to the user.

 

Information is transmitted via web pages via a form, and in HTML, there are specialized tags for dealing with these. HTML forms contain a set of HTML controls, such as textboxes, checkboxes, and drop-down lists, all of which aid the passage of information from the user to the server. On top of this, ASP.NET adds its own extra controls for dealing with forms. With these, ASP.NET introduces some new concepts to the control of forms. Previously with HTML forms, for example, when you selected a particular form control, the web browser dealt with the entire handling of the form data until it was passed to the web server. However, there are now new features of the .NET Framework, such as remembering what text you've typed into a textbox, or what selection you made in a listbox between page refreshes, which is carried out on the server.

 

While we're going to be using some ASP.NET code to handle the interchange of form data, we won't be explaining all of the facets of ASP.NET that we will use, only those that relate directly to forms. To keep things clear, we're not going to start explaining exactly how it all works until later chapters, but by the end of the chapter, you will understand how to send information between browser and server via ASP.NET code. You'll also be introduced to some of the terminology associated with this process.

 

In this chapter we will cover:

 

q        The client-server model of the web

q        HTML forms and web forms

q        HTML form controls

q        Server Controls


 

Forms in the Real World

The main focus of the chapter is using forms, andimplicitly, the transfer of data from the browser to the server. Before we start delving into the inner workings of the form, we'll describe a few situations in which forms would be required in the business world, to see what kind of things they are used for. If you take a look at a few commercial web sites, you'll find that forms are usually provided in the same kinds of situations, such as:

 

q        To take information from a user. This could be for the purpose of registration, the purchase of a product, or joining an e-mail list/form/newsgroup.

q        To take note of a user's preferences so that we can customize other pages in the site to include relevant information, and exclude things that don't interest them.

q        To provide a questionnaire or survey on how a business may go about improving the service that it offers.

q        To act as front end for a forum or newsgroup, where a user can enter and edit their text online.

 

These are just a few examples of some common everyday situations. In this chapter, we're just going to approach one facet of such a situation for a fictional business. Thisbusiness, Feiertag Holidays, requires a web site that will allow users to view different destinations within Europe, and then browse through details of different hotels at each destination. Our forms will allow the user to select a destination and send the details of their choice to the web server. We will see how ASP.NET forms can be used to expedite the process.

 

One of the main advantages of using this in a typical full-blown business application is that rather than having to create a unique page for each separate destination, as might have to be done in the real world, we can create a generic page, dynamically generated by ASP.NET, which fills in details about each destination, and therefore requires a lot less coding.

 

However, before we get into that, it's best to take a quick overview of forms, and see the ways in which ASP.NET affects them.

Web Pages, HTML Forms, and Web Forms

With the introduction of any newtechnology comes new terminology and jargon. ASP.NET is no different in this respect. With ASP.NET, even the terms you use to describe a simple web page have been updated to more accurately describe the processes that are going on within them. To avoid confusion, we're going to start by defining a few familiar concepts and their ASP.NET equivalents. Let's begin with a web page.

 

Everybody reading this should know what a web page is it's just a bundle of HTML code, made up of markup tags, nominally beginning and ending with <html> and </html> tags. The web page is placed on a machine, known as a web server, and it is the job of the web server to make that page available to all and sundry. Whether that page contains text, graphics, movies, sound, or bits and pieces of other languages/technologies or whether it was dynamically generated is of no concern to us.


An HTML form is a web page that contains one or more form controls (grouped together inside an HTML <form> element) that allow the user to enter information on the web page and send that information back to the web server. Commonly used form controls include buttons, textboxes, checkboxes, and drop-down lists. The user fills in details and presses a 'submit' button to send their data back to the web server.

 

Although you don't need anything more than HTML to send form data to the server, the server needs some sort of extra technology (in this case, ASP.NET) to actually do anything with the information it receives. HTML pages containing forms are typically saved with the suffix .html (or sometimes .htm).

 

ASP.NET goes one further than this and introduces a new concept, the web form. The web form is similar to the HTML forms we've seen before, and visually you wouldn't be able to differentiate between an HTML form and a web form, but it's what ASP.NET does behind the scenes with the web forms operation that make it quite a different entity. Firstly, the term web form refers to the grouping of two distinct blocks of code:

 

q        The HTML template containing page layoutinformation and ASP.NET server controls (see below). This is responsible for the presentation of the web form on the browser.

q        The ASP.NET code that providesthe web form's processing logic. This is responsible for generating dynamic content to be displayed within the web form. This content is typically exposed via server controls defined in the HTML presentation block.

Although a web form may also be an HTML form (that is, there's nothing to stop us using <form> elements inside an ASPX), remember that these two entities are defined in quite distinct terms.

When we start using ASP.NET within our web pages, and we create a web form, we can then use a new breed of ASP.NET server controls within our HTML (we looked at these very briefly in the last chapter). Not only do they duplicate the functionality of many HTML elements (including the form controls), but they also do a lot more besides. A server control has the appearance of an HTML-like element, but actually it only marks a point in the page at which the server needs to generate a corresponding true-HTML element. The advantage this offers over an HTML control is that we can create content for the form, before returning the form to the browser, and we can generate this content from just about anywhere in our code.

 

As we've already seen, the ASP.NET code can be specified in a <script> block that may occur at any point within the ASPX file. We're keeping it at the top of the code page, to help clarify the separation of presentation and content. As we'll see in Chapter 15 though, we can ultimately place the ASP.NET code into a completely separate file (a technique known as code behind). What's important is that you recognize that when we talk about a web form, we're referring to both these sections, regardless of where they are, or how they're organized.


So, we know that it is possible for web forms to use normal HTML form controls, but ASP.NET also comes with its own set of web form controls that are run on the server. We will be using these in preference most of the time, because they offer other advantages such as being able to remember the state of the different controls, such as what text has been typed into a textbox. These ASP.NET controls are run within specially modified HTML <form runat="server"> tags, and are ASP.NET forms.

There are four different terms here that we need to be clear about before we go any further:

 

q        A web page is any page that contains just HTML (they can also contain script/other languages not covered by this book, but in this book a web page will refer to pages containing only HTML)

q        An HTML form is an HTML element that contains HTML form controls

q        A web form is any page that combines ASP.NET code with an HTML template

q        An ASP.NET form is a form inside a web form, that contains ASP.NET server controls

 

Because our discussion of forms is essentially a discussion of how to transmit data from a browser back to the web server, we need to start out by considering the whole process of data transmission on the Web, so that we can put the role of forms into context. It's worth emphasizing first though that no matter which technology you use, the browser will always ultimately receive HTML.

 

So let's now look at how web browsers and web servers work together to make web pages available to the world.

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© Copyright 2002 Wrox Press This chapter is written by David Sussman, et al and taken from "Beginning ASP.NET with C#" published by Wrox Press Limited in June 2002; ISBN 1861007345; copyright Wrox Press Limited 2002; all rights reserved.

No part of these chapters may be reproduced, stored in a retrieval system or transmitted in any form or by any means -- electronic, electrostatic, mechanical, photocopying, recording or otherwise -- without the prior written permission of the publisher, except in the case of brief quotations embodied in critical articles or reviews.






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