Build Your Own ASP.NET 3.5 Website Using C# & VB (12 page)

developers:

■ They give us the ability to access HTML elements easily from within our code:

we can change these elements’ characteristics, check their values, or even update

them dynamically from our server-side programming language of choice.

■ ASP.NET controls retain their properties thanks to a mechanism called
view

state
. We’ll be covering view state later in this chapter. For now, you need to

know that view state prevents users from losing the data they’ve entered into a

form once that form has been sent to the server for processing. When the response

comes back to the client, text box entries, drop-down list selections, and so on

are all retained through view state.

■ With ASP.NET controls, developers are able to separate a page’s presentational

elements (everything the user sees) from its application logic (the dynamic portions of the ASP.NET page), so that each can be maintained separately.

■ Many ASP.NET controls can be “bound” to the data sources from which they

will extract data for display with minimal (if any) coding effort.

ASP.NET is all about controls, so we’ll be discussing them in greater detail as we

move through this book. In particular,
Chapter 4 explains many of the controls that

ship with ASP.NET. For now, though, let’s continue our dissection of an ASP.NET

page.

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ASP.NET Basics

35

Server-side Comments

Server-side comments allow you to include within the page comments or notes that

won’t be processed by ASP.NET. Traditional HTML uses the character

sequences to delimit comments; any information included between these tags won’t

be displayed to the user. ASP.NET comments look very similar, but use the sequences

<%--and --%>.

Our ASP.NET example contains two server-side comment blocks, the first of which

is the following:

LearningASP\VB\Hello.aspx
(excerpt)

<%--Display the current date and time --%>

The difference between ASP.NET comments and HTML comments is that ASP.NET

comments are not sent to the client at all; HTML comments are, so they’re not suited

to commenting out ASP.NET code. Consider the following example:

C#

<%= Title %>

-->

Here, it looks as though a developer has attempted to use an HTML comment to

stop a code render block from being executed. Unfortunately, HTML comments will

only hide information from the browser, not the ASP.NET runtime. So in this case,

although we won’t see anything in the browser that represents these two lines, they

will be processed by ASP.NET, and the value of the variable Title will be sent to

the browser inside an HTML comment, as shown here:

-->

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Build Your Own ASP.NET 3.5 Web Site Using C# & VB

The code could be modified to use server-side comments very simply:

C#

<%--<% string Title = "This is generated by a code render block."; %>

<%= Title %>

--%>

The ASP.NET runtime will ignore the contents of this comment, and the value of

the Title variable will not be output.

Literal Text and HTML Tags

The final elements of an ASP.NET page are plain old text and HTML. Generally,

you can’t do without these elements—after all, HTML allows the display of the information in your ASP.NET controls and code in a way that’s suitable for users and their browsers. Let’s take a look at the literal text and HTML tags that were used to

produce the display in the Visual Basic version of our sample page (the text and

HTML in the C# version is identical):

Visual Basic

LearningASP\VB\Hello.aspx
(excerpt)

<%@ Page Language="VB" %>

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

In the following sections, we’ll explore view state, discuss working with directives,

and shine a little light on the languages that can be used within ASP.NET.

View State

ASP.NET controls automatically retain their data when a page is sent to the server

in response to an event (such as a user clicking a button). Microsoft calls this persistence of data
view state
. In the past, developers would have had to resort to hacks to have the application remember the item a user had selected in a drop-down menu,

or store the content entered into a text box; typically, these hacks would have relied

on hidden form fields.

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Build Your Own ASP.NET 3.5 Web Site Using C# & VB

This is no longer the case. Once they’re submitted to the server for processing,

ASP.NET pages automatically retain all the information contained in text boxes and

drop-down lists, as well as radio button and checkbox selections. They even keep

track of dynamically generated tags, controls, and text. Consider the following code

written in the “ancient” ASP (not ASP.NET!) framework:

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

value="Click Me" />

<%

If Request.Form("nameTextBox") <> "" Then

Response.Write(Request.Form("nameTextBox"))

End If

%>

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Build Your Own ASP.NET 3.5 Web Site Using C# & VB