Intro to ASP.NET (VBL).pdf

Introduction to ASP.NET

Introduction to

ASP.NET

Objectives

•

Review the features and shortcomings of classic Active Server Pages.

•

Understand the advantages of an ASP.NET application.

•

Learn about server controls and events in ASP.NET.

•

Create a simple Web Service in ASP.NET.

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Introduction to ASP.NET

A Review of Classic ASP

You may be an experienced developer of Web applications that use a previous

version of Microsoft Active Server Pages, or “classic” ASP. Or, you may be

just getting started as a Web developer. In either case, it is useful to review the

Active Server Page technology that preceded ASP.NET.

Dynamically Creating Web Pages

ASP is a technology that Microsoft created to ease the development of

interactive Web applications. When the Internet began, it was used to transmit

static HTML pages in response to HTTP requests from a browser. These Web

pages included hyperlinks that allowed users to navigate easily from page to

page, but that was about the extent of their interactivity.

As the Internet evolved, Web publishers developed ever more sophisticated

ways of allowing users to have more control over the pages they received.

HTML forms allowed users to enter information using simple Windows-like

controls such as text boxes, list boxes, and check boxes. The data from these

controls is embedded in the HTTP request that is issued when the user clicks a

Submit button.

Web server software execution technologies like CGI (Common Gateway

Interface) evolved to meet the need for more interactivity by intercepting

certain Web requests and running programs that created and returned custom

Web pages, usually based on data retrieved from a database.

After a couple of false starts (anyone remember ADC/HTX files?), Microsoft

settled on Active Server Pages as their way of supporting dynamic creation of

HTML pages based on user input.

An ASP Example

See Products-

ASP.asp

Fig ure 1 sho ws the results of running a typical ASP application. Microsoft

installs an ISAPI application with IIS that intercepts all requests for pages with

an extension of .asp.

These requests and the pages they point to are then handled within the ASP

run-time environment. This execution environment allows pages to contain

code in special script blocks delimited by < % and % > characters or contained

within < script > elements that include the attribute, runat=server. This script

performs whatever processing tasks are necessary to generate a custom HTML

page in response to the user’s request.

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Web Pages and Web Services Professional Skills Development

A Review of Classic ASP

Figure 1. A typical “classic” Active Server Page.

The ASP page in this example begins with a couple of script-related lines that

identify the language and specify that all variables will be explicitly declared:

<%@ Language=VBScript %>

<%Option Explicit%>

The next lines are typical HTML that creates a title, defines an HTML form,

adds some text to the page, and begins a drop-down list box:

<html>

<head>

<title>Products-ASP</title>

</head>

<body>

Select a Category:

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Introduction to ASP.NET

Embedding Data Access Code in the Page

Now, the page gets more interesting (and more complicated!), using ADO

objects to retrieve a list of product categories from the Northwind database on

a local SQL Server:

Dim cnn, cmd, rst

Set cnn=Server.CreateObject("ADODB.Connection")

' Adjust user name and password, if necessary

cnn.Open "Provider=SQLOLEDB.1;Data Source=(local);" _

& "Initial Catalog=Northwind;User ID=sa;Password=;"

' Open a read-only, forward-only, server-side recordset.

Set cmd=Server.CreateObject("ADODB.Command")

cmd.CommandText= _

"SELECT CategoryID, CategoryName FROM Categories" _

& " ORDER BY CategoryName;"

cmd.ActiveConnection=cnn

Set rst=cmd.Execute

The ASP Object Model

The code above uses the CreateObject method of the ASP Server object to

instantiate the ADO objects used for data access. In addition to providing a

script execution engine, ASP also provides a set of six objects, including

Server, to facilitate the development of Web applications. Here is a brief

summary of these objects:

•

The Request object is used to read data that was packaged inside the

HTTP request for the page.

•

The Response object allows you to inject data, including HTML,

cookies, or redirection headers into the response stream that is sent

back to the client’s browser.

•

A Session object is created when the first request from a particular

client is processed, and it stays in scope until a timeout period expires

following the last request from that user, allowing you to store data

and objects that span multiple requests from one user.

•

The Application object is similar to the Session object, but its data is

shared across all client requests over the lifetime of the application,

and it also allows you to write code that runs automatically when the

applications starts or ends.

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Web Pages and Web Services Professional Skills Development

A Review of Classic ASP

•

The ObjectContext object is used to commit or abort transactions

managed by MTS or COM+.

•

The Server object provides a set of generic utility methods for

creating COM objects, encoding data as HTML or URL strings that

can be embedded within the HTML sent back to a browser, and

finding the actual file locations that correspond to virtual paths.

Getting back to our sample, the next lines of code use the ASP Response

object to inject HTML into the HTTP response being sent back to the browser.

The category ID from each row in the recordset is assigned as the value for

each row of the drop-down list box:

' Use the ADO recordset to populate the dropdown list.

Do Until rst.EOF

Response.Write( _

"<option value=""" & rst("CategoryID") & """")

Handling ASP Postbacks

A postback is what happens when an Active Server Page creates HTML that

allows the user to call the same ASP all over again. In the example, the page

Products-ASP.asp is reloaded every time the user clicks the Show Products

button. This happens because the page’s address appeared in the action

attribute of the opening tag for the HTML form:

When the user clicks the button a postback occurs, and the drop-down list is

populated all over again. But you want the users to see the category when they

get the new page, which will also now show them the products in that

category. The remainder of this section of code ensures that the category

matching the one in the HTTP request will be selected in the new page, and it

also adds the category name as the text for each row in the drop-down list box:

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