ASP.NET Practical Technique

1. Building ASP.NET Pages

ASP.NET and the .NET Framework

ASP.NET is part of Microsoft's overall .NET framework, which contains a vast set of programming classes designed to satisfy any conceivable programming need. In the following two sections, you learn how ASP.NET fits within the .NET framework, and you learn about the languages you can use in your ASP.NET pages.

The .NET Framework Class Library

Imagine that you are Microsoft. Imagine that you have to support multiple programming languages—such as Visual Basic, JScript, and C++. A great deal of the functionality of these programming languages overlaps. For example, for each language, you would have to include methods for accessing the file system, working with databases, and manipulating strings.

Furthermore, these languages contain similar programming constructs. Every language, for example, can represent loops and conditionals. Even though the syntax of a conditional written in Visual Basic differs from the syntax of a conditional written in C++, the programming function is the same.

Finally, most programming languages have similar variable data types. In most languages, you have some means of representing strings and integers, for example. The maximum and minimum size of an integer might depend on the language, but the basic data type is the same.

Maintaining all this functionality for multiple languages requires a lot of work. Why keep reinventing the wheel? Wouldn't it be easier to create all this functionality once and use it for every language?

The .NET Framework Class Library does exactly that. It consists of a vast set of classes designed to satisfy any conceivable programming need. For example, the .NET framework contains classes for handling database access, working with the file system, manipulating text, and generating graphics. In addition, it contains more specialized classes for performing tasks such as working with regular expressions and handling network protocols.

The .NET framework, furthermore, contains classes that represent all the basic variable data types such as strings, integers, bytes, characters, and arrays.

Most importantly, for purposes of this book, the .NET Framework Class Library contains classes for building ASP.NET pages. You need to understand, however, that you can access any of the .NET framework classes when you are building your ASP.NET pages.

Understanding Namespaces

As you might guess, the .NET framework is huge. It contains thousands of classes (over 3,400). Fortunately, the classes are not simply jumbled together. The classes of the .NET framework are organized into a hierarchy of namespaces.

ASP Classic Note

In previous versions of Active Server Pages, you had access to only five standard classes (the Response, Request, Session, Application, and Server objects). ASP.NET, in contrast, provides you with access to over 3,400 classes!

A namespace is a logical grouping of classes. For example, all the classes that relate to working with the file system are gathered together into the System.IO namespace.

The namespaces are organized into a hierarchy (a logical tree). At the root of the tree is the System namespace. This namespace contains all the classes for the base data types, such as strings and arrays. It also contains classes for working with random numbers and dates and times.

You can uniquely identify any class in the .NET framework by using the full namespace of the class. For example, to uniquely refer to the class that represents a file system file (the File class), you would use the following:

System.IO.File

System.IO refers to the namespace, and File refers to the particular class.

NOTE

You can view all the namespaces of the standard classes in the .NET Framework Class Library by viewing the Reference Documentation for the .NET Framework.

Standard ASP.NET Namespaces

The classes contained in a select number of namespaces are available in your ASP.NET pages by default. (You must explicitly import other namespaces.) These default namespaces contain classes that you use most often in your ASP.NET applications:

System— Contains all the base data types and other useful classes such as those related to generating random numbers and working with dates and times.

System.Collections— Contains classes for working with standard collection types such as hash tables, and array lists.

System.Collections.Specialized— Contains classes that represent specialized collections such as linked lists and string collections.

System.Configuration— Contains classes for working with configuration files (Web.config files).

System.Text— Contains classes for encoding, decoding, and manipulating the contents of strings.

System.Text.RegularExpressions— Contains classes for performing regular expression match and replace operations.

System.Web— Contains the basic classes for working with the World Wide Web, including classes for representing browser requests and server responses.

System.Web.Caching— Contains classes used for caching the content of pages and classes for performing custom caching operations.

System.Web.Security— Contains classes for implementing authentication and authorization such as Forms and Passport authentication.

System.Web.SessionState— Contains classes for implementing session state.

System.Web.UI— Contains the basic classes used in building the user interface of ASP.NET pages.

System.Web.UI.HTMLControls— Contains the classes for the HTML controls.

System.Web.UI.WebControls— Contains the classes for the Web controls.

.NET Framework-Compatible Languages

For purposes of this book, you will write the application logic for your ASP.NET pages using Visual Basic as your programming language. It is the default language for ASP.NET pages. Although you stick to Visual Basic in this book, you also need to understand that you can create ASP.NET pages by using any language that supports the .NET Common Language Runtime. Out of the box, this includes C#, JScript.NET, and the Managed Extensions to C++.

NOTE

The CD included with this book contains C# versions of all the code samples.

Dozens of other languages created by companies other than Microsoft have been developed to work with the .NET framework. Some examples of these other languages include Python, SmallTalk, Eiffel, and COBOL. This means that you could, if you really wanted to, write ASP.NET pages using COBOL.

Regardless of the language that you use to develop your ASP.NET pages, you need to understand that ASP.NET pages are compiled before they are executed. This means that ASP.NET pages can execute very quickly.

The first time you request an ASP.NET page, the page is compiled into a .NET class, and the resulting class file is saved beneath a special directory on your server named Temporary ASP.NET Files. For each and every ASP.NET page, a corresponding class file appears in the Temporary ASP.NET Files directory. Whenever you request the same ASP.NET page in the future, the corresponding class file is executed.

When an ASP.NET page is compiled, it is not compiled directly into machine code. Instead, it is compiled into an intermediate-level language called Microsoft Intermediate Language (MSIL). All .NET-compatible languages are compiled into this intermediate language.

An ASP.NET page isn't compiled into native machine code until it is actually requested by a browser. At that point, the class file contained in the Temporary ASP.NET Files directory is compiled with the .NET framework Just in Time (JIT) compiler and executed.

The magical aspect of this whole process is that it happens automatically in the background. All you have to do is create a text file with the source code for your ASP.NET page, and the .NET framework handles all the hard work of converting it into compiled code for you.

ASP CLASSIC NOTE

What about VBScript? Before ASP.NET, VBScript was the most popular language for developing Active Server Pages.

ASP.NET does not support VBScript, and this is good news. Visual Basic is a superset of VBScript, which means that Visual Basic has all the functionality of VBScript and more. So, you have a richer set of functions and statements with Visual Basic.

Furthermore, unlike VBScript, Visual Basic is a compiled language. This means that if you use Visual Basic to rewrite the same code that you wrote with VBScript, you can get better performance.

If you have worked only with VBScript and not Visual Basic in the past, don't worry. Since VBScript is so closely related to Visual Basic, you'll find it easy to make the transition between the two languages.

NOTE

Microsoft includes an interesting tool named the IL Disassembler (ILDASM) with the .NET framework. You can use this tool to view the disassembled code for any of the ASP.NET classes in the Temporary ASP.NET Files directory. It lists all the methods and properties of the class and enables you to view the intermediate-level code.

This tool also works with all the ASP.NET controls discussed in this chapter. For example, you can use the IL Disassembler to view the intermediate-level code for the TextBox control (located in a file named System.Web.dll).

Introducing ASP.NET Controls

ASP.NET controls provide the dynamic and interactive portions of the user interface for your Web application. The controls render the content that the users of your Web site actually see and interact with. For example, you can use controls to create HTML form elements, interactive calendars, and rotating banner advertisements.

ASP.NET controls coexist peacefully with HTML content. Typically, you create the static areas of your Web pages with normal HTML content and create the dynamic or interactive portions with ASP.NET controls.

The best way to understand how ASP.NET controls work in an HTML page is to look at a simple Web Forms Page.

Adding Application Logic to an ASP.NET Page

The second building block of an ASP.NET page is the application logic, which is the actual programming code in the page. You add application logic to a page to handle both control and page events.

If a user clicks a Button control within an HTML form, for example, the Button control raises an event (the Click event). Typically, you want to add code to the page that does something in response to this event. For example, when someone clicks the Button control, you might want to save the form data to a file or database.

Controls are not the only things that can raise events. An ASP.NET page itself raises several events every time it is requested. For example, whenever you request a page, the page's Load event is triggered. You can add application logic to the page that executes whenever the Load event occurs.

2. Building Forms with Web Server Controls

Building Smart Forms

You use several of the basic Web controls to represent standard HTML form elements such as radio buttons, text boxes, and list boxes. You can use these controls in your ASP.NET pages to create the user interface for your Web application. The following sections provide detailed overviews and programming samples for each of these Web controls.

Controlling Page Navigation

In the following sections, you learn how to control how a user moves from one ASP.NET page to another. First, you learn how to submit an HTML form to another page and retrieve form information. Next, you learn how to use the Redirect() method to automatically transfer a user to a new page. Finally, you learn how to link pages together with the HyperLink control.

Applying Formatting to Controls

In the following sections, you learn how to make more attractive Web forms. First, you look at an overview of the formatting properties common to all Web controls; they are the formatting properties of the base control class. Next, you learn how to apply Cascading Style Sheet styles and classes to Web controls.

3. Performing Form Validation with Validation Controls

Using Client-side Validation

Traditionally, Web developers have faced a tough choice when adding form validation logic to their pages. You can add form validation routines to your server-side code, or you can add the validation routines to your client-side code.

The advantage of writing validation logic in client-side code is that you can provide instant feedback to your users. For example, if a user neglects to enter a value in a required form field, you can instantly display an error message without requiring a roundtrip back to the server.

People really like client-side validation. It looks great and creates a better overall user experience. The problem, however, is that it does not work with all browsers. Not all browsers support JavaScript, and different versions of browsers support different versions of JavaScript, so client-side validation is never guaranteed to work.

For this reason, in the past, many developers decided to add all their form validation logic exclusively to server-side code. Because server-side code functions correctly with any browser, this course of action was safer.

Fortunately, the Validation controls discussed in this chapter do not force you to make this difficult choice. The Validation controls automatically generate both client-side and server-side code. If a browser is capable of supporting JavaScript, client-side validation scripts are automatically sent to the browser. If a browser is incapable of supporting JavaScript, the validation routines are automatically implemented in server-side code.

You should be warned, however, that client-side validation works only with Microsoft Internet Explorer version 4.0 and higher. In particular, the client-side scripts discussed in this chapter do not work with any version of Netscape Navigator.

Requiring Fields: The RequiredFieldValidator Control

You use RequiredFieldValidator in a Web form to check whether a control has a value. Typically, you use this control with a TextBox control. However, nothing is wrong with using RequiredFieldValidator with other input controls such as RadioButtonList.

Validating Expressions: The RegularExpressionValidator Control

You can use RegularExpressionValidator to match the value entered into a form field to a regular expression. You can use this control to check whether a user has entered, for example, a valid e-mail address, telephone number, or username or password. Samples of how to use a regular expression to perform all these validation tasks are provided in the following sections.

Comparing Values: The CompareValidator Control

The CompareValidator control performs comparisons between the data entered into a form field and another value. The other value can be a fixed value, such as a particular number, or a value entered into another control.

Summarizing Errors: The ValidationSummary Control

Imagine that you have a form with 50 form fields. If you use only the Validation controls discussed in the previous sections of this chapter to display errors, seeing an error message on the page might be difficult. For example, you might have to scroll down to the 48th form field to find the error message.

Fortunately, Microsoft includes a ValidationSummary control with the Validation controls. You can use this control to summarize all the errors at the top of a page, or wherever else you want.

4. Advanced Control Programming

Working with View State

By default, almost all ASP.NET controls retain the values of their properties between form posts. For example, if you assign text to a Label control and submit the form, when the page is rendered again, the contents of the Label control are preserved.

The magic of view state is that it does not depend on any special server or browser properties. In particular, it does not depend on cookies, session variables, or application variables. View state is implemented with a hidden form field called VIEWSTATE that is automatically created in every Web Forms Page.

When used wisely, view state can have a dramatic and positive effect on the performance of your Web site. For example, if you display database data in a control that has view state enabled, you do not have to return to the database each time the page is posted back to the server. You can automatically preserve the data within the page's view state between form posts.

Displaying and Hiding Content

Imagine that you are creating a form with an optional section. For example, imagine that you are creating an online tax form, and you want to display or hide a section that contains questions that apply only to married tax filers.

Or, imagine that you want to add an additional help button to the tax form. You might want to hide or display detailed instructions for completing form questions depending on a user's preferences.

Finally, imagine that you want to break the tax form into multiple pages so that a person views only one part of the tax form at a time.

In the following sections, you learn about the properties that you can use to hide and display controls in a form. You learn how to use the Visible and Enabled properties with individual controls and groups of controls to hide and display page content.

Using the Visible and Enabled Properties

Every control, including both HTML and Web controls, has a Visible property that determines whether the control is rendered. When a control's Visible property has the value False, the control is not displayed on the page; the control is not processed for either pre-rendering or rendering.

Web controls (but not every HTML control) have an additional property named Enabled. When Enabled has the value False and you are using Internet Explorer version 4.0 or higher, the control appears ghosted and no longer functions. When used with other browsers, such as Netscape Navigator, the control might not appear ghosted, but it does not function.

Disabling View State

In certain circumstances, you might want to disable view state for an individual control or for an ASP.NET page as a whole. For example, you might have a control that contains a lot of data (imagine a RadioButtonList control with 1,000 options). You might not want to load the data into the hidden __VIEWSTATE form field if you are worried that the form data would significantly slow down the rendering of the page.

Using Rich Controls

In the following sections, you learn how to use three of the more feature-rich controls in the ASP.NET framework. You learn how to use the Calendar control to display interactive calendars, the AdRotator control to display rotating banner advertisements, and the HTMLInputFile control to accept file uploads.