Session 6—XML: A Primer

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quotes, which are required. You can create attributes to help describe your elements. You could have also used another child element called <TYPE> rather than using an attribute. Either way is fine. It’s really a matter of preference.

XML and the .NET Framework

The important thing to remember about XML is that it is simply a way of describing data so that any application that knows the structure of the document can use it. Why do you need to know about XML when dealing with ASP.NET? Well, much of the .NET Framework revolves around the concept of universal data and application access. In fact, pretty much everything in the .NET world revolves around the premise of universal access.

Probably two of the most evident examples of XML usage in ASP.NET are the config.web and global.asax files. Without going into too much detail about these files, they store application and configuration data. Here is an example of a config.web file:

<configuration>

<sessionstate timeout=”120”/> <assemblies>

<add assembly=”mscorlib”/> <add assembly=”System.Web”/> <add assembly=”System.Data”/>

</assemblies>

<appsettings>

<add key=”DSN” value=”Server=127.0.0.1;Database=hootie; UID=sa”/> </appsettings>

</configuration>

From your knowledge of XML, you can see that the document element for the config.web file is <configuration>. The document element contains several child elements including

<sessionstate>, <assemblies>, and <appsettings>. The <sessionstate> element has an attribute named timeout that has a value of 120. The <sessionstate> element is empty, which means that it does not contain any character data. When an element is empty is must be closed with the /> or </ELEMENT> syntax. The same could have been accomplished by writing:

<sessionstate timeout=”120”></sessionstate>

XML is also used in ASP.NET with Web Services. A Web Service’s results are always returned in XML format. Suppose you created a Web Service named Math that has a method named Add that sums two numbers. If you invoke the Web Service and call the Add function by passing 2 and 6 as the parameters you might get the following result:

<?xml version=”1.0” ?>

<int xmlns=”http://tempuri.org/”>8</int>

You’ll notice that the result is returned in XML format. The XML document contains a prolog and a document element. That’s it.

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This brief introduction of XML and how it is used by the .NET Framework will help you understand some of the concepts introduced later in the book. For further information about XML, we recommend visiting the World Wide Web Consortium’s (W3C) XML Web site at www.w3c.com.

.NET Web Services, like the global.asax and config.web files, are based heavily on XML. All Web Services responses are serialized as XML. We’ll talk more about Web Services in Session 28.

REVIEW

“XML is the future . . .” If you’ve heard that once, you’ve heard it a million times. You may be sick of hearing it, but you accept it because it’s probably true. Simply put, the purpose of XML is to describe data so it can easily be exchanged. The nice thing about XML is that it’s easy to learn as its syntax is very similar to its cousin HTML.

QUIZ YOURSELF

1.What is the purpose of XML? (See session introduction.)

2.How many root elements are in an XML document? (See “The XML Design Specs.”)

3.What are the three logical parts of an XML document? (See “The Structure of XML Documents.”)

S E S S I O N

7

Developing ASP.NET Pages

Session Checklist

Handling ASP.NET events

Using page directives and namespaces

Choosing a language

In this session, we are going to walk you through writing an ASP.NET page. Writing and ASP.NET page is a little more complicated than writing an ASP page, but once you get the hang of it, you’ll end up writing a lot less code. ASP.NET pages are event-oriented

rather than procedural. This means that instead of starting at the top of a page and writing code that is executed as the page is interpreted, you write event-handling code. An event can be pretty much any type of action, for example, a user submitting a form, a page loading, or clicking a button, and so on.

ASP.NET Events

When an ASP.NET page is loaded, a structured series of events is fired in a set order. You write code that responds to these events rather than interspersing it with HTML as the general practice with ASP. Figure 7-1 shows the ASP.NET event order.

As you can see, the first event to be fired when a page is loaded is the Page_Load event and the last to be fired is the Page_Unload event. The Page_Load event is fired every time a page is loaded. In between the Page_Load and the Page_Unload events, control events are fired. A control event is an event that is wired to a control of some sort. ASP.NET provides many types of controls including HTML controls, Web controls, user controls, validation controls, and so on. Don’t concern yourself with the differences between controls right now (they’re discussed in greater detail in Session 8, “Using HTML Controls”, and Session 9, “Using Web Controls”.), just be aware that they can all respond to events, like being clicked.

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Figure 7-1 ASP.NET page events

In order to write code for these events, you need to include them in a code declaration block. In ASP.NET, a code declaration block looks like this:

<SCRIPT [LANGUAGE=”codeLangauge”] RUNAT=”SERVER” [SRC=”externalfilename”]> ‘ Event Handling Code

</SCRIPT>

The LANGUAGE attribute in the <SCRIPT> element specifies the language used in the code block. The value can be any .NET language like VB or C#. The RUNAT=”SERVER” attribute/value pair specifies that the script block should be executed on the server-side rather than the client-side. The SRC attribute enables you to specify an external file where the code is located.

So, based on what we know at this point, an ASP.NET page should look like this:

<SCRIPT LANGUAGE=”VB” RUNAT=”server”>

Sub Page_Load(Source As Object, E As EventArgs)

‘Page_Load Code End Sub

Sub Control_Click(Source As Object, E As EventArgs) ‘Control_Click Code

End Sub

Sub Page_Unload(Source As Object, E As EventArgs)

‘Page_Unload

End Sub </SCRIPT> <html> <head>

<title>ASP.NET Page</title> </head>

<body>

</body>

</html>

Note

ASP.NET files have an .aspx extension. The .aspx extension simply tells IIS that an ASP.NET page is being requested and should be handled accordingly. All code in this session should be written in files with an .aspx extension.

Session 7—Developing ASP.NET Pages

63

In ASP.NET, a page is an object, which means it has properties, events and methods. The Page object has one very important property: isPostBack. The isPostBack property returns a Boolean value indicating whether the page is being loaded in response to a client post back. This is important because in many cases you will be initializing controls when a page is loaded. Since ASP.NET manages control state between requests, you probably don’t want to initialize a control if the page is responding to a post back. Right? Listing 7-1 shows an example of using the isPostBack property:

Listing 7-1 An isPostBack property example

<SCRIPT LANGUAGE=”VB” RUNAT=”server”>

Sub Page_Load(Source As Object, E As EventArgs) ‘ Page_Load Code

lblTest.Text = Page.isPostBack End Sub

Sub Control_Click(Source As Object, E As EventArgs) ‘Control_Click Code

End Sub

Sub Page_Unload(Source As Object, E As EventArgs) ‘ Page_Unload

End Sub </SCRIPT> <html> <head>

<title>ASP.NET Page</title> </head>

<body>

<form ID=”frmTest” RUNAT=”SERVER”> <asp:Label ID=”lblTest” RUNAT=”SERVER”/> </BR>

<asp:Button ID=”btnSubmit” TEXT=”Submit” RUNAT=”SERVER”/> </form>

</body>

</html>

As shown in Listing 7-1, we added a snippet of code to the Page_Load event that checks if the page is being posted back using the isPostBack property. Something that may look a little foreign are the server-side control declarations a little further down the page. We declared three server controls including a Form HTML control, a Label Web control and a

Button Web control. You’ll notice that each of these declarations contains a “RUNAT=SERVER” attribute/value pair. This simply means that the control is rendered on the server and its events are handled on the server side rather than the client side. We’ll talk more about controls in later sessions.

The Page class contains many useful properties and methods. Refer to your

.NET documentation for a complete treatment of the Page class.

Note

OK, try running the previous example. When the page is first loaded the word “False” appears above the Submit button. If you click the Submit button, the form is posted and