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There are two key challenges to using Ajax. The first is complexity. Writing the JavaScript code needed to implement an Ajax application is a major feat. Fortunately, you ll sidestep this problem in this chapter, because you ll use ASP.NET s Ajax-enabled features. That means you ll let Microsoft manage the complexity instead of worrying about it yourself. The other challenge to using Ajax is browser support. The technology that supports Ajax has existed for several years, but it s only now found consistently in all major browsers. If you use the Ajax features that ASP.NET provides, they ll work in Internet Explorer 5 and newer, Netscape 7 and newer, Opera 7.6 and newer, Safari 1.2 and newer, and Firefox 1.0 and newer. This captures the overwhelming majority of web users. (The actual percentage depends on your audience, but over 90 percent is a good initial assumption.) But what about the minority of users who are using old browsers or have JavaScript switched off It all depends on the feature you re using and the way it s implemented. If you re using the

partial rendering support that s provided by ASP.NET s UpdatePanel control (which you ll learn about in this chapter), your page will continue to work with non-Ajax-enabled browsers it will simply use full postbacks instead of more streamlined partial updates. On the other hand, if you re using a more advanced Ajax-enabled web control, you may find that it doesn t work properly or at all. The only way to know is to switch JavaScript off in your browser and try it out. Either way, there s a price to be paid for slick Ajax features, and that price is increased web browser requirements. Finally, Ajax applications introduce a few quirks that might not be to your liking. Web pages that use Ajax often do a lot of work on a single page. This is different than traditional web pages, which often move the user from one page to another to complete a task. Although the multiplepage approach is a little more roundabout, it allows the user to place bookmarks along the way and use the browser s Back and Forward buttons to step through the sequence. These techniques usually don t work with Ajax applications, because there s only a single page to bookmark or navigate to, and the URL for that page doesn t capture the user s current state. This isn t a showstopper of an issue, but it might cause you to consider the design of your web application a little more closely.

There are a variety of ways to implement Ajax in any web application, including ASP.NET. To implement it on your own, you need to have a thorough understanding of JavaScript, because it s JavaScript code that runs in the browser, requesting the new information from the web server when needed and updating the page accordingly. Although JavaScript isn t terribly complex, it s remarkably difficult to program correctly, for two reasons: The implementation of key JavaScript details varies from browser to browser, which means you need a tremendous amount of experience to write a solid web page that runs equally well on all browsers. JavaScript is a notoriously loose language that tolerates many minor typos and mistakes. Catching these mistakes and removing them is a tedious process. Even worse, the mistakes might be fatal on some browsers and harmless in others, which complicates debugging. In this chapter, you won t use JavaScript directly. Instead, you ll use a higher-level model called ASP.NET AJAX. ASP.NET AJAX gives you a set of server-side components and controls that you can use when designing your web page. These components automatically render all the JavaScript you need to get the effect you want. The result is that you can create a page with Ajax effects while programming with a familiar (and much more productive) model of serverside objects. Of course, you won t get quite as much control to customize every last detail about the way your web pages work, but you will get some great functionality with minimal effort.