MSDN Magazine - August 2008 - (Page 37)

control tree. Nicely enough, the Add method of the Controls collection automatically updates the state of the control with any data found for that control in the view state. Which Control Posted Back? A page that loads some of its content from external resources needs to know why a postback is raised. In the sample code you see in Figure 4, the page may post back because the user selected a given element from the menu or because the user triggered a postback while working with another component on the user interface. In ASP.NET, there’s no property anywhere that can tell you which control caused the postback. The ID of the posting control, however, is not difficult to find. If the user posted back by clicking a Submit button, then the ID of the control is listed in the HTTP request. If no button control exists in the body of the HTTP request, then the user posted back by interacting with a Link button or an auto-postback control. In any case, the ID of the postback source is in the __EVENTTARGET hidden field. In ASP.NET AJAX, when partial rendering is used, things are easier, as the ScriptManager control exposes a custom property—the AsyncPostBackSourceElementID property. The following code, which is an excerpt of the code that produced Figure 4, shows how to determine whether the user clicked a menu item or posted back to the server through another control: bool IsPostingFromMenu() { ScriptManager sm = ScriptManager.GetCurrent(this); string ctlID = sm.AsyncPostBackSourceElementID; Control c = this.FindControl(ctlID); if (c == null) return false; } return (c.ID == "Menu1"); Figure 4 Interactive Control Dynamically Loaded into Page the @OutputCache attribute in the source of the ASCX, as shown in the following code. The code snippet caches the output of the control for one minute: When the user clicks repeatedly on the menu, a given ASCX user control may be loaded again. Loading a user control means downloading the control markup and going through the server lifecycle. However, since continuously reloading a user control could possibly have a negative impact on performance, you can use an ASP.NET feature such as output caching to help boost the overall performance of the page. Caching Output for User Controls A user control that provides some content to a site may be expensive to create because it could require one or more database calls and potentially a few million CPU cycles. Why then would you want to regenerate the same user control several times per second, especially if your content is not continuously changing? A better strategy is to create the user control once, cache its output, and give it a maximum duration. Once the cached snapshot of the user control becomes stale, the first incoming request will be served again in the standard way, running the control’s code and caching the generated markup again. The ASP.NET page output caching feature allows you to cache page and user control responses so that subsequent requests can be satisfied without executing code but rather by simply returning the cached output. To make a user control cacheable, you declare msdnmagazine.com Output caching is a nice feature, but it is far from being a silver bullet. It is simply a way to have the application serve more pages and user controls more quickly. It won’t necessarily make the application more efficient or scalable. With output caching, however, you certainly reduce the workload on the server, as pages and resources are cached downstream. Additionally, output caching is only a viable option for anonymous content. Requests for cached content are served by IIS directly and never make it to the ASP.NET pipeline, where the caller can be authenticated. Finally, pages and user controls that post back require some extra work. In particular, you need to instruct ASP.NET to save multiple copies of the output that vary by one or more parameters. For this purpose, you use the VaryByParam attribute on the @OutputCache directive. It’s about Pragmatism While partial rendering is often perceived as the poor man’s AJAX and reserved for those who can’t exert the energy required to create “true” AJAX applications, it is one of the best options at the moment for developers using ASP.NET. In the end, remember that AJAX involves many trade-offs, and effective AJAX results from the combination of a variety of techniques and patterns.  Dino Esposito is currently an architect at IDesign and the author of Programming ASP.NET 3.5 Core References. Based in Italy, Dino is a frequent speaker at industry events worldwide. You can join his blog at weblogs.asp.net/despos. August 2008 37 http://weblogs.asp.net/despos http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - August 2008

MSDN Magazine - August 2008 Toolbox CLR Inside Out Basic Instincts Cutting Edge Patterns in Practice Data 2.0 - Expose And Consume Data In A Web Services World Biztalk EDI - Build A Robust EDI Solution With BizTalk Server Silverlight - Create Animations With XAML And Expression Blend Write On! - Create Web Apps You Can Draw On With Silverlight 2 Wicked Code - Craft Custom Controls For Silverlight 2 Team System Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - August 2008

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