ASP.NET Applications and Configuration

Overview

Like ASP, ASP.NET encapsulates its entities within a web application. A web application is an abstract term for all the resources available within the confines of an IIS virtual directory. For example, a web application may consist of one or more ASP.NET pages, assemblies, web services configuration files, graphics, and more. In this section we explore two fundamental components of a web application, namely global application files (Global.asax) and configuration files (Web.config).

Global.asax

Global.asax is a file used to declare application-level events and objects. Global.asax is the ASP.NET extension of the ASP Global.asa file. Code to handle application events (such as the start and end of an application) reside in Global.asax. Such event code cannot reside in the ASP.NET page or web service code itself, since during the start or end of the application, its code has not yet been loaded (or unloaded). Global.asax is also used to declare data that is available across different application requests or across different browser sessions. This process is known as application and session state management.

The Global.asax file must reside in the IIS virtual root. Remember that a virtual root can be thought of as the container of a web application. Events and state specified in the global file are then applied to all resources housed within the web application. If, for example, Global.asax defines a state application variable, all .aspx files within the virtual root will be able to access the variable.

Like an ASP.NET page, the Global.asax file is compiled upon the arrival of the first request for any resource in the application. The similarity continues when changes are made to the Global.asax file; ASP.NET automatically notices the changes, recompiles the file, and directs all new requests to the newest compilation. A Global.asax file is automatically created when you create a new web application project in the VS.NET IDE.

Application Directives

Application directives are placed at the top of the Global.asax file and provide information used to compile the global file. Three application directives are defined, namely Application, Assembly, and Import. Each directive is applied with the following syntax:

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<%@ appDirective appAttribute=Value ...%> 

Web.config

In ASP, configuration settings for an application (such as session state) are stored in the IIS metabase. There are two major disadvantages with this scheme. First, settings are not stored in a human-readable manner but in a proprietary, binary format. Second, the settings are not easily ported from one host machine to another.(It is difficult to transfer information from an IIS�s metabase or Windows Registry to another machine, even if it has the same version of Windows.)

Web.config solves both of the aforementioned issues by storing configuration information as XML. Unlike Registry or metabase entries, XML documents are human-readable and can be modified with any text editor. Second, XML files are far more portable, involving a simple file transfer to switch machines.

Unlike Global.asax, Web.config can reside in any directory, which may or may not be a virtual root. The Web.config settings are then applied to all resources accessed within that directory, as well as its subdirectories. One consequence is that an IIS instance may have many web.config files. Attributes are applied in a hierarchical fashion. In other words, the web.config file at the lowest level directory is used.

Since Web.config is based on XML, it is extensible and flexible for a wide variety of applications. It is important, however, to note that the Web.config file is optional. A default Web.config file, used by all ASP.NET application resources, can be found on the local machine at:

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\%winroot%\Microsoft.Net\Framework\version\CONFIG\machine.config

Summary

ASP.NET is an evolution of Microsoft�s Active Server Page (ASP) technology. Using ASP.NET, you can rapidly develop highly advanced web applications based on the .NET framework. Visual Studio Web Form Designer, which allows the design of web applications in an intuitive, graphical method similar to Visual Basic 6. ASP.NET ships with web controls wrapping each of the standard HTML controls, in addition to several controls specific to .NET. One such example is validation controls, which intuitively validate user input without the need for extensive client-side script.

In many respects, ASP.NET provides major improvements over ASP, and can definitely be considered a viable alternative for rapidly developing web-based applications.