Principles of ASP.NET
Even though ASP.NET takes its name from Microsoft's old web development technology, ASP, the two differ significantly. Microsoft has completely rebuilt ASP.NET, based on the Common Language Runtime (CLR) shared by all Microsoft .NET applications. Programmers can write ASP.NET code using any of the different programming languages supported by the .NET Framework, usually C#, Visual Basic.NET, or JScript .NET, but also including open-source languages such as Perl and Python. ASP.NET has performance benefits over other script-based technologies because the server-side code is compiled to one or a few DLL files on a web server.
ASP.NET uses the .NET Framework as an infrastructure. The .NET Framework offers a managed runtime environment (like Java), providing a virtual machine with JIT and a class library.
The numerous .NET controls, classes and tools can cut down on development time by providing a rich set of features for common programming tasks. Data access provides one example, and comes tightly coupled with ASP.NET. A developer can make a page to display a list of records in a database, for example, significantly more readily using ASP.NET than with traditional web technologies like ASP or PHP.
ASPX file format
ASPX is a text file format used as web-form pages in the .NET environment. In programming jargon, the ASPX file typically contains only static HTML or XHTML code where the developer places all the required form fields and text content for the web page. The dynamic code that involves request and response from the server is placed in an HTML page with a tag or block <% -- dynamic code -- %> which is similar to other web development technologies such as PHP and JSP. ASP.NET does support inline code blocks inside an ASPX file, but this practice is generally discouraged. The ASPX and other resource files are placed in a virtual host on an Internet Information Services or other compatible ASP.NET servers. When a client requests information, the .NET framework parses and compiles the file into a .NET class and sends the response. Unlike some other web development technologies, which compile their files each time when responding the client, the ASPX files are compiled only for the first time when accessed and are then re-used to reduce the response time. Developers can also choose to pre-compile their code before deployment, eliminating the need for just-in-time compilation in a live environment.
Advantages of ASP.NET over ASP
Compiled code means applications run faster with more design-time errors trapped at the development stage.
Significantly improved run-time error handling, making use of exceptions and try-catch blocks.
User-defined controls allow commonly used templates, such as menus.
Similar metaphors to Windows applications such as controls and events, which make development of rich user interfaces, previously only found on the desktop, possible.
An extensive set of controls and class libraries allows the rapid building of applications.
ASP.NET leverages the multi-language capabilities of the .NET CLR, allowing web pages to be coded in VB.NET, C#, J#, etc.
Ability to cache the whole page or just parts of it to improve performance.
Ability to use the code-behind development model to separate business logic from presentation.
If an ASP.NET application leaks memory, the ASP.NET runtime unloads the AppDomain hosting the erring application and reloads the application in a new AppDomain.
Session state in ASP.NET can be saved in a SQL Server database or in a separate process running on the same machine as the web server or on a different machine. That way session values are not lost when the web server is reset or the ASP.NET worker process is recycled.
ASP.NET 2.0 produces markup that passes W3C validation, but it is debatable as to whether this increases accessibility, one of the benefits of a semantic xhtml page + css representation. Several controls (eg. the Login controls, the Wizard control) use html tables for layout by default.
Microsoft has now gone some way to solve this problem by releasing the ASP.NET 2.0 CSS Control Adapters, a free add-on that produces compliant accessible xhmtl+css markup.