Understanding Dynamic Compilation

Strangely enough, when you create an ASP.NET page, you are actually creating the source code for a .NET class. You are creating a new instance of the System.Web.UI.Page class. The entire contents of an ASP.NET page, including all script and HTML content, are compiled into a .NET class.

When you request an ASP.NET page, the ASP.NET Framework checks for a .NET class that corresponds to the page. If a corresponding class does not exist, the Framework automatically compiles the page into a new class and stores the compiled class (the assembly) in the Temporary ASP.NET Files folder located at the following path:

\WINDOWS\Microsoft.NET\Framework\[version]\Temporary ASP.NET Files

The next time anyone requests the same page in the future, the page is not compiled again. The previously compiled class is executed and the results are returned to the browser.

Even if you unplug your web server, move to Borneo for 3 years, and start up your web server again, the next time someone requests the same page, the page does not need to be re-compiled. The compiled class is preserved in the Temporary ASP.NET Files folder until the source code for your application is modified.

When the class is added to the Temporary ASP.NET Files folder, a file dependency is created between the class and the original ASP.NET page. If the ASP.NET page is modified in any way, the corresponding .NET class is automatically deleted. The next time someone requests the page, the Framework automatically compiles the modified page source into a new .NET class.

This process is called dynamic compilation. Dynamic compilation enables ASP.NET applications to support thousands of simultaneous users. Unlike an ASP Classic page, for example, an ASP.NET page does not need to be parsed and compiled each and every time it is requested. An ASP.NET page is compiled only when an application is modified.

Note

You can precompile an entire ASP.NET application by using the aspnet_compiler.exe command-line tool. If you precompile an application, users don't experience the compilation delay resulting from the first page request.