ASP.NET 4 Unleashed

Components and Dynamic Compilation

You are not required to explicitly compile (build) the component because ASP.NET Framework automatically compiles the component for you. Any component that you add to the App_Code folder is compiled dynamically in the same way as an ASP.NET page. If you add a new component to the App_Code folder and request any page from your website, the contents of the App_Code folder are compiled into a new assembly and saved to the Temporary ASP.NET Files folder, located at the following path:

C:\WINDOWS\Microsoft.NET\Framework\[version]\Temporary ASP.NET Files\[application name]

Whenever you modify the component, the existing assembly in the Temporary ASP.NET Files folder is deleted. The App_Code folder is compiled again ...