aspnet_isapi.dll handles all request to aspx file. actually all dotnet related file including .soap .axd .ascx files are handled by aspnet_isapi.dll .

Abul Asim M. R. Qarshi
Software Engineer
Merced Solutions Inc.
www.merced-solutions.com

When the Internet Information Service process (inetinfo.exe) receives an HTTP request it uses the filename extension of the requested resource to determine which Internet Server Application Programming Interface (ISAPI) program to run to process the request. When the request is for an ASP.NET page (.aspx file) IIS passes the request to the ISAPI DLL capable of handling the request for ASP.NET pages which is aspnet_isapi.dll.

When a browser initially calls up an ASP.NET page IIS recognizes that this is a ASPX file request and lets the ASP.NET module (aspnet_isapi.dll) deal with it. The aspnet_isapi.dll places the Aspx file we request into a new class definition. This new class is defined in a namespace called ASP; so the contents of the file say myPage.aspx end up in a class called Asp.myPage.aspx . The ASP Class inherits from the page class.

The new Asp class is then instantiated as an object in the CLR. A render method is then called on our new object that returns appropriated HTML via the aspnet_isapi.dll to IIS which then sends the HTML to the client than originally made the request.