ASP.NET and IIS Integration IIS 6.0 and II6.0+ (IIS 7.0, IIS 7.5, IIS 8.0…)

In these earlier releases, IIS processed a request to an ASP.NET content type and then forwarded that request to the ASP.NET ISAPI DLL, which hosted the ASP.NET request pipeline and page framework. Requests to non-ASP.NET content, such as ASP pages or static files, were processed by IIS or other ISAPI extensions and were not visible to ASP.NET.

The major limitation of this model was that services provided by ASP.NET modules and custom ASP.NET application code were not available to non-ASP.NET requests. In addition, ASP.NET modules were unable to affect certain parts of the IIS request processing that occurred before and after the ASP.NET execution path.

In IIS, the ASP.NET request-processing pipeline overlays the IIS pipeline directly, essentially providing a wrapper over it instead of plugging into it.

IIS processes requests that arrive for any content type, with both native IIS modules and ASP.NET modules providing request processing in all stages. This enables services that are provided by ASP.NET modules, such as Forms authentication or output cache, to be used for requests to ASP pages, PHP pages, static files, and so on.

The ability to plug in directly into the server pipeline allows ASP.NET modules to replace, run before, or run after any IIS functionality. This enables, for example, a custom ASP.NET Basic authentication module that is written to use the Membership service and SQL Server user database to replace the built-in IIS Basic authentication feature that works only with Windows accounts.

In addition, the expanded ASP.NET APIs use direct integration to enable more request-processing tasks. For example, ASP.NET modules can modify request headers before other components process the request, by inserting an Accept-Language header before ASP applications execute, which forces localized content to be sent back to the client based on user preference.


Because of the runtime integration, IIS and ASP.NET can use the same configuration to enable and order server modules, and to configure handler mappings. Other unified functionality includes tracing, custom errors, and output caching.

When you are creating app pool in IIS 7.0 and above, you get following options / modes for integrating ASP.Net Pipeline

  • Classic
  • Integrated


  • Classic mode (the only mode in IIS6 and below) is a mode where IIS only works with ISAPI extensions and ISAPI filters directly. In fact, in this mode, ASP.NET is just an ISAPI extension (aspnet_isapi.dll) and an ISAPI filter (aspnet_filter.dll). IIS just treats ASP.NET as an external plugin implemented in ISAPI and works with it like a black box (and only when it’s needs to give out the request to ASP.NET). In this mode, ASP.NET is not much different from PHP or other technologies for IIS.
  • Integrated mode, on the other hand, is a new mode in IIS7 where IIS pipeline is tightly integrated (i.e. is just the same) as ASP.NET request pipeline. ASP.NET can see every request it wants to and manipulate things along the way. ASP.NET is no longer treated as an external plugin. It’s completely blended and integrated in IIS. In this mode, ASP.NET HttpModules basically have nearly as much power as an ISAPI filter would have had and ASP.NET HttpHandlers can have nearly equivalent capability as an ISAPI extension could have. In this mode, ASP.NET is basically a part of IIS.

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