why is it said that ASP.NET depends on IIS?

Question

what is meant by this line?

Hosted within IIS, ASP.NET apps rely on IIS to instantiate certain objects and call certain methods when a request arrives. (https://learn.microsoft.com/en-us/dotnet/architecture/porting-existing-aspnet-apps/app-startup-differences)
I have read many articles, asked many developers, that how exactly ASP.NET apps rely on IIS and no one has given me a satisfactory answer.

Answer 1

It is unclear what kind of answers will be satisfactory to you, but ASP.NET is not completely open sourced (some classes are open sourced in .NET Framework Reference Source) and its internals are not well documented.

I wrote about the history around ASP.NET/IIS on Stack Overflow once, https://stackoverflow.com/a/46878663/11182 So the two evolved together in the past decades and have tight integration ever since.

The actual integration code is deep down inside aspnet_wp.exe, aspnet_isapi.dll and webengine4.dll, so saying "Hosted within IIS, ASP.NET apps rely on IIS to instantiate certain objects and call certain methods when a request arrives" is probably the only feasible way to summarize.

The "objects" are in fact instances of,

• HttpApplication, the actual HTTP request processing pipeline, https://learn.microsoft.com/en-us/dotnet/api/system.web.httpapplication?view=netframework-4.8
• HttpContext, data bag for everything around request/response, https://learn.microsoft.com/en-us/dotnet/api/system.web.httpcontext?view=netframework-4.8
• IHttpModule and IHttpHandler derived classes based on your ASP.NET configuration.
• Many other supporting classes.

Everything around ASP.NET page lifecycle can be answered once you know all the details, but that level of knowledge won't be easily gained unless you debug deep into your web apps and IIS.

The fortunate part is that everything around ASP.NET Core is open sourced, so once you migrate to that new platform you can dig the source code to easily know what's happening under the hood.

Answer 2

Hi @bhavna ,

In asp.net core application, kestrel is more like a micro server to handle request and execute the method.

There are two hosting models in asp.net core application on IIS. In process and out of process.

When using out of process hosting model, IIS will forward request to kestrel server and Kestrel handles the incoming HTTP traffic and a Kestrel connector hands of an HttpContext to the ASP.NET Core request middleware pipeline for processing.

In the in-process hosting model, what ASP.NET Core Module does is to load the CoreCLR and calls the Program.Main method to bootstrap your app’s logic. It then handles the lifetime of the IIS native request.

App’s logic takes in the HttpContext produced by IISHttpServer which is responsible for converting the native HTTP request to managed before passing the ASP.NET Core middleware pipeline. The middleware pipeline handles the request and passes it on as an HttpContext instance to your app’s logic. The IISHttpServer passes your app’s response back to IIS which then forwards it back to the client initiating the request.

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Best regards,
Bruce Zhang