I am starting a new project (ASP.NET 3.5) that will require at least one custom HTTP module. My two development computers are running 32-bit XP Pro/SP2, but I plan to reimage with Vista (already have it ready to be installed). My preference would be to NOT switch to Vista quite yet, but continue the project with my XP Pro systems.

Is it possible or reasonable to proceed with developing the custom HTTP module(s) with my XP Pro system (IIS 5.1) and expect that they will work just fine with IIS7 - and with no changes - when I upgrade to Vista? Or do I really need to switch to Vista before developing these custom HTTP modules?

One module will implement a custom authentication scheme. Another will load an assembly into a separate app domain, execute some code, marshal the results back, stick them into the ASP.NET application state, and then kill that separate app domain.


I think the choice of development platform should depend on the features you want to provide and the platforms you plan to support. Personal preference and upgrade schedule should not decide the development platform unless you also want them to decide your potential user/customer base.

Personally, I believe you should develop on Vista, and here is why.

First, IIS7 is finally the release where the development experience on the client OS matches the corresponding server OS. No more hassling over XP Pro and Windows Server 2003 differences in everything related to IIS, from security ACLs, security models, Application isolation between high/medium/low and Application Pools, Web Service Restrictions on CGI and ISAPI, configuration changes, TCP Connection limitations, etc... you get the picture. The same IIS7 server core is on Vista and Windows Server 2008, with the same security model, application isolation, configuration files, etc. Unity at last.

Now, for some more technical reasons. IIS7 has two "Pipeline modes", classic and integrated, that offer completely different behavior/abilities for certain events (authentication being one of them):

  • Classic mode is supposed to mirror ASP.Net behavior on IIS5x/6.0. With some minor tweaks while reconciling IIS and ASP.Net behavior.
  • Integrated mode is completely new and represents true managed code extensibility of IIS for the future that mostly matches the native code extensibility of IIS.

I must warn that Classic mode will NEVER match Integrated mode in terms of features and functionality - it exists only for legacy, compatibility reasons. Thus, by definition, the two modes are different.

So, yes, it should be relatively painless to author a Custom Authentication HttpModule in ASP.Net on IIS 5.1 and run it in Classic mode on IIS7. However, that module will likely fail when you switch to Integrated mode, especially for Authentication since it is one of those events whose ordering changes (intentionally, for the better) between Classic and Integrated mode.

In fact, the way you configure that module will be different in IIS7 between the two modes:

  • In Integrated mode, your simply add a new <module> using the type of your custom auth assembly in <modules>, and optionally add a preCondition to determine if the module should only run on requests involving managed code (i.e. only .aspx, .asmx, etc pages) or all requests (i.e. .html, .xml, .css, .asp, .php, pages).

    This is exactly what people tried (in vain) to do with ASP.Net 2.0 on IIS6 but could not, and certainly not with the elegance and compactness of preConditions.

    Instead, what one needs to do is the following set of kludges...
  • In Classic mode, you need configuration that emulates what it took on IIS6 to get similar (but not identical) behavior. This means:
    1. *-scriptmap to aspnet_isapi.dll -- this is now a *-<handler> to isapi.dll with aspnet_isapi.dll as the scriptProcessor
    2. <httpHandler> for * which derives from DefaultHttpHandler and tweaks some special settings on the HttpContext to cause it to call the HSE_REQ_EXEC_URL ISAPI ServerSupportFunction underneath the covers
    3. Order the remaining <httpHandlers> correctly relative to #2 to get reasonable behavior

The astute reader will remind you that I just rushed through a WHOLE lot of details there, which many people have dedicated countless number of hours of pain/frustration/anguish and countless fragments of articles describing how to get an ASP.Net 2.0 Custom Authentication Module to [not] work on IIS 6. And what I just described does not even apply or work on the IIS 5.1 on XP Pro 32bit scenario you just described, unless you intend to only implement Custom Authentication module for ASP.Net pages and static files.

Now, we could have made it easier on the users (but much harder on us) in implementation, but we chose the clean implementation you currently see PRECISELY because it starkly shows how much more elegent Integrated mode is in comparison to classic mode at resolving the issue of managed code extensibility of IIS.

I hope this helps frame the discussion for your future development .