(Updated 2-Jun-2011 – see the bottom of the post for some notes on 64 bit machines)

This problem can manifest itself in a number of ways:

• You get an error that indicates that ASPX files are mapped to a static file handler
• You have multiple versions of the framework installed, your site gives a 404, and you get errors/warnings in the eventlog along the lines of “Unable to find schema for config section 'system.web.extensions/scripting/scriptResourceHandler'“, “Unknown attribute 'type'” where these errors come from the web.config of the web application you’re trying to access.

There are other possibilities for these errors, but a big possibility is that particular version of the .NET framework that your application is built to use is not registered in IIS. The most common reason for this is that you install IIS after the framework is installed. Obviously the installer can’t do the business when IIS isn’t around.

Note: you can have the correct framework selected in the IIS application pool settings, and your web.config can be perfect but you may still be experiencing this error.

IIS looked right to me, but it wasn’t finding squat. You may not see the ASP section for the website if no framework version is installed, but if say, .NET 4 is configured, and the 3.5 version that comes with 7 isn’t, your 3.5 sites will mysteriously fail. What fun!

The magic application that allows you to diagnose whether this problem is affecting you is aspnet_regiis.exe. This little tool is found in the framework install folder. Importantly, and counter-intuitively, to check the status of a particular framework version, you need to run the executable that is specific to that version of the framework. For example, to check 1.1, you need to open a command prompt and change to the %windir%\Microsoft.NET\Framework\v1.1.4322 folder.

To check that the framework is installed, run the command: aspnet_regiis -lv.

To install this version of the framework, use aspnet_regiis -ir. This installs the framework version, but doesn’t change the settings of existing IIS application pools.

See the reference topic: ASP.NET IIS Registration Tool (Aspnet_regiis.exe) on MSDN for more information.

Update: The aspnet_iisreg tool may be in your v4.x folder. And if you’re running on a 64 bit machine, you need the version in the Framework64 folder. (Thanks to an article on breaking changes in ASP.NET 4 by Scott Hanselman via IIS Manager can’t configure .NET Compilation on .NET 4 Applications on StackOverflow.)