So this is one of those errors that I occasionally run into, and have not been sure what to do. I usually do an iisreset, kill the aspnet_wp.exe process, shut down visual studio, then open visual studio again, and it goes away.

Not today. And of course at the most inconvenient time, my usual kludge to get this error to disapeer did not work. So with some further research I found a KB Article on Microsoft, explaining why this can happen.

The indexing service is known to index temporary asp.net files! I thought it was too good to be true, as a quick fix, I paused the indexing service, did a rebuild on my solution, no error. Sweet! I then followed the instructions in the KB article to tell the indexing service not to index my temporary ASP.NET files.

Also handy if you want to keep the indexing service out of certain other areas.

http://support.microsoft.com/default.aspx?scid=kb;en-us;329065