Compilation error CS0016: Could not write to output file

Question

In Brief

When I attempt to browse my website, I get the following error message:

CS0016: Could not write to output file \'c:\

Answer 1

The answer was found using ProcMon:

http://technet.microsoft.com/en-us/sysinternals/bb896645

The temporary asp.net files are first created in the user's temp folder and then moved into this folder. For some as yet unknown reason, the compilation for my Web2 site (.NET 2 AppPool) now uses the same folders as my Web4 (.NET 4 AppPool) site.

I used ProcMon to identify all results for "access denied" and gave the IIS_IUSRS group write permissions (and in one case - C:\Users\ASP.NET V4.0 Integrated - modify permissions).

I have no idea why the compilation is using the "wrong" folders and I suspect I may have over-elevated the permissions in some cases, but for now it's working.