ASP.NET Error: Could not load file or assembly App_Web_z9w33txs

Wow, this one is a bit of a doozy. I did some research and apparently lots of people have been experiencing this problem for years. It's possible that you are caching types that are defined in your web site. Such types do not have an assembly, so one is randomly generated for them at runtime. The next time you recycle your web server, your types are given a different randomly generated assembly, and your cache won't be able to deserialize because the old assembly no longer exists. Here are some possible fixes you can try:

• Define all types in a separate assembly rather than in your web site.
• On your local box, see if running the site in webdev rather than iis has the same behavior.
• If you're using out-of-proc (SQL Server) session/cache storage, try using in-proc (local in-memory) session/cache storage
• Delete all subdirectories under C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files (as Dimi mentioned, use Unlocker to break locks any applications might have)
• If you precompile your web site when deploying, make sure your web server is shut down before deploying. (Apparently if users are requesting pages on the old version of the site, it will screw things up when the new version is deployed.)

Theory

When this issue is not caused by a bug in the application (e.g., duplicate class name):

This issue appears to present after a change is made to the application's project that results in a new build (e.g., code/reference/resource change). The issue appears to lie within the output of this new build: for various reasons Visual Studio is not replacing the entire contents of your application's obj/bin folders. This results in at least some of the contents of your application's bin folder being out of date.

When said issue occurs, clearing out the "Temporary ASP.NET Files" folder, alone, does not solve the problem. It cannot solve the problem, because the stale contents of your application's bin folder are copied back into the "Temporary ASP.NET Files" folder the next time your application is accessed, causing the issue to persist. The key is to remove all existing files and force Visual Studio to rebuild every object, so the next time your application is accessed the new bin files will be copied into the "Temporary ASP.NET Files" folder.

Solution

1. Close Visual Studio
2. Perform an iisreset
3. Delete all the folders and files within the "Temporary ASP.NET Files" folder (the path is referenced in the error message)
4. Delete the offending application's "obj" and "bin" folders
5. Restart Visual Studio and open the solution
6. Perform a "Clean Solution" followed by a "Rebuild Solution"

Explanation

• Steps 1-2: remove resource locks from the folders/files we need to delete.
• Steps 3-4: remove all of the old build files
• Steps 5-6: create new versions of the build files