Where does error CS0433 "Type 'X' already exists in both A.dll and B.dll " come from?

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 This results in at least some of the contents of your application's bin folder being out of date.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

Shut down w3svc and delete everything from c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\

added

• on Windows 7

  c:\Users\{username}\AppData\Local\Temp\Temporary ASP.NET Files\root\

• on IIS servers (64 bit) this can also occur. Look for:

  C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root

  (replace v4.0.30319 by the framework version you're using if newer on your server)

This might happen if you place .cs files in App_Code and changed their build action to compile in a Web Application Project.

Either have the build action for the .cs files in App_Code as Content or change the name of App_Code to something else. I changed the name since intellisense won't fix .cs files marked as content.

More info at http://vishaljoshi.blogspot.se/2009/07/appcode-folder-doesnt-work-with-web.html