I got an email from a former client a couple of days ago with roughly the following problem description:
I’m trying to run a unit test (with MSTest) which calls an interop function. The native DLL (written in C) is located in C:\Windows\System32. For some reason, when running the test Visual Studio can’t seem to find the DLL, and throws a FileNotFoundException.
Moreover, I can’t even see the DLL in the File—>Open dialog in Visual Studio! I tried launching Visual Studio as admin and as standard user but the result was the same. What am I missing here?
This was a good opportunity to demonstrate psychic debugging skills. Here’s what I wrote back:
The test probably runs as 32-bit and therefore it looks under C:\Windows\SysWOW64.
Indeed, VSTestHost.exe (the external executable that runs Visual Studio 2008 tests) is compiled with the /32BIT+ CLR header flag, forcing it to run as a 32-bit process even on 64-bit Windows. There are some instructions elsewhere that remove the 32-bit flag from the VSTestHost.exe file, in an obviously unsupported fashion. Finally, Visual Studio 2010 seems to offer a way to run unit tests in a 64-bit process.
Since Visual Studio is a 32-bit process, it is also not a big surprise that the File—>Open dialog “doesn’t work” by not showing the file. Here’s a simple experiment you can perform on your 64-bit Windows:
- Run two copies of Notepad: C:\Windows\System32\notepad.exe and C:\Windows\SysWOW64\notepad.exe
- Open an administrative command prompt and copy some text file to C:\Windows\System32
- Now try opening the file in both Notepad instances using the File—>Open dialog
Bonus reading: More than two years I resolved a similar problem over the phone.
More bonus reading: User-account control redirection can cause similar inconsistencies. A 32-bit application without a manifest that runs without administrative privileges will write a text file to C:\Program Files successfully, but upon closer examination with Windows Explorer you won’t find the file there. Instead, it will be in a redirected VirtualStore folder under %LOCALAPPDATA%. See this great post for more resources, labs, and presentations.