How to Change SharePoint 2010 Project from Sandboxed Solution to Farm Solution

For Visual Studio/Express 2010 only. See other (easier) answers for VS2012, VS2013, etc

To find the missing file(s), use info from the article Enable C++ project system logging to enable debug logging in Visual Studio and let it just tell you what's causing the rebuild:

1. Open the devenv.exe.config file (found in %ProgramFiles%\Microsoft Visual Studio 10.0\Common7\IDE\ or in %ProgramFiles(x86)%\Microsoft Visual Studio 10.0\Common7\IDE\). For Express versions the config file is named V*Express.exe.config.

2. Add the following after the </configSections> line:

   <system.diagnostics>
     <switches>
       <add name="CPS" value="4" />
     </switches>
   </system.diagnostics>
   

3. Restart Visual Studio
4. Open up DbgView and make sure it's capturing debug output
5. Try to debug (hit F5 in Visual Studio)

6. Search the debug log for any lines of the form:

   devenv.exe Information: 0 : Project 'Bla\Bla\Dummy.vcxproj' not up to date because build input 'Bla\Bla\SomeFile.h' is missing.

   (I just hit Ctrl+F and searched for not up to date) These will be the references causing the project to be perpetually "out of date".

To correct this, either remove any references to the missing files from your project, or update the references to indicate their actual locations.

Note: If using 2012 or later then the snippet should be:

<system.diagnostics>
  <switches>
   <add name="CPS" value="Verbose" />
  </switches>
</system.diagnostics>

I fixed thus by going to Central Administration > System Settings > Manager user solutions. Under Load Balancing, I selected "All Sandboxed code runs on the same machine as a request" option and it is all working now.

For some reason I am still missing this service - "Microsoft SharePoint Foundation User Code Service". However I have always had "Microsoft SharePoint Foundation Sandboxed Code Service" service listed under Services on central administration site. And I am using SharePoint Server Standard Edition so it may be that for Standard edition I have this service instead.