I have updated once more one of the most popular articles that I have ever written to include information about Visual Studio 2010:

INFO: Default .AddIn file locations for Visual Studio add-ins
http://www.mztools.com/Articles/2008/MZ2008001.aspx

This is a problematic area in Visual Studio extensibility that is increasingly complicated with each new Visual Studio version or Windows version:

– Actual folder names changed between Windows XP and Windows Vista (fortunately Windows 7 doesn’t change them).

– Folders can be localized.

– If Visual Studio 2005 provided 5 folders (too many) where to search for .AddIn files, Visual Studio 2008 introduced a 6th one (%ALLUSERSDOCUMENTS%\Microsoft\MSEnvShared\AddIns), which actually never worked.

Visual Studio 2010 has fixed that bug with the 6th folder introduced by VS 2008, but Microsoft couldn’t resist to introduce a new (the 7th!) folder: %ALLUSERSPROFILE%\Microsoft Visual Studio\Addins (which at least works on its debut).

Furthermore, the long standing bug about the “Application Data” part hardcoded in one of the locations(%ALLUSERSPROFILE%\Application Data\Microsoft\MSEnvShared\AddIns) that I reported and that Microsoft reported that would be fixed, hasn’t been fixed. The fix would be a new folder where “Application Data” is not hardcoded (while keeping the old one to avoid breaking existing add-ins). If the new %ALLUSERSPROFILE%\Microsoft Visual Studio\Addins folder is the fix for this problem, it isn’t exactly a fix since it won’t fix add-ins whose setup use CSIDL_COMMON_APPDATA and on Windows XP only work in English.