The strange case of empty Add-In Manager (not showing XML-based add-ins)

There is a problem that I have never experienced but it was happening for a few people in Visual Studio 2005 / 2008. The symptoms were:
  • The Add-In Manager is empty
  • The Add-In only shows registry-based add-ins, not XML-based add-ins (using an .AddIn file)

Of course the first difficulty was to notice that the problem happened to several add-ins, not only a specific add-in. When you don’t see your add-in in the Add-in Manager, the first thought is that your add-in is not correctly registered, specially because folders to put the .AddIn file are a bit tricky, as I explained in the article INFO: Default .AddIn file locations for Visual Studio add-ins (COM-registration is also tricky, BTW).

Using a file-monitoring tool like Process Monitor revealed that Visual Studio actually found .AddIn files, so there was no registration problem.

After almost one year with this problem unsolved, someone discovered yersterday the cause: a bad installation of MSXML 6.0, which apparently is used by Visual Studio to read the actual contents of the XML .AddIn files used by add-ins. After uninstalling MSXML 6.0 and installing the latest MSXML 6.0 SP1 the problem was solved. Here is the whole discussion thread with the problem:

VS 2008: Add-In Manager is empty
http://social.msdn.microsoft.com/Forums/en-US/vsx/thread/6778e3a2-2783-4826-a5f2-7089e1c8fb90

I have updated my article:

HOWTO: Troubleshooting Visual Studio and Office add-ins
http://www.mztools.com/articles/2007/MZ2007009.aspx

I would be nice if Visual Studio showed an error message rather than failing silently when parsing .AddIn files, and better yet, used the XML parser of the .NET framework rather that depending on an external component.