I have created an Excel Add-in project (a VSTO project). The VSTO is signed with a Domain Certificate that the client machines trust. On the client machines, the add-in works well if you double click the spreadsheet or if you are in Excel and ‘File>Open’ the spreadsheet.
However, when the spreadsheet is sent via an email attachment, the VSTO is disabled. Excel will open and the spreadsheet is loaded but, the Add-in is disable. Excel displays this message,
This document contains code that cannot be loaded because the location is not in your trusted locations list: WrkBook.xlsx. If you trust this document you can save and open it from a location on your computer. For more information, see http://go.microsoft.com/fwlink/?LinkId=99104
I have looked into the link above and tried the solution that it specifies. That ‘solution’ will not work because Outlook saves attachments to a sub folder of Internet Explorer and Excel will not allow that folder to be trusted. The details of where Outlook saves attachments is documented here: http://support.microsoft.com/kb/817878
One thing that does work but is horrendous from desktop management prespective is to move Outlook’s temp storage folder by changing the registry key HKEY_CURRENT_USER\Software
But we are talking about thousands of desktops. To have to update and maintain all those registry entries and ensure they point to valid temp folders is a disaster waiting to happen.
So, my question.
Is there some way through Windows' policies, a more trusted VSTO compile, or perhaps some other way to make Excel trust a VSTO Add-in when the spreadsheet is delivered as an Outlook attachment?