Question

My VSTO Outlook Add-in suddenly stopped working on one customer machine (it does not load, no error message) and I’m stuck with troubleshooting. The machine is Windows 7 x86, Outlook 2007. The add-in is written with Visual Studio 2008 and uses VSTO 2005 with the 2003 PIAs (because we need to support Outlook 2003 as well). It works perfectly fine on other machines.

Here’s what I tried to get useful troubleshooting output:

• Add the VSTO_SUPPRESSDISPLAYALERTS environment variable (set to 0).
• Start Outlook with administrative permissions.
• Look into the event log.

Nothing. The add-in just won’t load without giving any indication of the reason. I’ve also checked the “usual suspects” (CAS policy, PIAs installed, LoadBehavior in the registry, reinstalling VSTO and the add-in).

Some other observations:

• LoadBehavior in the registry stays at 3.
• The add-in shows up as “disabled” in Outlook. Checking its checkbox in “COM add-ins” just does nothing (no error, checkbox is cleared again when entering the form a second time).
• It works perfectly fine on other customer machines, and it worked perfectly fine on this machine. (No, the customer cannot tell me what changed on his machine.)
• I have a Trace.WriteLine message at the very top of my code (first line in the ThisAddIn_Startup handler), which is not reached (I checked with DebugView). Thus, the reason for not loading is not an exception in my add-in but rather a failure of VSTO to load the add-in or of Outlook to load VSTO.

Instead of more random debugging (“try this…”, “try that…”), I’d really like to force Outlook and/or VSTO to tell me what’s wrong, i.e. to give me a useful error message instead of just doing nothing when trying to enable the add-in. Any ideas?

Answer 1

Did you try enabling the add-in again? It won’t run after its in the disabled queue. After you re-enable it from the disabled add-in screen, you can then check the box in the COM-AddIn screen to have it load which then should prompt you more detail since you set the VSTO_SUPPRESSDISPLAYALERTS variable about what may have happened in the first place.