One of the more annoying aspects of developing a VSTO solution is debugging deployment issues.  The Office host application (Word/Excel) often returns error messages along the lines of:

 

“Can’t find or load the <assembly> assembly in <url>”, or:

 

The custom macros in <url> could not be initialized correctly

 

These wonderfully unhelpful messages are sometimes related to environmental issues.  Other than verifying that the URL to the VSTO assembly is correct, some other things to try:

 

  1. Clear the IE cache on the client
  2. Clear the Fusion download cache (using “GACUTIL /CDL”) on the client
  3. If the URL is on the local network, clear the IE proxy settings on the client
  4. Browse the client Task Manager process list to ensure that there are no “invisible” WINWORD.EXE instances floating around.