I can get around this of course by writing the file under a different
name and then renaming it. I'm still hoping though to find some
easy way to use the task ID to directly signal the .NET process
from Excel, or even pass it a string. But if I can't then
FileSystemWatcher it is.

If your .NET process has a window then you can use FindWindow to find
the window handle then SendMessage to send it a message. These are
Win32 API functions, but you should be able to call them from VBA with
suitable declarations.

http://msdn.microsoft.com/en-us/libr...50(VS.85).aspx

http://msdn.microsoft.com/en-us/libr...99(VS.85).aspx

http://msdn.microsoft.com/en-us/libr...31(VS.85).aspx