Assuming you really do have an Automation addin rather than some other kind
of addin:

Basically, my c# add-in does not know that the VBA add-in exists.

Of course not, but its so easy: all it has to do is open it as if it was a
workbook ...

****** The VBA addin checks whether the c# add-in is loaded - and loads
it programmatically, if necessary.

something like this (not tested), where Myprojectname.classname is the
ProgId of your Automation addin

dim oAddin as addin

on error resume next
set oaddin=nothing
set oaddin=addins("MyProjectname.classname")
on error goto 0
if oaddin is nothing then
addins.add("MyProjectname.classname")
set oaddin=addins("MyProjectname.classname")
endif

if not oaddin.installed then oaddin.installed=true


Charles
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Decision Models
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