If both apps are dot net, use remoting. create an object
you can remote from your host app that takes as a shared
variable the xyobject instance you want to share. then
either make the xyobject inherit marshalbyobjectref and
have a function on the remoted class to pass it back, or
in the remoted object have methods that wrap the methods
on the xyobject.

remoting creates instances of the remoted onject for each
method call so dont rely on any persistance other than
shared (static) objects.