can you clarify? Are authoring user interface in Flex - so it is exported as flash movie (.swf) -> which can be embedded in your C# based native application - and then scripted with flash.external.ExternalInterface - or you are authoring user interface in Flex - which will be run in AIR runtime - so it cannot be embedded - but can communicate with your C# front end via some native system process or local low level socket api.
(I assume that you're *NOT* developing native ActionScript extensions as of AIR 2.5 or newer)
Thanks for your reply,
Actualy now I'm trying what you have mentioned. I knew I could embed flash with .net, but didn't try to do it with flex.