If you use google, you'll find almost a million references to doing it. Here's a link to some results (searched for "using flash interface in c# app"):
http://www.google.com/search?q=using+flash+interface+in+c%23+app&sourceid=ie7&rls=com.microsoft:en-us:IE-SearchBox&ie=&oe=[
^]
After saying that, take my word for this - it's a pain in the ass. I did it once, and there were all kinds of problems because the Flash programmer wasn't adhering to the agreed-upon interface hooks. Further, you have to take steps to make it look decent. I hated it, and will do anything to avoid having to do it again.