The SwfDotNet framework is a full .Net open source library for writing, reading and generating Flash(tm) Swf files on-the-fly with C# .Net or VB.Net. It's composed of packages for IO writing/reading process. Written by Olivier Carpentier.
Thanks for Swfdotnet, it's the best!
Makes writing vector SWF files almost as painless as drawing to GDI!