code for converting avi to mp4

Just to add a clarification to a good answer by Simon: AVI and MPEG-4 are the entities of different orders of hierarchy. AVI is a file extension and a format for the media container; but MPEG-4 is more essential thing: this is a method of compression of audio and video. In principle, AVI container can contain different kinds of video+audio streams including MPEG4.

See for more detail:
http://en.wikipedia.org/wiki/Audio_Video_Interleave[^],
http://en.wikipedia.org/wiki/MPEG-4[^].

Also, look at the links at the bottom of the first article to see how many different MPEG-4 standards are there. Which one do you need? :-)

—SA

Converting extension from avi to mp4:

File.Move(filePath, string.Format(@"{0}\{1}.mp4",
                                  Path.GetDirectoryPath(filePath),
                                  Path.GetFileName(filePath));

All jokes aside, I don't think you realize the size of your question. But if you really want code that can convert between the two I would look at FFmpeg[^]