How can I avoid user from being able to record app's audio in C#? It's due to some copyright issues and must be done in my project.
In general, this isn't possible. If you can hear something, you can record it. Obviously that process would be lossy, but there is absolutely nothing you can do to prevent it (the laws of physics say so).
I should clarify that it appears not to be possible to prevent digital recording via WASAPI on windows machines, either. Even if this was possible, it would not be possible to prevent someone plugging in their own sound card which supports digital recording (and some do).