random drawing of a folder without delivery

I am a beginner in C # and I learn a On the job that's why I turn to you to try to solve my problem that I can not solve I would like that, in a selected folder, I can then randomly select a subfolder but, for the rest of the execution of the program, the subfolders that have been selected never stand out.

I tell myself that it will surely be practical to give a number to each sub-folder and then to draw a number at random. if the number is in the list of "already out" then we restart. or simply delete the numbers from the list of those that can be drawn randomly. but I don't see how to do this in C #

Each of my subfolders contains a sound file (but all of which have a different name). I would then like to recover the path of the sound file in the sub-folder which was chosen at random

Thank you