How to best break out of this while loop?

[Mubanga]

I’m trying to write a random level picker, the idea is to never have the same level twice until you’ve had all of them. The levels are Game Objects that I’ve stored in a list. This is the code:

		bool unique = false;
		if (usedLevels.Count > 0) {
			while (!unique) {
				levelID = Random.Range (0, levels.Count);
				for (int i = 0; i < usedLevels.Count; i++) {
					if (i == usedLevels*) {*

_ if (usedLevels == levelID) {_
* unique = false;*
* break;*
* }*
* else {*
* unique = true;*
* }*
* }*
* }*
* }*
* }*

* Instantiate (levels[levelID]);*
* usedLevels.Add (levelID);*
* if (usedLevels.Count >= levels.Count)*
* usedLevels = new List ();*
This works for one round of levels but every time I get to the second level of the second round of levels Unity freezes. Which probably means it get’s stuck in the while loop, right? From making the usedLevels list public it seems to be reseting allright.
So how would I stop this from happening?

[Bunny83]

This is an unefficient and dangerous approach. It’s better to do it the other way round:

if (availableLevels.Count == 0)
    for(int i = 0; i < levels.Count; i++)
        availableLevels.Add(i);

int index = Random.Range (0, availableLevels.Count);
levelID = availableLevels[index];
availableLevels.RemoveAt(index);
Instantiate(levels[levelID]);