public void Add(Level level)
{
if (level == null)
throw new ArgumentNullException("level");
if (_levels.ContainsValue(level))
throw new Exception("This level is already added.");
level.SetLevelNumber(LastLevelNumber + 1);
_levels.Add(level.LevelNumber, level);
}
public void Replace(Level oldLevel, Level newLevel)
{
newLevel.SetLevelNumber(oldLevel.LevelNumber);
_levels[oldLevel.LevelNumber] = newLevel;
}