var folders = GetLeafFolders(DirFolder);
Console.WriteLine(folders[random.Next(folders.Count)]);

List<string> GetLeafFolders(string topFolder)
{
	var result = new List<string>();
	
	var subFolders = Directory.GetDirectories(topFolder);
	
	if (subFolders.Length == 0)
	{
		result.Add(topFolder);
		return result;
	}
	
	foreach (var folder in subFolders)
	{
		result.AddRange(GetLeafFolders(folder));
	}
	return result;
}