public class RandomEx
{
    static Random r = new Random();
    /// <summary>
    /// 递归 生成不重复的随机数 ///不存在死循环
    /// </summary>
    /// <param name="start">随机数范围：开始</param>
    /// <param name="end">随机数范围：结束</param>
    /// <param name="len">随机数个数</param>
    /// <param name="list">生成的随机数</param>
    public static void Random(int start, int end, int len, List<int> list)
    {        
        if (list.Count == len||start>end) return;
        int i= r.Next(start, end);
        if (!list.Contains(i))
            list.Add(i);
        if (i > start)
            Random(start, i-1, len, list);
        if (i < end)
            Random(i+1, end, len, list);
    }
}