c# 生成2开头共16位数的随机方法

生成随机数，共16位第一个数值为2，写成一个方法，怎么实现和调用呢？

/// <summary>
        /// 生成16位随机数字
        /// </summary>
        /// <returns></returns>
        public static string Nmrandom()
        {
            string rm = "2";
            Random ra = new Random();
            for (int i = 0; i < 15; i++)
            {
                rm += ra.Next(0, 9).ToString();
            }
            return rm;
        }

 private ulong rand(int len=16)
        {
            ulong sum = 2;
            Random random = new Random();
            for (int i = 1; i < len; i++)
            {
                sum = sum * 10 + (ulong)random.Next(0, 9);
            }
            return sum;

        }