List<string> list = new List<string>(){ "1","2","3","4","5","6"};
            //方式一、
            string result = "";
            for (int i = 0; i < list.Count; i++)
            {
                result = result + list[i] + ",";
            }
            Console.WriteLine(result);
            Console.WriteLine(result.TrimEnd(','));//去掉最后一个","
            //方式二、
            string result2= String.Join(",", list);
            Console.WriteLine(result2);
            //方式三、
            string result3 = list.Aggregate("", (current, s) => current + (s + ","));
            Console.WriteLine(result3.TrimEnd(','));