public static List<T>  Swap<T>(this List<T> list, int index1,int index2)

        {

            if(index1<0||index1>=list.Count)

            {

                throw new Exception("index1");

            }

            if (index2 < 0 || index2 >= list.Count)

            {

                throw new Exception("index2");

            }

            var temp = list[index1];

            list[index1] = list[index2];

            list[index2] = temp;

            return list;

        }