using System.Collections.Generic;
public static void Main()
List<int> list = new List<int> { 1, 2, 3, 4, 5, 6 };
int nextNumber = list[(list.IndexOf(currentNumber) + 1) % list.Count];
int previousNumber = list[(list.IndexOf(currentNumber) - 1 + list.Count) % list.Count];
Console.WriteLine(nextNumber);
Console.WriteLine(previousNumber);