using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string smallInput = "1,2,3,4,s,6";

        var result = ConvertToIntListNoLinq(smallInput);

​

        Console.WriteLine(string.Join(",", result));

    }

    public static List<int> ConvertToIntListNoLinq(string input)

    {

        List<int> output = new List<int>();

        foreach(string s in input.Split(','))

        {

            if(int.TryParse(s, out int result))

            {

                output.Add(result);

            }           

        }

        return output;

    }

}