using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

using System.Text;

public class Program

{

    public static void Main()

    {

        var strings = new List<string> {

        "123.99999", // must become 124

        "129.99999", // must become 130

        "123.823467283762378",  // must become 123.82346728376238

        "123.823467283762398",  // must become 123.8234672837624 (notice the last 0 has gone)

        "123.09999999", // must become 123.1 (notice no trailing zeros needed..)

        "122.00000009", // must become 122.0000001

        "124.81379281", // must become simply 124.8137928

        "129.07872345", // must become 129.0787235

        "129.07872344", // must become 129.0787234

        };

​

        IEnumerable<string> results = strings.Select(s => s.RoundLongNumber());

        foreach(var res in results)

        {

            Console.WriteLine(res);

        }

    }

}

​

public static class Extensions

{

    public static string RoundLongNumber(this string input)

    {

        if (!ValidLongNumber(input, out bool isInteger) || isInteger) return input;

        int index = input.IndexOf('.');

        string part1 = input.Remove(index);

        string part2 = input.Substring(index + 1);

        StringBuilder sb = new StringBuilder(part2);

​

        while(true)

        {

            if (LastInt() <= 4)

            {

                return BuildNumber();

            }

​

            sb.Length = sb.Length - 1; // remove last

            int lastInt = LastInt() + 1;

            while (lastInt == 10)

            {

                sb.Length = sb.Length - 1;

                if (sb.Length == 0)

                {

                    // just integer remaining

                    int num = int.Parse(part1);

                    return (++num).ToString();

                }

                lastInt = LastInt() + 1;

            }

​

            sb[sb.Length - 1] = (char)(lastInt + '0');

            if (lastInt != 9) return BuildNumber();

        }

​

        int LastInt() => sb[sb.Length - 1] - '0';

        string BuildNumber() => part1 + "." + sb.ToString();

    }

​

    private static bool ValidLongNumber(string number, out bool isInteger)

    {

        isInteger = true;

        if (string.IsNullOrWhiteSpace(number)) return false;

        int pointCount = 0;

        foreach(char c in number)

        {

            bool isDigit = char.IsDigit(c);

            bool isPoint = c == '.';

            if (!isDigit) isInteger = false;

            if (isPoint) pointCount++;

            if(pointCount > 1 || (!isPoint && !isDigit)) return false;

        }

        return true;

    }

}