using System;

public class Program

{

    static decimal RoundFirstSignificantDigit(string input) {

        decimal significantDigit;

        if (!decimal.TryParse(input,out significantDigit))

        {

            throw new ArgumentException("Invalid input!!");

        }

        var floatLength = input.Split('.')[1].Length;

​

        return Math.Round(significantDigit, floatLength - 1, MidpointRounding.AwayFromZero);

    }

​

    public static void Main()

    {

        Console.WriteLine(RoundFirstSignificantDigit("123.99999")); // must become 124

        Console.WriteLine(RoundFirstSignificantDigit("123.823467283762378"));  // must become 123.82346728376238

        Console.WriteLine(RoundFirstSignificantDigit("123.823467283762398"));  // must become 123.8234672837624 (notice the last 0 has gone)

        Console.WriteLine(RoundFirstSignificantDigit("123.09999999")); // must become 123.1 (notice no trailing zeros needed..)

        Console.WriteLine(RoundFirstSignificantDigit("122.00000009")); // must become 122.0000001

        Console.WriteLine(RoundFirstSignificantDigit("124.81379281")); // must become simply 124.8137928

        Console.WriteLine(RoundFirstSignificantDigit("129.07872345")); // must become 129.0787235

        Console.WriteLine(RoundFirstSignificantDigit("129.07872344")); // must become 129.0787234

    }

}