using System;

    public class Example

    {

        public string round(string LargeDecimal)

        {

            Console.WriteLine("Number as string is: " + LargeDecimal);

            int lastDigit = (int)char.GetNumericValue(LargeDecimal[LargeDecimal.Length -1]);   // get last character

            Console.WriteLine("lastDigit = " + lastDigit.ToString());

            string number = LargeDecimal.Remove(LargeDecimal.Length - 1);  // delete last character

            Console.WriteLine("Now number is " + number);

            if (lastDigit > 4)

            {

                Console.WriteLine("Last digit {0} was >4", lastDigit.ToString());

                int newLastDigit = (int)char.GetNumericValue(number[number.Length -1]);

                Console.WriteLine("Next to left last digit is {0} which will be raised by 1 and become {1}", newLastDigit.ToString(), (newLastDigit +1).ToString());

                newLastDigit += 1;                             //increase by one

                number = number.Remove(number.Length - 1);  // delete ex-penultimate (and now last) character

                number = number + newLastDigit.ToString(); 

                return number;       // and add a digit increased by 1

            }

            else

            {

                return LargeDecimal.Remove(LargeDecimal.Length - 2);

            }

        }

        public Example()

        {}

    }

​

public class Program

{

    public static void Main(string[] args)

    {

        string myNumber = "124.2398478278268985738276523548769";

​

        Example myExample = new Example();

        string result = myExample.round(myNumber);

​

        Console.WriteLine("Now I have " + result);

    }

}