using System;

public class Program

{

    public static void Main()

    {

        string qd = "50.5439999";

        Console.WriteLine(TruncateDigits(qd,3));

​

    }

    private static string TruncateDigits(string value, int places)

    {

        if (places <= 0)

        {

            throw new InvalidOperationException("Places to shift must be greater then zero!");

        }

​

        decimal valueDecimal = Convert.ToDecimal(value);

​

        if (Decimal.Round(valueDecimal, places) != valueDecimal)

        {

            var multiplier = (decimal)Math.Pow(10, places);

​

            return (Math.Truncate(Convert.ToDecimal(value) * multiplier) / multiplier).ToString();

        }

        else

        {

            return value;

        }

    }

}