using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        double val = 0.00500008;

        Console.WriteLine(SignificantFiguresFormatter.FormatWithSignificantFigures(val,3));

        Console.WriteLine(SignificantFiguresFormatter.FormatWithSignificantFigures(val,4));

        Console.WriteLine(SignificantFiguresFormatter.FormatWithSignificantFigures(val,5));

        Console.WriteLine(SignificantFiguresFormatter.FormatWithSignificantFigures(val,6));

        Console.WriteLine(SignificantFiguresFormatter.FormatWithSignificantFigures(val,7));

    }

}

​

public static class SignificantFiguresFormatter

{

    public static string FormatWithSignificantFigures(double value, int significantFigures)

    {

        if (double.IsNaN(value) || double.IsInfinity(value) || value == 0.0)

        {

            return value.ToString("G", CultureInfo.InvariantCulture);

        }

​

        // Determine the number of digits before the decimal point

        int digitsBeforeDecimal = (int)Math.Floor(Math.Log10(Math.Abs(value))) + 1;

        int totalDigits = significantFigures;

​

        // Calculate how many decimal places are needed

        int decimalPlaces = totalDigits - digitsBeforeDecimal;

​

        // Use scientific notation if number is too small or large

        if (Math.Abs(digitsBeforeDecimal) > 5)

        {

            return value.ToString("E" + (significantFigures - 1), CultureInfo.InvariantCulture);

        }

​

        // Round and format the number with the correct number of decimal places

        string formatString = "F" + Math.Max(decimalPlaces, 0);

        double rounded = Math.Round(value, Math.Max(decimalPlaces, 0), MidpointRounding.AwayFromZero);

        string result = rounded.ToString(formatString, CultureInfo.InvariantCulture);

​

        // Ensure trailing zeros are preserved (important for whole numbers)

        if (!result.Contains(".") && decimalPlaces > 0)

        {

            result += "." + new string('0', decimalPlaces);

        }

​

        return result;

    }

}