using System;

public class Program

{

    public static void Main()

    {

        //User Inputs

        string min = "0.200";

        string max = "0.215";

        string userInput = "0.21150";

        //Get Significant Digits

        int significantDigitsMin =Methods.GetSignificantNumberOfDecimalPlaces(min); 

        int significantDigitsMax =Methods.GetSignificantNumberOfDecimalPlaces(max); 

        int significantDigitsUserInput =Methods.GetSignificantNumberOfDecimalPlaces(userInput); 

        //Check if significantDigits are enough

        if (significantDigitsMax > significantDigitsUserInput) 

        {

            Console.WriteLine("ERROR");

        }

        Console.WriteLine(Methods.TruncateToSignificantDigits(double.Parse(userInput), significantDigitsMax));

    }

}

​

public static class Methods 

{

    public static double TruncateToSignificantDigits(this double d, int digits){

        if(d == 0)

            return 0;

​

        double scale = Math.Pow(10, Math.Floor(Math.Log10(Math.Abs(d))) + 1);

        return scale * Math.Round(d / scale, digits);

    }

    public static int GetSignificantNumberOfDecimalPlaces(string d)

    {

        string inputStr = d.ToString();

        inputStr = inputStr.Replace(".", string.Empty);

        inputStr = inputStr.TrimStart('0');

        Console.WriteLine(inputStr.Length);

        return inputStr.Length;

    }

}