using System;

public class Program

{

    public static void Main()

    {

        double d = 999999999999999;

        Console.WriteLine(d.TruncateToSignificantDigits(3)); // wrong - shows 99000...

        d = 999999999999996;

        Console.WriteLine(d.TruncateToSignificantDigits(3)); // right - shows 99900...

    }

}

​

public static class Extensions

{

    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 - digits);

        return scale * Math.Truncate(d / scale);

    }

}