using System;
using System.Linq;

public class Test
{
	public static void Main()
	{
		Console.WriteLine(RoundToSignificantFigures(20.051M, 4));
		Console.WriteLine(RoundToSignificantFigures(20.001M, 4));
	}
	
private static readonly decimal[] Pows = Enumerable.Range(-28, 57).Select(p => (decimal)Math.Pow(10, p)).ToArray();

/// <remarks>https://stackoverflow.com/a/18146056/3194005</remarks>
public static decimal RoundToSignificantFigures(decimal value, int significantFigures)
{
    if (value == 0)
        return 0;

    int d = Log10Ceiling(Math.Abs(value));
    int power = significantFigures - d;

    decimal magnitude = (decimal)Math.Pow(10, power);

    var res = Math.Round(value * magnitude) / magnitude;
    var invMag = 1/magnitude;
    return res + invMag - invMag;
}

private static int Log10Ceiling(decimal value)
{
    int log10 = Array.BinarySearch(Pows, value);

    return (log10 >= 0 ? log10 : ~log10) - 28;
}	
	
	
}