using System;

public class Program

{

    public static void Main()

    {

        var example1 =  1.12345638282382384;

        var example2 = 12.12;

        var example3 = 30.19999;

        Console.WriteLine(string.Format("{0:N4}", TruncateDigits(example1, 4)));

        Console.WriteLine(string.Format("{0:N4}", TruncateDigits(example2, 4)));

        Console.WriteLine(string.Format("{0:N4}", TruncateDigits(example3, 4)));

    }

    public static double TruncateDigits(double value, int places)

    {

        var integral = Math.Truncate(value);

        var fraction = value - integral;

​

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

        var truncatedFraction = Math.Truncate(fraction * multiplier) / multiplier;

​

        return integral + truncatedFraction;

    }

}

​