using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        int? numeroDecimali = 2;

         var  NumberFormatInfoWithoutGroupSeparator = (NumberFormatInfo)CultureInfo.CurrentUICulture.NumberFormat.Clone();

                        NumberFormatInfoWithoutGroupSeparator.NumberGroupSeparator = "";

​

                 var  InvariantNumberFormatInfoWithoutGroupSeparator = (NumberFormatInfo)CultureInfo.InvariantCulture.NumberFormat.Clone();

                        InvariantNumberFormatInfoWithoutGroupSeparator.NumberGroupSeparator = "";

        decimal a = 0;

        decimal b = 123456789.123456789M;

        Console.WriteLine(string.Format(NumberFormatInfoWithoutGroupSeparator, string.Concat("{0:n", (numeroDecimali ?? 0), "}"), a));

        Console.WriteLine(string.Format(NumberFormatInfoWithoutGroupSeparator, string.Concat("{0:n", (numeroDecimali ?? 0), "}"), b));

            Console.WriteLine(string.Format(InvariantNumberFormatInfoWithoutGroupSeparator, string.Concat("{0:n", (numeroDecimali ?? 0), "}"), a));

        Console.WriteLine(string.Format(InvariantNumberFormatInfoWithoutGroupSeparator, string.Concat("{0:n", (numeroDecimali ?? 0), "}"), b));

    }

}