using System;

using System.Globalization;

using System.Text.RegularExpressions;

#nullable enable

public class Program

{

    public static void Main()

    {

        decimal test = 100_000_000.123m;

        var str = test.ToString("N2", new CultureInfo("pl")).Replace(" ", ".", StringComparison.InvariantCultureIgnoreCase);

        Console.WriteLine(test);

        Console.WriteLine(str);

    }       

}