using System;
using System.Globalization;
using static System.Console;

namespace System.Runtime.CompilerServices
{
    public class FormattableStringFactory
    {
        public static IFormattable Create(string messageFormat, params object[] args)
        {
            return new FormattableString(messageFormat, args);
        }

        public class FormattableString : IFormattable
        {
            private readonly string _messageFormat;
            private readonly object[] _args;

            public FormattableString(string messageFormat, object[] args)
            {
                _messageFormat = messageFormat;
                _args = args;
            }

            public string ToString(string format, IFormatProvider formatProvider)
            {
                return string.Format(formatProvider, _messageFormat, _args);
            }
        }
    }
}

namespace CsTests
{
    public static class FormattableExt
    {
        public static string AsFr(this IFormattable format)
        {
            return format.ToString(null, CultureInfo.GetCultureInfo("fr-FR"));
        }
    }
    class Program
    {
        static string FR(IFormattable src)
        {
            return src.ToString(null, CultureInfo.GetCultureInfo("fr-FR"));
        }

        static string US(IFormattable src)
        {
            return src.ToString(null, CultureInfo.GetCultureInfo("en-US"));
        }
        static string INV(IFormattable src)
        {
            return src.ToString(null, CultureInfo.InvariantCulture);
        }

        private static void Main(string[] args)
        {
            WriteLine($"Number: {999 * 2:N}; the current date is {DateTime.Now + TimeSpan.FromHours(12)}");
            WriteLine(FR($"Number: {999 * 2:N}; the current date is {DateTime.Now + TimeSpan.FromHours(12)}"));
            WriteLine(US($"Number: {999 * 2:N}; the current date is {DateTime.Now + TimeSpan.FromHours(12)}"));
            WriteLine(INV($"Number: {999 * 2:N}; the current date is {DateTime.Now + TimeSpan.FromHours(12)}"));
        }
    }
}