FormatNumberStrings.cs

using System;
using System.Globalization;

namespace myApp
{
    class Program
    {
        static void Main()
        {
            double value = 123456.78d;

            IFormatProvider usFormatProvider = 
                new System.Globalization.CultureInfo("en-US");
            
            Console.WriteLine(value.ToString("#,##0.00", usFormatProvider));
            // => 123,456.78 (American format)

            IFormatProvider frenchFormatProvider = 
                new System.Globalization.CultureInfo("fr-FR");
            
            Console.WriteLine(value.ToString("#,##0.00", frenchFormatProvider));
            // => 123 456,78 (French format)
        }
    }
}