using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        var ni = new CultureInfo(CultureInfo.CurrentCulture.Name).NumberFormat;

        ni.NumberDecimalDigits = 0;     //Keep the ".00" from appearing

        ni.NumberGroupSeparator = " ";  //Set the group separator to a space

        ni.NumberGroupSizes = new int[] { 3 };  //Groups of 3 digits

        var dig = 123456789;

        Console.WriteLine(dig.ToString("N", ni));

    }

}