using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        decimal myDecimal = 12345.67m;

​

        NumberFormatInfo nfi = new NumberFormatInfo { NumberDecimalSeparator = "," };

​

        string formattedNumber = string.Format(nfi, "{0:### ### ###.###}", myDecimal);

​

        Console.WriteLine(formattedNumber);

    }

}