using System;

public class Program

{

    public static void Main()

    {

        float floatValue = 123000.18f;

        decimal decimalValueFromString = decimal.Parse(floatValue.ToString("R"));

        Console.WriteLine("123000.18f: " + String.Format("{0:### ### ###.##}", decimalValueFromString));

        floatValue = 1023000.18f;

        decimalValueFromString = decimal.Parse(floatValue.ToString("R"));

        Console.WriteLine("1023000.18f: " + String.Format("{0:### ### ###.##}", decimalValueFromString));

        Console.WriteLine("");

        // приведение типа

        var t = 10000018.18f;

        Console.WriteLine("float: " + String.Format("{0:### ### ###.##}", t));

        Console.WriteLine("double: " + String.Format("{0:### ### ###.##}", (double)1000018.18f));

        Console.WriteLine("decimal: " + String.Format("{0:### ### ###.##}", (decimal)1000018.18f));

        Console.WriteLine("test: " + String.Format("{0:### ### ###.##}", 10000018.18d));

        Console.WriteLine("");

        Console.WriteLine(1234567f.ToString("#.##"));

        Console.WriteLine(12345678f.ToString("#.##"));

        Console.WriteLine(123456789f.ToString("#.##"));

        Console.WriteLine(1023456789f.ToString("#.##"));

        Console.WriteLine("");

        // с большими числами не поможет

        Console.WriteLine(String.Format("{0:### ### ###.##}", decimal.Parse(16777217f.ToString("R"))));     

        Console.WriteLine(String.Format("{0:### ### ###.##}", decimal.Parse(16777218f.ToString("R"))));     

    }

}