NumberFormatInfo.NumberGroupSeparator Property

using System;
using System.Globalization;

class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      // Gets a NumberFormatInfo associated with the en-US culture.
      NumberFormatInfo nfi = new CultureInfo("en-US").NumberFormat;

      // Displays a value with the default separator (",").
      Int64 myInt = 123456789;
      outputBlock.Text += String.Format(myInt.ToString("N", nfi)) + "\n";

      // Displays the same value with a blank as the separator.
      nfi.NumberGroupSeparator = " ";
      outputBlock.Text += String.Format(myInt.ToString("N", nfi)) + "\n";
   }
}
/* 
This code produces the following output.

123,456,789.00
123 456 789.00
*/