Use the static String.Format() method to build a new string.

   
  

using System;
using System.Collections.Generic;
using System.Text;

class Program {
    static void Main(string[] args) {

        string formatStr;
        formatStr = String.Format("Don't you wish you had {0:C} in your account?", 99989.987);
        Console.WriteLine(formatStr);
    }
}