IFormatable

Code
namespace RobotH.MyFormat
{
    using System;
    using System.Globalization;
    class MyClass : System.IFormattable
    {
        private double _d;
        public MyClass(double d)
        {
            this._d = d;
        }


        #region IFormattable 成员

        public string ToString(string format, IFormatProvider formatProvider)
        {
            if (formatProvider != null)
            {
                ICustomFormatter cust = formatProvider.GetFormat(this.GetType()) as ICustomFormatter;
                if (cust != null)
                {
                    return cust.Format(format, this, formatProvider);
                }
                else
                {
                    return _d.ToString(format, formatProvider);
                }
            }
            else
            {
                switch (format)
                {
                    case "special":
                        return "special";
                    default:
                        return _d.ToString(format, formatProvider);
                }
            }
        }

        #endregion
    }
    class App
    {
        static void Main()
        {
            CultureInfo ci = null;
            MyClass cls = new MyClass(4);        
            Console.WriteLine("China Currency's Format"+cls.ToString("C",null));
            ci=CultureInfo.CurrentCulture;
            Console.WriteLine("system Currency's Format"+cls.ToString("C",ci));
            ci=new CultureInfo("zh-HK");
            Console.WriteLine("system Currency's Format" + cls.ToString("C", ci));
            Console.Read();
        }
    }