通过实现IFormatProvider, ICustomFormatter接口可以实现自定义的格式输现，这里有实现一个例子，以输出日期格式为例(显示今天、明天、后天和"x月x日"等)

1、假设我们有多种显示日期格式的需求，我们可以定义一个枚举如下：

    public enum DateTimeFormat
    {
        WithDayName,//含"前天、昨天、今天、明天、后天的显示"
        WithWeek//含星期几的显示
    }

2、每种类型实现 IFormatProvider, ICustomFormatter接口。

internal class WithDayNameFormat : IFormatProvider, ICustomFormatter
    {
        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
            {
                return this;
            }
            else
            {
                return Thread.CurrentThread.CurrentCulture.GetFormat(formatType);
            }
        }public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            string str;
            IFormattable formatable = arg as IFormattable;
            if (formatable == null)
            {   str = arg.ToString();}
            else
            {
                str = formatable.ToString(format, formatProvider);
            }
            if (arg.GetType() == typeof(DateTime))
            {
                DateTime dt = (DateTime)arg;
                int days = (DateTime.Now - dt).Days;
                switch(days)
                {
                    case -2:
                        str="前天";
                        break;
                    case -1:
                        str = "明天";
                        break;
                    case 0:
                        str = "今天";
                        break;
                    case 1:
                        str = "明天";
                        break;
                    case 2:
                        str = "后天";
                        break;
                    default:
                        str = string.Format("{0}月{1}日", dt.Month, dt.Day);
                        break;
                }
            }
            return str;
        }

    }

internal class WithWeekFormat : IFormatProvider, ICustomFormatter
    {
        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
            {
                return this;
            }
            else
            {
                return Thread.CurrentThread.CurrentCulture.GetFormat(formatType);
            }
        }public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            string str;
            IFormattable formatable = arg as IFormattable;
            if (formatable == null)
            { str = arg.ToString(); }
            else
            {
                str = formatable.ToString(format, formatProvider);
            }
            if (arg.GetType() == typeof(DateTime))
            {
                DateTime dt = (DateTime)arg;
                string dayOfWeef = dt.DayOfWeek.ToString().Trim();
                switch (dayOfWeef)
                {
                    case "Monday":
                        dayOfWeef = "一";
                        break;
                    case "Tuesday":
                        dayOfWeef = "二";
                        break;
                    case "Wednesday":
                        dayOfWeef = "三";
                        break;
                    case "Thursday":
                        dayOfWeef = "四";
                        break;
                    case "Friday":
                        dayOfWeef = "五";
                        break;
                    case "Saturday":
                        dayOfWeef = "六";
                        break;
                    case "Sunday":
                        dayOfWeef = "日";
                        break;
                }
                return string.Format("{0}年{1}月{2}日 星期{3}", dt.Year, dt.Month, dt.Day, dayOfWeef);
            }
            return str;
        }
    }

3、使用扩展方法对使用进行封装

    public static class DateTimeExtention
    {
        public static string ToCustomFormat(this DateTime dt, DateTimeFormat format)
        {
            string str;
            switch (format)
            {
                case DateTimeFormat.WithDayName:
                    str = string.Format(new WithDayNameFormat(), "{0}", dt);
                    break;
                case DateTimeFormat.WithWeek:
                      str = string.Format(new WithWeekFormat(), "{0}", dt);
                      break;
                default:
                    throw new NotSupportedException("不支持的日期格式类型DateTimeFormat");
                    break;
            }
            return str;
        }
    }

4、客户端使用：

        static void Main(string[] args)
        {
            Console.WriteLine(DateTime.Now.AddDays(-3).ToCustomFormat(DateTimeFormat.WithDayName));
            Console.WriteLine(DateTime.Now.AddDays(-2).ToCustomFormat(DateTimeFormat.WithDayName));
            Console.WriteLine(DateTime.Now.AddDays(-1).ToCustomFormat(DateTimeFormat.WithDayName));
            Console.WriteLine(DateTime.Now.AddDays(0).ToCustomFormat(DateTimeFormat.WithDayName));
            Console.WriteLine(DateTime.Now.AddDays(1).ToCustomFormat(DateTimeFormat.WithDayName));
            Console.WriteLine(DateTime.Now.AddDays(2).ToCustomFormat(DateTimeFormat.WithDayName));
            Console.WriteLine(DateTime.Now.AddDays(3).ToCustomFormat(DateTimeFormat.WithDayName));

            Console.WriteLine(DateTime.Now.AddDays(