定制字符串Format

   class Program 
    {
        static void Main()
        {
            StringBuilder sb = new StringBuilder();
            IFormatProvider f=new BoldInt();
            BoldInt b = new BoldInt();
            sb.AppendFormat(f, "{0}  {1}  {2:M}", "Xiaobai", 21, DateTime.Now);
            Console.WriteLine(sb.ToString());
            Console.ReadKey();
        }
    }
    sealed class BoldInt : IFormatProvider, ICustomFormatter
    {
        #region ICustomFormatter 成员

        string ICustomFormatter.Format(string format, object arg, IFormatProvider formatProvider)
        {
            string s;
            IFormattable formattable = arg as IFormattable;
            if (formattable == null) s = arg.ToString();
            else s = formattable.ToString(format, formatProvider);
            if (arg.GetType() == typeof(Int32))                
                return "<B>"+s+"</B>";
            if(arg.GetType()==typeof(DateTime))
                return "<DateTime>"+s+"</DateTime>";
            if (arg.GetType() == typeof(String))
                return "<String>" + s + "</String>";
            return s;
        }

        #endregion
        #region IFormatProvider 成员
        object IFormatProvider.GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter)) return this;
            return Thread.CurrentThread.CurrentCulture.GetFormat(formatType);
        }
        #endregion
    }