定製字串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 成員

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