示例: 定制格式化器

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;

namespace Test5
{
    class Program
    {
        static void Main(string[] args)
        {
            StringBuilder sb = new StringBuilder();
            sb.AppendFormat(new BoldInt32s(), "{0} {1} {2:M}", "Jeff", 123, DateTime.Now);
            Console.WriteLine(sb);
            Console.Read();
        }
    }

    internal sealed class BoldInt32s : IFormatProvider, ICustomFormatter
    {
        public Object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
                return this;
            //使用线程的语言文化信息来获取默认格式
            return Thread.CurrentThread.CurrentCulture.GetFormat(formatType);
        }

        public String 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>";
            return s;
        }
    }
}

运行测试