public static    string  get_uft8(string unicodeString)
        {
            UTF8Encoding utf8 = new UTF8Encoding();
            Byte[] encodedBytes = utf8.GetBytes(unicodeString);
            String decodedString = utf8.GetString(encodedBytes);
            return decodedString;
        }

   这边我以big5转换gb2312为例
   Encoding big5 =Encoding.GetEncoding("big5");
   Encoding gb2312 = Encoding.GetEncoding("gb2312");

   byte[] big5b= big5.GetBytes("編程無悔！");
   //关键也就是这句了
   byte[] gb2312b= Encoding.Convert(big5,gb2312,big5b);

   string strGb2312 = gb2312.GetString(gb2312b)