来源：http://www.cnblogs.com/mahatmasmile/p/3467495.html
/// <summary>
        /// GB2312转换成UTF8
        /// </summary>
        /// <param name="text"></param>
        /// <returns></returns>
        public static string gb2312_utf8(string text)
        {
            //声明字符集   
            System.Text.Encoding utf8, gb2312;
            //gb2312   
            gb2312 = System.Text.Encoding.GetEncoding("gb2312");
            //utf8   
            utf8 = System.Text.Encoding.GetEncoding("utf-8");
            byte[] gb;
            gb = gb2312.GetBytes(text);
            gb = System.Text.Encoding.Convert(gb2312, utf8, gb);
            //返回转换后的字符   
            return utf8.GetString(gb);
        }

        /// <summary>
        /// UTF8转换成GB2312
        /// </summary>
        /// <param name="text"></param>
        /// <returns></returns>
        public static string utf8_gb2312(string text)
        {
            //声明字符集   
            System.Text.Encoding utf8, gb2312;
            //utf8   
            utf8 = System.Text.Encoding.GetEncoding("utf-8");
            //gb2312   
            gb2312 = System.Text.Encoding.GetEncoding("gb2312");
            byte[] utf;
            utf = utf8.GetBytes(text);
            utf = System.Text.Encoding.Convert(utf8, gb2312, utf);
            //返回转换后的字符   
            return gb2312.GetString(utf);
        }