net core 导出内容带微信表情emoji 后乱码问题

#region [解决微信表情emoji]
        /// <summary>
        /// 过滤emoji 或者 其他非文字类型的字符
        /// </summary>
        /// <param name="source">param source</param>
        /// <returns></returns>
        public static String filterEmoji(String source)
        {
            if (string.IsNullOrWhiteSpace(source))
            {
                return "";
            }
            source = source.Replace("[^\\u0000-\\uFFFF]", "").Replace("??", "");
            if (!containsEmoji(source))
            {
                return source; //如果不包含，直接返回
            }
            //到这里铁定包含
            StringBuilder buf = null;

            int len = source.Length;

            for (int i = 0; i < len; i++)
            {
                char codePoint = source[i];

                if (!isEmojiCharacter(codePoint))
                {
                    if (buf == null)
                    {
                        buf = new StringBuilder(source.Length);
                    }

                    buf.Append(codePoint);
                }
                else
                {
                }
            }

            if (buf == null)
            {
                return source; //如果没有找到 emoji表情，则返回源字符串
            }
            else
            {
                if (buf.Length == len)
                {
                    //这里的意义在于尽可能少的toString，因为会重新生成字符串
                    buf = null;
                    return source;
                }
                else
                {
                    return buf.ToString();
                }
            }

        }

        /// <summary>
        /// 检测是否有emoji字符
        /// </summary>
        /// <param name="source"></param>
        /// <returns></returns>
        public static bool containsEmoji(String source)
        {
            if (string.IsNullOrEmpty(source))
            {
                return false;
            }

            int len = source.Length;

            for (int i = 0; i < len; i++)
            {
                char codePoint = source[i];

                if (isEmojiCharacter(codePoint))
                {
                    //do nothing，判断到了这里表明，确认有表情字符
                    return true;
                }
            }

            return false;
        }

        /// <summary>
        /// 去掉表情符号
        /// </summary>
        /// <param name="codePoint"></param>
        /// <returns></returns>
        public static bool isEmojiCharacter(char codePoint)
        {
            return (codePoint >= 0x2600 && codePoint <= 0x27BF) // 杂项符号与符号字体
                   || codePoint == 0x303D
                   || codePoint == 0x2049
                   || codePoint == 0x203C
                   || (codePoint >= 0x2000 && codePoint <= 0x200F) //
                   || (codePoint >= 0x2028 && codePoint <= 0x202F) //
                   || codePoint == 0x205F //
                   || (codePoint >= 0x2065 && codePoint <= 0x206F) //
                   /* 标点符号占用区域 */
                   || (codePoint >= 0x2100 && codePoint <= 0x214F) // 字母符号
                   || (codePoint >= 0x2300 && codePoint <= 0x23FF) // 各种技术符号
                   || (codePoint >= 0x2B00 && codePoint <= 0x2BFF) // 箭头A
                   || (codePoint >= 0x2900 && codePoint <= 0x297F) // 箭头B
                   || (codePoint >= 0x3200 && codePoint <= 0x32FF) // 中文符号
                   || (codePoint >= 0xD800 && codePoint <= 0xDFFF) // 高低位替代符保留区域
                   || (codePoint >= 0xE000 && codePoint <= 0xF8FF) // 私有保留区域
                   || (codePoint >= 0xFE00 && codePoint <= 0xFE0F) // 变异选择器
                                                                   //   || (codePoint >= U + 2600 && codePoint <= 0xFE0F)
                   || codePoint >= 0x10000; // Plane在第二平面以上的，char都不可以存，全部都转

        }

        #endregion

来源：https://www.cnblogs.com/lilo202/p/8058012.html