编码转换

 static string UNICODE_ToChinese(string content)
        {
            string GBK_Code = "";
            string GBK_Value = "";
            try
            {
                string pattern = @"\\u([A-Fa-f0-9]{1,4})";
                if (!Regex.IsMatch(content, pattern))
                    return content;
                MatchCollection collection = Regex.Matches(content, pattern);
                HashSet<string> hs = new HashSet<string>();
                foreach (Match item in collection)
                {
                    GBK_Code = item.Value;
                    if (hs.Contains(GBK_Code))
                        continue;
                    hs.Add(GBK_Code);
                    GBK_Value = item.Groups[1].Value;
                    string chinese = ((char)Convert.ToInt32(GBK_Value, 16)).ToString();
                    content = content.Replace(GBK_Code, chinese);
                }
                return content;
            }
            catch (Exception ex)
            {
                var t1 = GBK_Value;
                var t = GBK_Code;
                return "";
            }
        }
        static string GBK_X_ToChinese(string content)
        {
            string GBK_Code = "";
            string GBK_Value = "";
            try
            {
                string pattern = @"&#x([\dA-Fa-f]{1,4});";
                if (!Regex.IsMatch(content, pattern))
                    return content;
                MatchCollection collection = Regex.Matches(content, pattern);
                HashSet<string> hs = new HashSet<string>();
                foreach (Match item in collection)
                {
                    GBK_Code = item.Value;
                    if (hs.Contains(GBK_Code))
                        continue;
                    hs.Add(GBK_Code);
                    GBK_Value = item.Groups[1].Value;
                    string chinese = ((char)Convert.ToInt32(GBK_Value, 16)).ToString();
                    content = content.Replace(GBK_Code, chinese);
                }
                return content;
            }
            catch (Exception ex)
            {
                var t1 = GBK_Value;
                var t = GBK_Code;
                return "";
            }
        }
        static string GBKToChinese(string content)
        {
            string GBK_Code = "";
            string GBK_Value = "";
            try
            {
            string pattern = "&#(\\d{1,4});";
            if (!Regex.IsMatch(content, pattern))
                return content;
            MatchCollection collection = Regex.Matches(content, pattern);
            HashSet<string> hs = new HashSet<string>();
            foreach (Match item in collection)
            {
                
                 GBK_Code = item.Value;
                 if (hs.Contains(GBK_Code))
                     continue;
                 hs.Add(GBK_Code);
                 GBK_Value = item.Groups[1].Value;
                string chinese = ((char)Convert.ToInt32(GBK_Value, 10)).ToString();
                content = content.Replace(GBK_Code, chinese);
            }
            return content;
            }
            catch (Exception ex)
            {
                var t1 = GBK_Value;
                var t = GBK_Code;
                return "";
            }
        }

        private static string conventString(string str)
        {
            StringBuilder sb = new StringBuilder();
            foreach (char c in str)
                sb.AppendFormat("\\u{0}", Convert.ToString((int)c, 16).PadLeft(4,'0'));
            return sb.ToString();
        }

备注：PadLeft(4,'0')不足4位以0补其，很重要