Add / Update url paramter

UrlParamterAddUpdate

        public static string AddToUrlQuerystring(HttpRequest req, string paramName, object paramValue, bool encodeValue)
        {
            var sb = new StringBuilder();
            string ParamValue;

            // Encode parameter value or not
            if (encodeValue)
                ParamValue = SPEncode.UrlEncode(paramValue.ToString());
            else
                ParamValue = paramValue.ToString();

            // Add url without query string
            sb.AppendFormat("{0}{1}{2}{3}", req.Url.Scheme,Uri.SchemeDelimiter, 
                req.Url.Authority, req.Url.AbsolutePath);

            // Add '?' to url
            sb.Append("?");

            bool IsParamExist = false;
            foreach(string key in req.QueryString.AllKeys)
            {
                // make sure key exist or it is not empty
                if (string.IsNullOrEmpty(key)) { continue; }

                string KeyUrlEncoded = SPEncode.UrlEncode(key);
                string KeyValueUrlEncoded = SPEncode.UrlEncode(req.QueryString[key]);

                sb.Append("&");
                sb.Append(SPEncode.UrlEncode(key));
                sb.Append("=");
                if (key == paramName)
                {
                    // Key already exist, so replace value only
                    sb.Append(ParamValue);
                    IsParamExist = true;
                }
                else
                    sb.Append(KeyValueUrlEncoded);
                        
            }

            if (!IsParamExist)
            {
                sb.Append("&" + SPEncode.UrlEncode(paramName) + "=" + ParamValue);
            }

            // remove the first occurance of "?&"
            string pattern = Regex.Escape("?&");
            var rx = new Regex(pattern);
            return rx.Replace(sb.ToString(), "?", 1);
        }