nMail.Attachment.GetHeaderField C# (CSharp) Method

        public string GetHeaderField(string field_name)
        {
            _field_name = field_name;
            SetHeaderSize();
            _field = new StringBuilder(_header_size);
            _err = Pop3GetHeaderField(_field, _header.ToString(), _field_name, _header_size);
            if(_err < 0)
            {
                throw new nMailException("GetHeaderField", _err);
            }
            return _field.ToString();
        }

        /// <summary>
        /// 重要度取得
        /// </summary>
        /// <param name="header">ヘッダ</param>
        /// <returns>重要度(urgent/normal/non-urgent)</returns>
        private string GetPriority(string header)
        {
            string _priority = "normal";
            string priority = "";

            nMail.Attachment attach = new nMail.Attachment();

            // ヘッダにX-Priorityがあるとき
            if (header.Contains("X-Priority:")) {
                priority = attach.GetHeaderField("X-Priority:", header);

                if (priority == "1" || priority == "2") {
                    _priority = "urgent";
                }
                else if (priority == "3") {
                    _priority = "normal";
                }
                else if (priority == "4" || priority == "5") {
                    _priority = "non-urgent";
                }
            }
            else if (header.Contains("X-MsMail-Priotiry:")) {
                priority = attach.GetHeaderField("X-MsMail-Priotiry:", header);

                if (priority.ToLower() == "High") {
                    _priority = "urgent";
                }
                else if (priority.ToLower() == "Normal") {
                    _priority = "normal";
                }
                else if (priority.ToLower() == "low") {
                    _priority = "non-urgent";
                }
            }
            else if (header.Contains("Importance:")) {
                priority = attach.GetHeaderField("Importance:", header);

                if (priority.ToLower() == "high") {
                    _priority = "urgent";
                }
                else if (priority.ToLower() == "normal") {
                    _priority = "normal";
                }
                else if (priority.ToLower() == "low") {
                    _priority = "non-urgent";
                }
            }
            else if (header.Contains("Priority:")) {
                _priority = attach.GetHeaderField("Priority:", header);
            }

            return _priority;
        }