读取XML文件中获取特定值

        /// <summary>
        /// 获取子系统驱动获取DCM配置文件的特定值
        /// </summary>
        /// <param name="configurationFile"></param>
        /// <param name="key"></param>
        /// <returns></returns>
        public static string ReadValueByKey(ConfigurationFile configurationFile, string key)
        {
            string value = string.Empty;
            string filename = string.Empty;
            if (configurationFile.ToString() == ConfigurationFile.AppXml.ToString())
            {
                //string exeroot = "F:\DCM\DLAX.DataCollectorMiddleware\DLAX.DataCollectorMiddleware\bin\Debug\DeviceLib\视频子系统\北京电科院\BJDKY\Start.exe";
                string exeroot = System.Windows.Forms.Application.ExecutablePath;
                if (exeroot.IndexOf("DeviceLib") > 0)
                {
                    string fileroot = exeroot.Substring(0, exeroot.IndexOf("DeviceLib"));
                    filename = fileroot + "dcm.xml";
                }

                else
                {
                    filename = System.AppDomain.CurrentDomain.BaseDirectory + "dcm.xml";
                }
                    
            }

            XmlDocument doc = new XmlDocument();
            try
            {
                doc.Load(filename); //加载配置文件
            }
            catch (Exception ex)
            {
                return null;
            }
            XmlNode node = doc.SelectSingleNode("//appSettings");   //得到[appSettings]节点

            ////得到[appSettings]节点中关于Key的子节点
            XmlElement element = (XmlElement)node.SelectSingleNode("//add[@key='" + key + "']");

            if (element != null)
            {
                value = element.GetAttribute("value");
            }

            return value;
        }