XmlDocument xDoc = new XmlDocument();

             try

             {

                 xDoc.Load(配置文件路径);

                 XmlNode xNode;

                 XmlElement xElem;

                 xNode = xDoc.SelectSingleNode("//appSettings");　　　　//补充，需要在你的app.config 文件中增加一下，<appSetting> </appSetting>

                 xElem = (XmlElement)xNode.SelectSingleNode("//add[@key='" + 读取的配置KEY+ "']");

                 if (xElem != null)

                     return xElem.GetAttribute("value");

                 else

                     return "";

             }

             catch (Exception)

             {

                 return "";

             }