c# - How to create a xmlnode as in given format in asp.net web.config file -

i want add new rule in web.config file programmatically, not able , error

"system.exception: expression must evaluate node-set.".

my code follows..

 xmldocument doc = new xmldocument();           bool change = false;          string configfile = path.combine( server.mappath("~").tostring(), "web.config");          doc.load(configfile);   xmlnode xmlnode1 = cfg.xmlsection.selectsinglenode("rewriterules");             xmlnode xmlnode = xmlnode1.selectsinglenode("rule");             if (xmlnode != null)             {                  string nodeformat = string.format("<rule source='{0}'  destination='{1}' />", source, destination); try                 {                      xmlelement xmlelement = (xmlelement)(xmlnode.selectsinglenode(nodeformat));//error occured                     if (xmlelement != null)                     {                         //xmlelement.setattribute("source", source);                         //xmlelement.setattribute("destination", destination);                         //xmlnode.appendchild(xmlelement);                     }                      else                     {                         xmlelement = doc.createelement("rule");                         xmlelement.setattribute("source", source);                         xmlelement.setattribute("destination", destination);                         xmlnode.appendchild(xmlelement);                     }                     doc.save(configfile);                     //savewebconfig( xmldoc );                 }                 catch (exception ex)                 {                     throw new exception(ex.message);                 } 

please me...

can not use linq xml? it's easier querying/updating xml files.

for example, have method query xml , return true/false if element exists depending on source , destination attributes

public static bool ruleexists(string source, string destination) {      xdocument doc = xdocument.load(httpcontext.current                               .server.mappath("your file"));       return doc.descendants("rewriterules").elements()                .where(e => e.attribute("source").value == source                 && e.attribute("destination").value == destination).any(); } 

this tell if rule exists. modify existing code this:

xdocument xml = xdocument.load(httpcontext.current.server.mappath("your file"));  //replace actual source/destination if (ruleexists("about/about-ashoka", "about/about.aspx")) {      //element in config file      //do something... } else {      xelement elem = new xelement("rule");      elem.setattributevalue("source", source);      elem.setattributevalue("destination", destination);      xml.element("rewriterules").add(elem);      xml.save(httpcontext.current.server.mappath("your file"));  }