C#操作XML文档

XmlDocument xmldoc = new XmlDocument();
string filename = Server.MapPath("..//") + "Web.config";
xmldoc.Load(filename);
XmlNode root = xmldoc.SelectSingleNode("configuration");
XmlNode conNode = root.SelectSingleNode("connectionStrings");
XmlNodeList conNodeList = conNode.ChildNodes;
foreach (XmlNode node in conNodeList)
{
XmlElement element = (XmlElement)node;

if (element.GetAttribute("providerName") == "System.Data.OleDb")
{

element.SetAttribute("connectionString", sb.ToString());

element.SetAttribute("name", "connectionString");
}
xmldoc.Save(filename);
}