C#

XmlDocument doc = new XmlDocument();
doc.Load(<the file="">);

XmlNodeCollection nodes = doc.SelectNodes("//<element name="">");</element></the>

XmlDocument doc = new XmlDocument();
doc.Load();


XmlNode editing = doc.SelectSingleNode(treeView1.SelectedNode.Text); //here I'm trying to find a node with selected single node's text

editing.NextSibling.InnerText = "changedText"; //and here I'm trying to change the inner text of the arg or ges nodes' innertext

XmlNode editing = doc.SelectSingleNode(@"\\" +treeView1.SelectedNode.Text); 

XmlNode editing = dom.SelectSingleNode(@"//" + treeView1.SelectedNode.Text);  //this is not a problem any more
editing.NextSibling.InnerText = "changedText"; //this line says : "NullReferenceException was unhandled....  Object reference not set to an instance of an object.

foreach(MyClass obj in MyClassCollection)
     {
         dataRow[dataColumn] = obj;
     }

dataRow.Tag = myClass;

MyClass myClass = (MyClass)dataRow.Tag;