Reading and Writing XML files using C# .NET

using System.Xml;

      void WriteXML()
      {
          try
          {
                              //pick whatever filename with .xml extension
              string filename = "XML"+DateTime.Now.Day + ".xml";

              XmlDocument xmlDoc = new XmlDocument();

              try
              {
                  xmlDoc.Load(filename);
              }
              catch(System.IO.FileNotFoundException)
              {
                  //if file is not found, create a new xml file
                  XmlTextWriter xmlWriter = new XmlTextWriter(filename, System.Text.Encoding.UTF8);
                  xmlWriter.Formatting = Formatting.Indented;
                  xmlWriter.WriteProcessingInstruction("xml", "version='1.0' encoding='UTF-8'");
                  xmlWriter.WriteStartElement("Root");
                  //If WriteProcessingInstruction is used as above,
                  //Do not use WriteEndElement() here
                  //xmlWriter.WriteEndElement();
                  //it will cause the <Root> to be &ltRoot />
                  xmlWriter.Close();
                  xmlDoc.Load(filename);
              }
              XmlNode root = xmlDoc.DocumentElement;
              XmlElement childNode = xmlDoc.CreateElement("childNode");
              XmlElement childNode2 = xmlDoc.CreateElement("SecondChildNode");
              XmlText textNode = xmlDoc.CreateTextNode("hello");
              textNode.Value = "hello, world";
            
              root.AppendChild(childNode);
              childNode.AppendChild(childNode2);
              childNode2.SetAttribute("Name", "Value");
              childNode2.AppendChild(textNode);
            
              textNode.Value = "replacing hello world";
              xmlDoc.Save(filename);
          }
          catch(Exception ex)
          {
              WriteError(ex.ToString());
          }
      }

      void WriteError(string str)
      {
          outputBox.Text = str;
      }