Add a root node to an XML Document

public static void Main()
 {
   XmlDocument docA = new XmlDocument();
   XmlDocument docB = new XmlDocument();

   docA.LoadXml("<?xml version=""1.0"" encoding=""utf-8""?><root>some text here<pre lang=""text""></pre></root>");
   docB.loadXml("<?xml version=""1.0"" encoding=""utf-8""?><data></data>");

    System.IO.StringWriter sw;
    sw = new System.IO.StringWriter();
    docA.Save(sw);
   //Create a CData section.
   XmlCDataSection CData;
   CData = docB.CreateCDataSection(sw.ToString());

   //Add the new node to the document.
   XmlElement root = docB.DocumentElement;
   root.AppendChild(CData);

   docB.save("C:\\result.xml");

 }

XmlDocument origXml = new XmlDocument();
origXml.Load("YourXml.Xml");

XmlDocument newXml = new XmlDocument();
newXml.LoadXml("<data></data>");


XmlNode rootNode= newXml.ImportNode(origXml.DocumentElement, true);

newXml.DocumentElement.AppendChild(rootNode);

Here's the XML File:

<?xml version="1.0" encoding="utf-8"?>
<CategoryList>
  <Category ID="01">
    <MainCategory>XML</MainCategory>
    <Description>This is a list my XML articles.</Description>
    <Active>true</Active>
  </Category>
</CategoryList>
Here's the code:

<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Xml" %>
<%@ Page Language="C#" Debug="true" %>

<script  runat="server">
void Page_Load(object sender, System.EventArgs e){
    if(!Page.IsPostBack){
        XmlDocument xmlDoc = new XmlDocument();


        // Write down the XML declaration
        XmlDeclaration xmlDeclaration = xmlDoc.CreateXmlDeclaration("1.0","utf-8",null);

        // Create the root element
        XmlElement rootNode  = xmlDoc.CreateElement("CategoryList");
        xmlDoc.InsertBefore(xmlDeclaration, xmlDoc.DocumentElement);
        xmlDoc.AppendChild(rootNode);

        // Create a new <Category> element and add it to the root node
        XmlElement parentNode  = xmlDoc.CreateElement("Category");

        // Set attribute name and value!
        parentNode.SetAttribute("ID", "01");

        xmlDoc.DocumentElement.PrependChild(parentNode);

        // Create the required nodes
        XmlElement mainNode  = xmlDoc.CreateElement("MainCategory");
        XmlElement descNode  = xmlDoc.CreateElement("Description");
        XmlElement activeNode  = xmlDoc.CreateElement("Active");

        // retrieve the text
        XmlText categoryText= xmlDoc.CreateTextNode("XML");
        XmlText descText  = xmlDoc.CreateTextNode("This is a list my XML articles.");
        XmlText activeText  = xmlDoc.CreateTextNode("true");

        // append the nodes to the parentNode without the value
        parentNode.AppendChild(mainNode);
        parentNode.AppendChild(descNode);
        parentNode.AppendChild(activeNode);

        // save the value of the fields into the nodes
        mainNode.AppendChild(categoryText);
        descNode.AppendChild(descText);
        activeNode.AppendChild(activeText);

        // Save to the XML file
        xmlDoc.Save( Server.MapPath("categories.xml"));

        Response.Write("XML file created");
    }
}
</script>