How to add CDATA section in XML file?

it is!

look at the example that located in the MSDN: (http://msdn.microsoft.com/en-us/library/system.xml.xmldocument.createcdatasection.aspx)

using System;
using System.IO;
using System.Xml;

public class Sample
{
  public static void Main()
  {
    XmlDocument doc = new XmlDocument();
    doc.LoadXml("" +
                "" +
                "");

    //Create a CData section.
    XmlCDataSection CData;
    CData = doc.CreateCDataSection("All Jane Austen novels 25% off starting 3/23!");   

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

    Console.WriteLine("Display the modified XML...");       
    doc.Save(Console.Out);
  }
}

this function

 cxml.CreateCDataSection("content of cdata");

does not exist in system.xml ?

did you try:

XmlDocument cxml = new XmlDocument();

cxml.CreateCDataSection("content of cdata");

????????????????????????