<?xml version="1.0" encoding="utf-8"?>
<School sname="aa">
  <Class cname="c1">
    <Student name="bb" age="21">
    </Student>
    <Stu name="stu1" />
  </Class>
  <Class cname="c2">
    <Student name="cc" age="22">
    </Student>
  </Class>
</School>

public List<string> count_list = new List<string>();
            public void GetNodeName(XmlNode xn)
            {
                string name = xn.Name;
                if (!count_list.Contains(name))
                    count_list.Add(name);
                if (xn.HasChildNodes)
                {
                    foreach (XmlNode temp in xn.ChildNodes)
                    {
                        GetNodeName(temp);
                    }
                }
            }