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private void button1_Click(object sender, EventArgs e)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(@"d:\category.xml");
            XmlNode book;
            XmlNode root = doc.DocumentElement;
            book = root.SelectSingleNode("/Category/CategoryNode[@name='kaifa1']");
            label1.Text = book.Attributes["value"].Value;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(@"d:\category.xml");
            XmlNode book;
            XmlNode root = doc.DocumentElement;
            book = root.SelectSingleNode("/Category[CategoryNode='8888']");
            label1.Text = book.InnerText;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(@"d:\category.xml");
            XmlNode root = doc.DocumentElement;
            XmlElement element = doc.CreateElement("CategoryNode");
            element.InnerText = "123";
            root.AppendChild(element);
            doc.Save(@"d:\category.xml");
        }

Xml文件：

<?xml version="1.0"?>
<Category>
  <CategoryNode name="kaifa" value="2.jpg">8888</CategoryNode>
  <CategoryNode name="kaifa1" value="21.jpg" />
  <CategoryNode name="kaifa2" value="22.jpg" />
  <CategoryNode>123</CategoryNode>
</Category>