读取
 if (File.Exists("Book.xml")) 
            {
                XmlDocument doc = new XmlDocument();
                doc.Load("Book.xml");
                XmlElement root = doc.DocumentElement;
                XmlNodeList nodelist = root.ChildNodes;
                foreach (XmlNode item in nodelist) 
                {
                    Console.WriteLine(item.InnerText);
                }
                Console.ReadKey();

            }

 if (File.Exists("Book.xml")) 
            {
                XmlDocument doc = new XmlDocument();
                doc.Load("Book.xml");
                XmlNodeList nodelist = doc.SelectNodes("/order/Items/OrderItem");
                foreach (XmlNode item in nodelist) 
                {
                    Console.WriteLine(item.Attributes["Name"].Value);
                    Console.WriteLine(item.Attributes["Count"].Value);
                }
                Console.ReadKey();
            }

追加
static void Main(string[] args)
        {
            //XmlDocument xm = new XmlDocument();
            //XmlDeclaration doc = xm.CreateXmlDeclaration("1.0", "utf-8", "yes");
            //xm.AppendChild(doc);
            //XmlElement t1 = xm.CreateElement("order");
            //xm.AppendChild(t1);
            //XmlElement t2 = xm.CreateElement("CustomerName");
            //t1.AppendChild(t2);
            //t2.InnerXml = "<p>我是一个P标签</p>";
            //XmlElement t3 = xm.CreateElement("CustomerNumber");
            //t1.AppendChild(t3);
            //t3.InnerText = "<p>我是一个P标签</p>";
            //XmlElement t4 = xm.CreateElement("Items");
            //t1.AppendChild(t4);
            //XmlElement i1 = xm.CreateElement("OrderItem");
            //t4.AppendChild(i1);
            //i1.SetAttribute("Name","码表");
            //i1.SetAttribute("Count", "10");
            //XmlElement i2 = xm.CreateElement("OrderItem");
            //t4.AppendChild(i2);
            //i2.SetAttribute("Name", "雨衣");
            //i2.SetAttribute("Count", "5");
            //XmlElement i3 = xm.CreateElement("OrderItem");
            //t4.AppendChild(i3);
            //i3.SetAttribute("Name", "手套");
            //i3.SetAttribute("Count", "10");
            //xm.Save("a.xml");


            //有追加 没有 创建

            XmlDocument xm = new XmlDocument();
            XmlElement t1;
            XmlElement t2;
            XmlElement t3;
           

            if (File.Exists("1.xml"))
            {
               
                //加载xml文档到doc
               xm.Load("1.xml");
                
                //获取根节点
                t1 = xm.DocumentElement;
               

                
            }
            else 
            {
                XmlDeclaration doc = xm.CreateXmlDeclaration("1.0", "utf-8", "yes");
                xm.AppendChild(doc);
                t1 = xm.CreateElement("order");
                xm.AppendChild(t1);
                
            }
           
        
           
           
            xm.Save("1.xml");
            
           
           
        }

 if (File.Exists("Book.xml")) 
            {
                doc.Load("Book.xml");
                //XmlNodeList nodelist = doc.SelectNodes("/order/Items");
                XmlNode nodelist = doc.SelectSingleNode("/order/Items");
                XmlElement orderitems = doc.CreateElement("orderitems");
                orderitems.SetAttribute("Name", "雨衣");
                orderitems.SetAttribute("Count", "10");
                nodelist.AppendChild(orderitems);
                //foreach (XmlNode item in nodelist) 
                //{
                //    item.AppendChild(orderitems);
                    
                //}
                Console.ReadKey();
                doc.Save("Book.xml");
                
   
            }

删除XML
 if (File.Exists("Book.xml")) 
            {
                XmlDocument doc = new XmlDocument();
                doc.Load("Book.xml");
                XmlNode nodelist = doc.SelectSingleNode("/order/Items");
                nodelist.RemoveAll();
                doc.Save("Book.xml");
                Console.ReadKey();
            }