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>

MSDN 文档

[C#]
using System;
using System.IO;
using System.Xml;

public class Sample
{
  public static void Main()
  {

      XmlDocument doc = new XmlDocument();
      doc.Load("booksort.xml");

      //Create an XmlNamespaceManager for resolving namespaces.
      XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
      nsmgr.AddNamespace("bk", "urn:samples");

      //Select the book node with the matching attribute value.
      XmlNode book;
      XmlElement root = doc.DocumentElement;
      book = root.SelectSingleNode("descendant::book[@bk:ISBN='1-861001-57-6']", nsmgr);

      Console.WriteLine(book.OuterXml);

  }
}


<?xml version="1.0"?>
<!-- A fragment of a book store inventory database -->
<bookstore xmlns:bk="urn:samples">
  <book genre="novel" publicationdate="1997" bk:ISBN="1-861001-57-8">
    <title>Pride And Prejudice</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>24.95</price>
  </book>
  <book genre="novel" publicationdate="1992" bk:ISBN="1-861002-30-1">
    <title>The Handmaid's Tale</title>
    <author>
      <first-name>Margaret</first-name>
      <last-name>Atwood</last-name>
    </author>
    <price>29.95</price>
  </book>
  <book genre="novel" publicationdate="1991" bk:ISBN="1-861001-57-6">
    <title>Emma</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>19.95</price>
  </book>
  <book genre="novel" publicationdate="1982" bk:ISBN="1-861001-45-3">
    <title>Sense and Sensibility</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>19.95</price>
  </book>
</bookstore>

 Xml中SelectSingleNode方法中的xpath用法

最常见的XML数据类型有:Element, Attribute,Comment, Text.
  Element, 指形如<Name>Tom<Name>的节点。它可以包括:Element, Text, Comment, ProcessingInstruction, CDATA, and EntityReference.
  Attribute, 指在<Employee id=”12345”>中的粗体部分。
  Comment,指形如:<!-- my comment --> 的节点。
  Text,指在<Name>Tom<Name>的粗体部分。
  在XML中,可以用XmlNode对象来参照各种XML数据类型。
  2.1 查询已知绝对路径的节点(集)
  objNodeList = objDoc.SelectNodes(“Company/Department/Employees/Employee”)
  或者
  objNodeList = objNode.SelectNodes(“/Company/Department/Employees/Employee”)
  以上两种方法可返回一个NodeList对象,如果要返回单个节点可使用SelectSingleNode方法,该方法如果查询到一个或多个节点,返回第一个节点;如果没有查询的任何节点返回 Nothing。例如:
  objNode = objNode.SelectSingleNode(“/Company/Department/Employees/Employee”)
  If Not (objNode is Nothing) then
   ‘- Do process
  End If
  2.2 查询已知相对路径的节点(集)
  可使用类似于文件路径的相对路径的方式来查询XML的数据
  objNode = objDoc.SelectSingleNode(“Company/Department”)
  objNodeList = objNode.SelectNodes(“../Department)
  objNode = objNode.SelectNode(“Employees/Employee”)
  2.3 查询已知元素名的节点(集)
  在使用不规则的层次文档时,由于不知道中间层次的元素名,可使用//符号来越过中间的节点,查询其子,孙或多层次下的其他所有元素。例如:
  objNodeList = objDoc.SelectNodes(“Company//Employee”)
  2.4 查询属性(attribute)节点
  以上的各种方法都返回元素(element)节点(集),返回属性(attribute),只需要采用相应的方法,在属性名前加一个@符号即可,例如:
  objNodeList = objDoc.SelectNodes(“Company/Department/Employees/Employee/@id”)
  objNodeList = objDoc.SelectNodes(“Company//@id”)
  2.5 查询Text节点
  使用text()来获取Text节点。
  objNode = objDoc.SelectSingleNode(“Company/Department/Deparmt_Name/text()”)
  2.6 查询特定条件的节点
  使用[]符号来查询特定条件的节点。例如:
  a. 返回id号为 10102的Employee节点
   objNode = objDoc.SelectSingleNode(“Company/Department/Employees/Employee[@id=’10102’]”)
  b. 返回Name为Zhang Qi的Name 节点
   objNode = objDoc.SelectSingleNode(“Company/Department/Employees/Employee/Name[text()=’Zhang Qi’]”)
  c. 返回部门含有职员22345的部门名称节点
  objNode = objDoc.SelectSingleNode("Company/Department[Employees/Employee/@id='22345']/Department_Name")
  2.7 查询多重模式的节点
  使用 | 符号可以获得多重模式的节点。例如:
  objNodeList = objDoc.SelectNodes(“Company/Department/Department_Name | Company/Department/Manager”)
  2.8 查询任意子节点
  使用*符号可以返回当前节点的所有子节点。
  objNodeList = objDoc.SelectNodes(“Company/*/Manager)
  或者
  objNodeList = objNode.ChildNodes
  
  3 XML数据的编辑
  3.1 增加一个元素的属性(attribute)节点
  Dim objNodeAttr As XmlNode
   objNodeAttr = objDoc.CreateAttribute("id", Nothing)
   objNodeAttr.InnerXml = "101"
  objNode.Attributes.Append(objNodeAttr)
  3.2 删除一个元素的属性
  objNode.Attributes.Remove(objNodeAttr)
  3.3 增加一个子元素(Element)
  Dim objNodeChild As XmlNode
  objNodeChild = objDoc.CreateElement(Nothing, "ID", Nothing)
  objNodeChild.InnerXml = "101"
  objNode.AppendChild(objNodeChild)
  3.4 删除一个子元素
  objNode.RemoveChild(objNodeChild)
  3.5 替换一个子元素
  objNOde.ReplaceChild(newChild,oldChild)
  
  4 参考数据
  <?xml version="1.0" encoding="UTF-8"?>
  <Company>
   <Department id="101">
   <Department_Name>Cai WuBu</Department_Name>
   <Manager>Zhang Bin</Manager>
   <Employees>
   <Employee id="12345">
   <Employee_ID>12345</Employee_ID>
   <Name>Zhang Bin</Name>
   <Gender>male</Gender>
   </Employee>
   <Employee id="10101">
   <Employee_ID>10101</Employee_ID>
   <Name>Zhang QI</Name>
   <Gender>female</Gender>
   </Employee>
   <Employee id="10102">
   <Employee_ID>10102</Employee_ID>
   <Name>Zhang Xia</Name>
   <Gender>male</Gender>
   </Employee>
   <Employee id="10201">
   <Employee_ID>10201</Employee_ID>
   <Name>ZhangChuang</Name>
   <Gender>male</Gender>
   </Employee>
   <Employee id="10202">
   <Employee_ID>10202</Employee_ID>
   <Name>Zhang Jun</Name>
   <Gender>male</Gender>
   </Employee>
   </Employees>
   </Department>
   <Department id="102">
   <Department_Name>KaiFa Bu</Department_Name>
   <Manager>Wang Bin</Manager>
   <Employees>
   <Employee id="22345">
   <Employee_ID>22345</Employee_ID>
   <Name>Wang Bin</Name>
   <Gender>male</Gender>
   </Employee>
   <Employee id="20101">
   <Employee_ID>20101</Employee_ID>
   <Name>Wang QI</Name>
   <Gender>female</Gender>
   </Employee>
   <Employee id="20102">
   <Employee_ID>20102</Employee_ID>
   <Name>Wang Xia</Name>
   <Gender>male</Gender>
   </Employee>
   <Employee id="20201">
   <Employee_ID>20201</Employee_ID>
   <Name>Wang Chuang</Name>
   <Gender>male</Gender>
   </Employee>
   <Employee id="20202">
   <Employee_ID>20201</Employee_ID>
   <Name>Wang Jun</Name>
   <Gender>male</Gender>
   </Employee>
   </Employees>
   </Department>
  </Company>


XML操作类  XmlOp.cs

  1using System;
  2using System.Data;
  3using System.Configuration;
  4using System.Web;
  5using System.Web.Security;
  6using System.Web.UI;
  7using System.Web.UI.WebControls;
  8using System.Web.UI.WebControls.WebParts;
  9using System.Web.UI.HtmlControls;
10using System.IO;
11//引用命名空间
12using System.Xml;
13//51aspx.c_o_m
14
15namespace xmlOp
16{
17    /// <summary>
18    /// xmlOp 类提供对XML文件的读/写/查询/更新/添加/删除 等操作! 19    /// </summary>

20    public class XmlOp
21    {
22        //声明一个XmlDocument空对象
23        protected XmlDocument doc = new XmlDocument();
24        public XmlOp(string xmlFile)
25        {
26            doc.Load(GetXmlFilePath(xmlFile));//载入Xml文档
27        }

28        //保存文件
29        public void Save(string filePath)
30        {
31            doc.Save(GetXmlFilePath(filePath));
32        }

33        //属性查询,返回属性值
34        public string SelectAttrib(string XmlPathNode, string Attrib)
35        {
36            string _strNode = "";
37            _strNode = doc.SelectSingleNode(XmlPathNode).Attributes[Attrib].Value;
38            return _strNode;
39        }

40        // 添加属性
41        public bool InsertAttrib(string MainNode, string Attrib, string AttribContent)
42        {
43            try
44            {
45                XmlElement objNode = (XmlElement)doc.SelectSingleNode(MainNode); //强制转化成XmlElement对象
46                objNode.SetAttribute(Attrib, AttribContent);    //XmlElement对象添加属性方法   
47                return true;
48            }

49            catch
50            {
51                return false;
52            }

53        }

54        //节点值查询与判断
55        public bool SelectNode(string XmlPathNode, int index, string NodeText)
56        {
57            XmlNodeList _NodeList = doc.SelectNodes(XmlPathNode);
58            //循环遍历节点,查询是否存在该节点
59            for (int i = 0; i < _NodeList.Count; i++)
60            {
61              if (_NodeList[i].ChildNodes[index].InnerText == NodeText)
62              {
63                  return true;
64                  break;
65              }

66            }

67            return false;
68        }

69        //获取子节点个数
70        public int NodeCount(string XmlPathNode)
71        {
72            int i = 0;
73            try
74            {
75                i = doc.SelectSingleNode(XmlPathNode).ChildNodes.Count;
76            }

77            catch
78            {
79                i = 0;
80            }

81            return i;
82        }

83        //插入一个节点,带N个子节点
84        public bool InsertNode(string MainNode, string ChildNode, string[] Element, string[] Content)
85        {
86            try
87            {
88                XmlNode objRootNode = doc.SelectSingleNode(MainNode);    //声明XmlNode对象
89                XmlElement objChildNode = doc.CreateElement(ChildNode);  //创建XmlElement对象
90                objRootNode.AppendChild(objChildNode);
91                for (int i = 0; i < Element.Length; i++)    //循环插入节点元素
92                {
93                    XmlElement objElement = doc.CreateElement(Element[i]);
94                    objElement.InnerText = Content[i];
95                    objChildNode.AppendChild(objElement);
96                }

97                return true;
98            }

99            catch
100            {
101                return false;
102            }

103        }

104        // 根据Xml文件的节点路径,返回一个DataSet数据集
105        public DataSet GetDs(string XmlPathNode)
106        {
107            DataSet ds = new DataSet();
108            try
109            {
110             StringReader read = new StringReader(doc.SelectSingleNode(XmlPathNode).OuterXml);
111                ds.ReadXml(read);   //利用DataSet的ReadXml方法读取StringReader文件流
112                read.Close();
113            }

114            catch
115            { }
116            return ds;
117        }

118        // 返回Xml文件实际路径
119        public string GetXmlFilePath(string xmlFile)
120        {
121            return xmlFile;
122        }

123    }

124}

  1using System;
  2using System.Data;
  3using System.Configuration;
  4using System.Web;
  5using System.Web.Security;
  6using System.Web.UI;
  7using System.Web.UI.WebControls;
  8using System.Web.UI.WebControls.WebParts;
  9using System.Web.UI.HtmlControls;
10using System.IO;
11//引用命名空间
12using System.Xml;
13//51aspx.c_o_m
14
15namespace xmlOp
16{
17    /// <summary>
18    /// xmlOp 类提供对XML文件的读/写/查询/更新/添加/删除 等操作! 19    /// </summary>

20    public class XmlOp
21    {
22        //声明一个XmlDocument空对象
23        protected XmlDocument doc = new XmlDocument();
24        public XmlOp(string xmlFile)
25        {
26            doc.Load(GetXmlFilePath(xmlFile));//载入Xml文档
27        }

28        //保存文件
29        public void Save(string filePath)
30        {
31            doc.Save(GetXmlFilePath(filePath));
32        }

33        //属性查询,返回属性值
34        public string SelectAttrib(string XmlPathNode, string Attrib)
35        {
36            string _strNode = "";
37            _strNode = doc.SelectSingleNode(XmlPathNode).Attributes[Attrib].Value;
38            return _strNode;
39        }

40        // 添加属性
41        public bool InsertAttrib(string MainNode, string Attrib, string AttribContent)
42        {
43            try
44            {
45                XmlElement objNode = (XmlElement)doc.SelectSingleNode(MainNode); //强制转化成XmlElement对象
46                objNode.SetAttribute(Attrib, AttribContent);    //XmlElement对象添加属性方法   
47                return true;
48            }

49            catch
50            {
51                return false;
52            }

53        }

54        //节点值查询与判断
55        public bool SelectNode(string XmlPathNode, int index, string NodeText)
56        {
57            XmlNodeList _NodeList = doc.SelectNodes(XmlPathNode);
58            //循环遍历节点,查询是否存在该节点
59            for (int i = 0; i < _NodeList.Count; i++)
60            {
61              if (_NodeList[i].ChildNodes[index].InnerText == NodeText)
62              {
63                  return true;
64                  break;
65              }

66            }

67            return false;
68        }

69        //获取子节点个数
70        public int NodeCount(string XmlPathNode)
71        {
72            int i = 0;
73            try
74            {
75                i = doc.SelectSingleNode(XmlPathNode).ChildNodes.Count;
76            }

77            catch
78            {
79                i = 0;
80            }

81            return i;
82        }

83        //插入一个节点,带N个子节点
84        public bool InsertNode(string MainNode, string ChildNode, string[] Element, string[] Content)
85        {
86            try
87            {
88                XmlNode objRootNode = doc.SelectSingleNode(MainNode);    //声明XmlNode对象
89                XmlElement objChildNode = doc.CreateElement(ChildNode);  //创建XmlElement对象
90                objRootNode.AppendChild(objChildNode);
91                for (int i = 0; i < Element.Length; i++)    //循环插入节点元素
92                {
93                    XmlElement objElement = doc.CreateElement(Element[i]);
94                    objElement.InnerText = Content[i];
95                    objChildNode.AppendChild(objElement);
96                }

97                return true;
98            }

99            catch
100            {
101                return false;
102            }

103        }

104        // 根据Xml文件的节点路径,返回一个DataSet数据集
105        public DataSet GetDs(string XmlPathNode)
106        {
107            DataSet ds = new DataSet();
108            try
109            {
110             StringReader read = new StringReader(doc.SelectSingleNode(XmlPathNode).OuterXml);
111                ds.ReadXml(read);   //利用DataSet的ReadXml方法读取StringReader文件流
112                read.Close();
113            }

114            catch
115            { }
116            return ds;
117        }

118        // 返回Xml文件实际路径
119        public string GetXmlFilePath(string xmlFile)
120        {
121            return xmlFile;
122        }

123    }

124}