如何查出指定元素节点中的指定属性值的那条节点

[OICQshenshi]

bookstore.xml

<?xml version="1.0" encoding="utf-8"?>
<bookstore>
  <book genre="fantasy" ISBN="2-3631-4">
    <title>Oberon's Legacy</title>
    <author>Corets, Eva</author>
    <price>5.95</price>
  </book>
  <applenote genre="santase" ISBN="2-3631-4">
    <title>Oberon's Legacy</title>
    <author>Corets, Eva</author>
    <price>5.95</price>
  </applenote>
  <book genre="fantasy" ISBN="2-3631-5">
    <title>Oberon's Legacy</title>
    <author>Corets, Eva</author>
    <price>5.95</price>
  </book>
</bookstore>

OPXML.cs

        private void button3_Click(object sender, EventArgs e)
        {

            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load(xmlFile);
            XmlNode selnode = xmlDoc.SelectSingleNode("//book[@genre='fantasy']");
            string txt = selnode.InnerText;
            MessageBox.Show(txt);
        }

我要找出<book>节点的属性名为genre且属性值等于fantasy的那个节点，请教xpath该如何写

[OICQshenshi]

string nodeName;//节点名称
string nodeAttName;//节点属性名
string oldNodeAttValue;//属性值

xmlDoc.SelectSingleNode("//"+nodeName+"[@"+nodeAttName+"='"+oldNodeAttValue+"']");
oldeAttValue的变量写法正确不？