因为你有

xmlns="http://xml"

在您的XML文件中，您需要添加XmlNamespaceManager，以允许导航器找到节点。如果从XML中删除xmlns="http://xml"，则无需使用XmlNamespaceManager。

另外，Select方法返回节点的集合-您需要调用SelectSingleNode来获取所需的节点。例如

XPathDocument pdoc = new XPathDocument("Courses.xml");
XPathNavigator pNav = pdoc.CreateNavigator();

var manager = new XmlNamespaceManager(pNav.NameTable);
manager.AddNamespace("cs","http://xml");

XPathNodeIterator iterator = pNav.Select("/cs:Courses/cs:Course",manager);

while(iterator.MoveNext())
{
    var nameNode = iterator.Current.SelectSingleNode("cs:Name",manager);
    string courseName = nameNode.Value;

    var codeNode = iterator.Current.SelectSingleNode("cs:Code",manager);

    codeNode.MoveToFirstAttribute();

    string courseCode = codeNode.Value;
    Console.WriteLine("{0} {1}",courseName,courseCode);
}

当您进入Code元素时，需要移至第一个属性以获取该值，否则Value属性将返回一个空字符串

它需要将namespace resolver传递给选择方法：

const string xml = @"
    <Courses xmlns=""http://xml"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xsi:schemaLocation=""Courses.xsd"">
        <Course>
            <Code Undergrad=""240""/>
            <Name>Biology</Name>
            <Instructor>
                <Name>
                    <First>John</First>
                    <Last>Doe</Last>
                </Name>
                <Contact>
                <Phone>898-989-8989</Phone>
                </Contact>
            </Instructor>
            <Room>515</Room>
        </Course>
        <Course>
            <Code Undergrad=""000""/>
            <Name>Math</Name>
            <Instructor>
                <Name>
                    <First>John</First>
                    <Last>Doe</Last>
                </Name>
                <Contact>
                <Phone>898-989-8989</Phone>
                </Contact>
            </Instructor>
            <Room>515</Room>
        </Course>
    </Courses>";

using (var stream = new MemoryStream(Encoding.ASCII.GetBytes(xml)))
{
    var pdoc = new XPathDocument(stream);
    var pNav = pdoc.CreateNavigator();

    var manager = new XmlNamespaceManager(pNav.NameTable);
    manager.AddNamespace("cs","http://xml");

    var iterator = pNav.Select("/cs:Courses/cs:Course",manager);

    foreach (XPathNavigator node in iterator)
    {
        var courseName = node.SelectSingleNode("cs:Name",manager)?.Value;
        var courseCode = node.SelectSingleNode("cs:Code",manager)?.GetAttribute("Undergrad",string.Empty);

        Console.WriteLine("{0} {1}",courseCode);
    }
}