由于部分节点位于"http://www.loc.gov/MARC21/slim/" 命名空间中，但是您的XPath仅在空名称空间中查找元素.

Because part of your nodes are in the "http://www.loc.gov/MARC21/slim/" namespace, but your XPath looks for elements in the empty namespace only.

要解决此问题，请通过调用名称空间管理器使您的环境知道该名称空间:

To fix this, make the namespace known to your environment by invoking a namespace manager:

XmlNamespaceManager nsmgr = new XmlNamespaceManager(r.ResponseXmlDocument);
nsmgr.AddNamespace("marc", "http://www.loc.gov/MARC21/slim/");
string xpath = "marc:record/marc:datafield[@tag='520']/marc:subfield[@code='a']";

// ...
string information = xmlResource.SelectSingleNode(xpath).InnerText;

虽然选择

//marc:datafield[@tag='520']/marc:subfield[@code='a']

摆脱现有的分两步走的方法.

and get rid of the two-step approach you currently have altogether.