以这种方式进行操作:

HtmlNode mdnode = htmlDoc.DocumentNode.SelectSingleNode("//meta[@name='description']");

              if (mdnode != null)
              {
                 HtmlAttribute desc;

                 desc = mdnode.Attributes["content"];
                 string fulldescription = desc.Value;
                 Console.Write("DESCRIPTION: " + fulldescription);
              }