这个例外是如何引起的，可以采用哪种替代方案？

Question

挑选一个我不理解的例外。如果有人能把它放在心上，真的会有所帮助。使用其他经过测试的RSS源并且没有问题，但是最新的测试候选人让我头疼。

Line 499 is the one in the query: select new {
Line 517 is the opening statement of the foreach.

  

----在System.Xml.Linq.XNamespace.op_Addition（XNamespace ns，String localName）at at   rssRepub.MainForm。＆lt;＆gt; c__DisplayClassd.b__2（XElement item）in   c：\ Documents and Settings \ Andrew Seabrook \ My   文档\研发\ rssPub \ rssRepub_0.5 \ rssRepub \ MainForm.cs：行   499在   System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext（）
  在rssRepub.MainForm.Completed（Object sender，AsyncCompletedEventArgs   e）在c：\ Documents and Settings \ Andrew Seabrook \ My   文档\研发\ rssPub \ rssRepub_0.5 \ rssRepub \ MainForm.cs：行   517

     

----值不能为空。参数名称：ns

        try
        {                       

            toolStripStatusLabelMain.Text = "Download completed!";
            XslCompiledTransform transform = new XslCompiledTransform();
            transform.Load(INSTALLPATH + "id.xslt");
            transform.Transform(INSTALLPATH + "tempfeed.txt", INSTALLPATH + "clean.xml");

            XDocument doc = XDocument.Load(INSTALLPATH + "clean.xml");

            Dictionary<String, String> dbdata = new Dictionary<String, String>();
            var db = new MSAccess();
            string feedID =  db.ExecuteScalar("SELECT feedID FROM Feeds WHERE feedURL = '" + tvMain.SelectedNode.Tag.ToString() + "'");


            XNamespace dcNs = doc.Element("rss").GetNamespaceOfPrefix("dc");
            XNamespace slashNs = doc.Element("rss").GetNamespaceOfPrefix("slash");
            //XNamespace slashNs = "http://purl.org/rss/1.0/modules/slash/";

            var data = from item in doc.Root.Descendants().First(i => i.Name.LocalName == "channel").Elements().Where(i => i.Name.LocalName == "item")
                      select new {
                            Title = item.Elements().First(i => i.Name.LocalName == "title").Value,
                            Link = item.Elements().First(i => i.Name.LocalName == "link").Value,
                            PublishDate = ParseDate(item.Elements().First(i => i.Name.LocalName == "pubDate").Value),
                            Creator = item.Element(dcNs + "creator").Value,
                            Guid = item.Elements().First(i => i.Name.LocalName == "guid").Value,                
                            Description = item.Elements().First(i => i.Name.LocalName == "description").Value,
                            //Content = item.Elements().First(i => i.Name.LocalName == "content").Value,
                            Content = item.Elements().First(i => i.Name.LocalName == "description").Value,
                            //Comments = item.Element("slash:comments").Value
                            Comments = item.Element(slashNs + "comments").Value,
                            //Comments = item.SelectSingleNode("slash:comments", nameSpace).InnerText
                            Categories = (from category in item.Elements("category")
                                          orderby category.Value
                                          select category.Value).ToList()

            };

            foreach (var item in data)
            {

                var csv = string.Join( ";", item.Categories );

                //add feed to database
                dbdata.Add("fiFeedID", feedID);
                dbdata.Add("fiTitle", item.Title);
                dbdata.Add("fiLink", item.Link);
                dbdata.Add("fiPubDate", item.PublishDate.ToString());
                //if (item.Creator != null) 
                dbdata.Add("fiCreator", item.Creator);
                dbdata.Add("fiGuid", item.Guid);
                dbdata.Add("fiDescription", item.Description);                  
                dbdata.Add("fiContent", item.Content);  
                //if (item.Comments != null) 
                dbdata.Add("fiComments", item.Comments);
                dbdata.Add("fiTags",csv);

                try
                {
                    db.Insert("FeedItems", dbdata);
                }
                catch(Exception crap)
                {
                    OutCrap(crap);
                } 
                dbdata.Clear();

            }
            UpdateStatus("Fetching " + tvMain.SelectedNode.Name);
            UpdateDGVMain();
        }
        catch (Exception crap)
        {
            OutCrap(crap);
        }