Question

I have the following XDocument called XDoc:

<?xml version="1.0" encoding="utf-8"?>
   <DatabaseList>
      <Database DatabaseName="c2501_data">
      <Plugin PluginName="FooPlugin" LastRun="1/21/2013 3:22:08 PM" />
      <Plugin PluginName="SpecialPlugin" LastRun="2013-01-21T15:22:09.3791103-05:00" />
      <Plugin PluginName="BarPlugin" LastRun="2013-01-21T15:23:13.0964814-05:00" />
   </Database>
</DatabaseList>

I’m writing a program that searches to see when the last time a plugin was run on a database, if at all. I use the following two pieces of code to figure out if an entry exists for a plugin on a database:

        var h = (from el in XDoc.Root.Elements("Database")
                 where el.Element("Plugin").Attribute("PluginName").Value=="FooPlugin" 
                 && el.Attribute("DatabaseName").Value=="c2501_data"
                 select el.Element("Plugin"));

        var e = (from el in XDoc.Root.Elements("Database")
                 where el.Element("Plugin").Attribute("PluginName").Value=="BarPlugin"
                 && el.Attribute("DatabaseName").Value == "c2501_data"
                 select el.Element("Plugin"));


        if ((from el in XDoc.Root.Elements("Database")
             where el.Element("Plugin").Attribute("PluginName").Value == "BarPlugin"
             && el.Attribute("DatabaseName").Value == "c2501_data"
             select el.Element("Plugin")).Count() == 0)
        {
            XElement SpecialPlugin = new XElement("Plugin",
                new XAttribute("PluginName", "BarPlugin"),
                new XAttribute("LastRun", DateTime.Now));

            var CurNode = from node in XDoc.Root.Elements("Database")
                          where (string)node.Attribute("DatabaseName").Value == "c2501_data"
                          select node;

            foreach (var node in CurNode)
                node.Add(SpecialPlugin);

            XDoc.Save(RuntimesPath);
            //XDoc.Root.Elements("Database").Attribute("DatabaseName").
        }

The problem that I’m having is that even though there is clearly an entry for BarPlugin, the count will always return 0 and e will always be unable to create an enumberable. Can anyone explain to me why this might be? FooPlugin always works correctly and returns the Plugin information for h.

Thanks for any help.

Answer 1

You’re selecting a Database element where it contains a child element called Plugin with a given name. Since you have only one Database element, you’re getting the same outer element each time. You then take that database element and return the first Plugin child, which will always be Foo, in this case. You need to find the appropriate Database element and then query through each of the child elements so you can return them:

public static XElement GetPlugin(XDocument XDoc, string databaseName, string pluginName)
{
    var h = from database in XDoc.Root.Elements("Database")
            where database.Attribute("DatabaseName").Value == databaseName
            from plugin in database.Elements("Plugin")
            where plugin.Attribute("PluginName").Value == pluginName
            select plugin;

    return h.FirstOrDefault();
}

Or, if you prefer, in method syntax:

var q = XDoc.Root.Elements("Database")
    .Where(db => db.Attribute("DatabaseName").Value == databaseName)
    .SelectMany(db => db.Elements("Plugin"))
    .Where(plugin => plugin.Attribute("PluginName").Value == pluginName);

return q.FirstOrDefault();