Question

I have this XML file that I parse into its elements and create a list of a custom object Module.

XDocument kobra = XDocument.Load(new StringReader(results.OuterXml));
XNamespace ns = "#RowsetSchema";

var kobraNodeList = from s in kobra.Descendants(ns + "row")
                    select new Module
                    {
                         id = s.Attribute("ows_ID").Value,
                         name = s.Attribute("ows_Title").Value,
                         sourceFile = s.Attribute("ows_Source_x0020_Message_x0020_File_").Value,
                         scope = Scope.KOBRA,
                         component = string.Empty
                    };

and here’s my Module struct:

public struct Module
{
    public string name;
    public Scope scope;
    public string component;
    public int wordCound;
    public string id;
    public string sourceFile;
}

The code works fine, but things get weird when I try to convert the var kobraNodeList into a list of Modules, I get a System.NullReferenceException at the AddRange line:

this.moduleList = new List<Module>;
this.moduleList.AddRange(kobraNodeList);

When trying to debug, I notice that although kobraNodeList.Count() also returns System.NullReferenceException, a kobraNodeList.Any() returns true, and kobraNodeList.First() returns a perfectly valid and correct Module struct with the desired data.

The XML file is valid, and if I replace the linq query with this:

var kobraNodeList = from s in kobra.Descendants(ns + "row")
                    select  s;

I get a valid list of XElement, which I can Count() ok.
Can someone explain me what’s wrong? BTW, I’m using .NET 3.5.

Answer 1

That looks like one (or more) of kobra.Descendants has ows_ID, ows_Title or ows_Source_x0020_Message_x0020_File_ attribute missing.

Linq uses deferred execution, so it won’t try to build the sequence until you ask for the items. When you call Any() or First(), it only needs the first item in the sequence to work, which tells me that the first item in kobra.Descendants does have all of the required nodes.

However, one of the items after the first is probably missing at least one of those attributes – so you end up asking for the Value of a NULL attribute.