Question

According to my LINQ book, this slightly altered example should work.

Why does it tell me “Object reference not set to an instance of an object“?

using System;
using System.Xml.Linq;

namespace TestNoAttribute
{
    class Program
    {
        static void Main(string[] args)
        {

            XDocument xdoc = new XDocument(
                new XElement("employee",
                    new XAttribute("id", "23"),
                    new XElement("firstName", new XAttribute("display", "true"), "Jim"),
                    new XElement("lastName", new XAttribute("display", "false"), "Smith")));

            XElement element = xdoc.Element("firstName");
            XAttribute attribute = element.Attribute("display"); //error

            Console.WriteLine(xdoc);

            Console.ReadLine();

        }
    }
}

Partial Answer:

I figured out if I change XDocument to XElement, then it works. Could anyone explain why?

Answer 1

You are accessing a child element of xdoc that doesn’t exist. Try one level down:

XElement element = xdoc.Element("employee").Element("firstName");

or

XElement element = xdoc.Descendants("firstName").FirstOrDefault();