C# ASP.NET：“对象引用未设置为 object 的实例。” System.Xml.Linq.XElement.Attribute(...) 返回 null

Question

Creating an application for getting,updating,deleting movies

I'm attempting to get my movies list from an XML file but everytime I go on the WebService tester and try getting the list of movies I get hit with this error

'Object reference not set to an instance of an object.' System.Xml.Linq.XElement.Attribute(...) returned null

My code

public class Movie
{
    private static string _xmlDataPath = HttpContext.Current.Server.MapPath("~/App_Data/Movies.xml");

    public int Id { get; set; }
    public string Title { get; set; }
    public string Director { get; set; }
    public string Synopsis { get; set; }

    public Movie(int id, string title, string director, string sypnosis)
    {
        this.Id = id;
        this.Title = title;
        this.Director = director;
        this.Synopsis = sypnosis;
    }

    public XElement toXML(XNamespace ns)
    {
        return new XElement(ns + "Movie",
            new XElement(ns + "Id", this.Id),
            new XElement(ns + "Title", this.Title),
            new XElement(ns + "Director", this.Director),
            new XElement(ns + "Sypnosis", this.Synopsis)
            );

    }

    public static List<Movie> GetAllMovies()
    {
        List<Movie> movieList = new List<Movie>();

        //Load XML file using LINQ
        XDocument doc = XDocument.Load(_xmlDataPath);
        var xmlMoviesList = doc.Root.Elements();
        XNamespace ns = doc.Root.GetDefaultNamespace();
        //Get each movie element from XML
        foreach (var xmlMovies in xmlMoviesList)
        {
            Movie movie = new Movie(
                int.Parse(xmlMovies.Attribute("Id").Value),
                xmlMovies.Element(ns + "Title").Value,
                xmlMovies.Element(ns + "Director").Value,
                xmlMovies.Element(ns + "Sypnosis").Value
                );
            //Add to the list
            movieList.Add(movie);
        }

        //Return the list
        return movieList;
    }

Answer 1

It would be great if you could include sample XML, otherwise people have to take a guess.

Your code works fine parsing this XML file, with Id as an attribute.

<Root>
  <Movie Id="1">
    <Title>Alien</Title>
    <Director>Ridley Scott</Director>
    <Sypnosis>In space no one can hear you scream.</Sypnosis>
  </Movie>
</Root>

However the toXML function you have written produces this XML which has Id as a node

<Root>
  <Movie>
    <Id>1</Id>
    <Title>Alien</Title>
    <Director>Ridley Scott</Director>
    <Sypnosis>In space no one can hear you scream.</Sypnosis>
  </Movie>
</Root>

Decide if you want the Id to be an attribute or a node, and adjust the code accordingly. Either change new XElement(ns + "Id", this.Id), to new XAttribute(ns + "Id", this.Id) , or change xmlMovies.Attribute("Id").Value to xmlMovies.Element(ns + "Id").Value

On a side note, have a look at the documentation for int.TryParse , and try to use it in place of int.Parse . Best to make sure that you first have an integer in the data being parsed