Question

I am trying to create a function to parse an XML file like this:

<?xml version="1.0" encoding="utf-8"?>
<list name="Grocery List" author="Ian" desc="Saturday grocery list">

        <item color="black" done="false">Milk</item>
        <item color="black" done="false">Eggs</item>
        <item color="blue" done="false">Water</item>

</list>

It parses the attributes correctly, but it fails to return the values of the list items. Here is the function and class it uses:

class List
{
    public string[] listItems;
    public string[] colorArray;
    public string[] doneArray;
    public string listName;
    public string listAuthor;
    public string listDesc;
    public string err;       
}

Reader definition:

class ListReader
{
    public List doListParse(string filename)
    {
         List l = new List();

         int arrayCount = 0;

         try
         {
             XmlReader r = XmlReader.Create(filename);

             while (r.Read())
             {
                 if (r.NodeType == XmlNodeType.Element && r.Name == "list")
                 {
                     //Get the attributes of the list
                     l.listName = r.GetAttribute("name");
                     l.listAuthor = r.GetAttribute("author");
                     l.listDesc = r.GetAttribute("desc");

                     while (r.NodeType != XmlNodeType.EndElement)
                     {
                         r.Read();
                         if (r.Name == "item")
                         {
                             r.Read();
                             if (r.NodeType == XmlNodeType.Text)
                             {
                                 //Get The Attributes
                                 l.colorArray[arrayCount] = r.GetAttribute("color");
                                 l.doneArray[arrayCount] = r.GetAttribute("done");

                                 //Get The Content
                                 l.listItems[arrayCount] = r.Value.ToString();

                                 arrayCount++;
                             }
                             r.Read();
                         }
                     }
                 }
             }
         }
         catch (Exception e)
         {
             l.err = e.ToString();
         }

         return l;
    }
}

When I execute the program, it gives this exception:

System.NullReferenceException: Object reference not set to an instance of an object.

What is going on here?

Answer 1

I’d recommend you using a serializer. The XmlSerializer class is pretty decent. It will simplify your code.

So start by defining the models that will map to this XML structure:

[XmlRoot("list")]
public class GroceryList
{
    [XmlAttribute("name")]
    public string Name { get; set; }

    [XmlAttribute("author")]
    public string Author { get; set; }

    [XmlAttribute("desc")]
    public string Description { get; set; }

    [XmlElement("item")]
    public Item[] Items { get; set; }
}

public class Item
{
    [XmlAttribute("color")]
    public string Color { get; set; }

    [XmlAttribute("done")]
    public bool Done { get; set; }

    [XmlText]
    public string Value { get; set; }
}

and then simply deserialize the XML:

class Program
{
    static void Main()
    {
        var serializer = new XmlSerializer(typeof(GroceryList));
        using (var reader = XmlReader.Create("groceriesList.xml"))
        {
            var list = (GroceryList)serializer.Deserialize(reader);
            // you could access the list items here
        }
    }
}