XmlReader ReadStartElement causes XmlException

Question

I'm writing a file reader using the XmlReader in a Silverlight project. However, I'm getting some errors (specifically around the XmlReader.ReadStartElement method) and it's causing me to believe that I've misunderstood how to use it somewhere along the way.

Basically, here is a sample of the format of the Xml I am using:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<root>
    <EmptyElement />
    <NonEmptyElement Name="NonEmptyElement">
        <SubElement Name="SubElement" />
    </NonEmptyElement>
</root>

And here is a sample of some code used in the same way as how I am using it:

public void ReadData(XmlReader reader)
{
    // Move to root element
    reader.ReadStartElement("root");

    // Move to the empty element
    reader.ReadStartElement("EmptyElement");

    // Read any children
    while(reader.ReadToNextSibling("SubEmptyElement"))
    {
        // ...
    }

    // Read the end of the empty element
    reader.ReadEndElement();

    // Move to the non empty element
    reader.ReadStartElement("NonEmptyElement");    // NOTE: This is where I get the error.

    // ...
}

So, essentially, I am simply trying to read each element and any contained children. The error I get at the highlighted point is as follows:

Error Description

[Xml_InvalidNodeType] Arguments: None,10,8 Debugging resource strings are unavailable. Often the key and arguments provide sufficient information to diagnose the problem. See http://go.microsoft.com/fwlink/?linkid=106663&Version=4.0.51204.0&File=System.Xml.dll&Key=Xml_InvalidNodeType

Error Stack Trace

at System.Xml.XmlReader.ReadStartElement(String name) at ----------------

Any advice or direction on this would be greatly appreciated.

EDIT Since this reader needs to be fairly generic, it can be assumed that the Xml may contain elements that are children of the EmptyElement. As such, the attempt at reading any SubEmptyElements should be valid.

Answer 1

<SubElement/> is not a sibling of <EmptyElement>, so <NonEmptyElement> is going to get skipped entirely, and your call to ReadEndElement() will read the end element </root>. When you try to subsequently read "NonEmptyElement", there are no elements left, and you'll get an XmlException: {"'None' is an invalid XmlNodeType. Line 8, position 1."}

Note also that since <EmptyElement/> is empty, when you ReadStartElement("EmptyElement"), you'll read the whole element, and you won't need to use ReadEndElement().

I'd also recommend that you configure your reader settings to IgnoreWhitespace (if you're not already doing so), to avoid any complications introduced by reading (insignificant) whitespace text nodes when you aren't expecting them.

Try moving the Read of NonEmptyElement up:

public static void ReadData(XmlReader reader)
{
    reader.ReadStartElement("root");

    reader.ReadStartElement("EmptyElement");

    reader.ReadStartElement("NonEmptyElement");

    while (reader.ReadToNextSibling("SubEmptyElement"))
    {
        // ...
    }

    reader.ReadEndElement(/* NonEmptyElement */);

    reader.ReadEndElement(/* root */);
    // ...
}

If you just want to skip anything in <EmptyElement>, regardless of whether or not its actually empty, use ReadToFollowing:

public static void ReadData(XmlReader reader)
{
    reader.ReadStartElement("root");

    reader.ReadToFollowing("NonEmptyElement");

    Console.WriteLine(reader.GetAttribute("Name"));

    reader.ReadStartElement("NonEmptyElement");

    Console.WriteLine(reader.GetAttribute("Name"));
    while (reader.ReadToNextSibling("SubEmptyElement"))
    {
        // ...
    }

    reader.ReadEndElement(/* NonEmptyElement */);

    reader.ReadEndElement(/* root */);
    // ...
}

Update: Here's a fuller example with a clearer data model. Maybe this is closer to what you're asking for.

XMLFile1.xml:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<root>
  <Person Type="Homeless"/>
  <Person Type="Developer">
    <Home Type="Apartment" />
  </Person>
  <Person Type="Banker">
    <Home Type="Apartment"/>
    <Home Type="Detached"/>
    <Home Type="Mansion">
      <PoolHouse/>
    </Home>
  </Person>
</root>

Program.cs:

using System;
using System.Xml;

namespace ConsoleApplication6
{
    internal class Program
    {
        public static void ReadData(XmlReader reader)
        {
            reader.ReadStartElement("root");

            while (reader.IsStartElement("Person"))
            {
                ReadPerson(reader);
            }

            reader.ReadEndElement( /* root */);
        }

        public static void ReadPerson(XmlReader reader)
        {
            Console.WriteLine(reader.GetAttribute("Type"));
            bool isEmpty = reader.IsEmptyElement;
            reader.ReadStartElement("Person");
            while (reader.IsStartElement("Home"))
            {
                ReadHome(reader);
            }
            if (!isEmpty)
            {
                reader.ReadEndElement( /* Person */);
            }
        }

        public static void ReadHome(XmlReader reader)
        {
            Console.WriteLine("\t" + reader.GetAttribute("Type"));
            bool isEmpty = reader.IsEmptyElement;
            reader.ReadStartElement("Home");

            if (!isEmpty)
            {
                reader.Skip();
                reader.ReadEndElement( /* Home */);
            }
        }

        private static void Main(string[] args)
        {
            var settings = new XmlReaderSettings { IgnoreWhitespace = true };
            using (var xr = XmlReader.Create("XMLFile1.xml", settings))
            {
                ReadData(xr);
            }
            Console.ReadKey();
        }
    }
}