Deserializing a single element in a large XML document: xmlSerializer.Deserialize(xmlReader.ReadSubtree()) fails due to namespace issues

Question

I am attempting to process a large XML document (using a XmlReader) in a single pass, and deserialize only certain elements in it using an XmlSerializer.

Below is some code and a tiny mock XML document showing how I have attempted to do this.

^{Rationale for using XmlReader: 1. I am dealing with very large XML documents (10–250 MB), which for this reason I do not want to load into memory. So XmlDocument is out of the question. 2. I want to extract only certain elements. Typically I will be able to ignore most other content. XmlReader appears to give me an efficient means of skipping irrelevant content. 3. I do not know in advance whether any and all elements that I can deal with will be present; therefore I am not using a bunch of Xpath/XQuery or LINQ to XML-based queries, because I want to make only a single pass over the XML files (due to their size).}

public class ElementOfInterest { }
…

var xml = @"<?xml version='1.0' encoding='utf-8' ?>
            <Root xmlns:ex='urn:stakx:example'
                  xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
              <ElementOfInterest xsi:type='ex:ElementOfInterest' />
            </Root>";

var reader = System.Xml.XmlReader.Create(new System.IO.StringReader(xml));
reader.ReadToFollowing("ElementOfInterest");

var serializer = new System.Xml.Serialization.XmlSerializer(typeof(ElementOfInterest));
serializer.Deserialize(reader.ReadSubtree());

The last line of code throws the following inner exception:

InvalidOperationException: "Namespace prefix ex is not defined."

Obviously, the XmlSerializer doesn't recognise the ex namespace prefix inside the xsi:type attribute's value.

This is just one error I am having, but frankly, the larger problem is that I have no idea how to go about the whole namespace issue. I am simply looking for a convenient way to de-serialize just a single node out of the XML document, but that seems to entail having to manually register/manage namespaces, and to somehow forward them from the XmlReader to the XmlSerializer.

Can someone demonstrate how to deserialize a single node from a XML document read with an XmlReader, either by pointing out the error in my code, or by showing an alternative approach?

Answer 1

The following works:

using System.IO;
using System.Xml;
using System.Xml.Serialization;

static void Main()
{
    var xml = @"<?xml version='1.0' encoding='utf-8' ?>
                <Root
                  xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
                  xmlns:ex='urn:stakx:example'
                >
                  <ex:ElementOfInterest xsi:type='ex:ElementOfInterest' />
                </Root>";

    var nt = new NameTable();
    var mgr = new XmlNamespaceManager(nt);
    mgr.AddNamespace("ex", "urn:stakx:example");
    var ctxt = new XmlParserContext(nt, mgr, "", XmlSpace.Default);
    var reader = XmlReader.Create(new StringReader(xml), null, ctxt);
    var serializer = new XmlSerializer(typeof(ElementOfInterest));

    reader.ReadToFollowing("ElementOfInterest", "urn:stakx:example");
    var eoi = (ElementOfInterest)serializer.Deserialize(reader.ReadSubtree());
}

[XmlRoot(Namespace = "urn:stakx:example")]
public class ElementOfInterest { }

Note the namespace in the input: <ex:ElementOfInterest>.