Deciding on when to use XmlDocument vs XmlReader

Question

I'm optimizing a custom object -> XML serialization utility, and it's all done and working and that's not the issue.

It worked by loading a file into an XmlDocument object, then recursively going through all the child nodes.

I figured that perhaps using XmlReader instead of having XmlDocument loading/parsing the entire thing would be faster, so I implemented that version as well.

The algorithms are exactly the same, I use a wrapper class to abstract the functionality of dealing with an XmlNode vs. an XmlReader. For instance, the GetChildren methods yield returns either a child XmlNode or a SubTree XmlReader.

So I wrote a test driver to test both versions, and using a non-trivial data set (a 900kb xml file with around 1350 elements).

However, using JetBrains dotTRACE, I see that the XmlReader version is actually slower than the XmlDocument version! It seems that there is some significant processing involved in XmlReader.Read calls when i'm iterating over child nodes.

So I say all that to ask this:

What are the advantages/disadvantages of XmlDocument and XmlReader, and in what circumstances should you use either?

My guess is that there is a file size threshold at which XmlReader becomes more economical in performance, as well as less memory-intensive. However, that threshold seems to be above 1MB.

---

Response to Robert Rossney: I'm calling ReadSubTree every time to process child nodes:

 public override IEnumerable<IXmlSourceProvider> GetChildren ()
 {
  XmlReader xr = myXmlSource.ReadSubtree ();
  // skip past the current element
  xr.Read ();

  while (xr.Read ())
  {
   if (xr.NodeType != XmlNodeType.Element) continue;
   yield return new XmlReaderXmlSourceProvider (xr);
  }
 }

---

Response to Aaron: That test applies to a lot of objects at a single level (ie wide & shallow) - but I wonder how well XmlReader fares when the XML is deep & wide? IE the XML i'm dealing with is much like a data object model, 1 parent object to Many child objects, etc: 1..M..M..M

I also don't know beforehand the structure of the XML i'm parsing, so I can't optimize for it.

Answer 1

There is a size threshold at which XmlDocument becomes slower, and eventually unusable. But the actual value of the threshold will depend on your application and XML content, so there are no hard and fast rules.

If your XML file can contain large lists (say tens of thousands of elements), you should definitely be using XmlReader.

Answer 2

XmlDocument is an in-memory representation of the entire XML document. Therefore if your document is large, then it will consume much more memory than if you had read it using XmlReader.

This is assuming that when you use XmlReader you read and process the elements one-by-one then discard it. If you use XmlReader and construct another intermediary structure in memory then you have the same problem, and you're defeating the purpose of it.

Google for "SAX versus DOM" to read more about the difference between the two models of processing XML.

Answer 3

I've generally looked at it not from a fastest perspective, but rather from a memory utilization perspective. All of the implementations have been fast enough for the usage scenarios I've used them in (typical enterprise integration). However, where I've fallen down, and sometimes spectacularly, is not taking into account the general size of the xml I'm working with. :) If you think about it up front you can save yourself some grief.

Xml tends to bloat when loaded into memory, at least with a DOM reader like XmlDocument or XPathDocument. Something like 10:1? Exact amount is hard to quantify, but if its 1MB on disk it will be 10MB in memory, or more, for example.

A process using any reader that loads the whole document into memory in its entirety (XmlDocument/XPathDocument) can suffer from large object heap fragmentation, which can ultimately lead to OutOfMemoryExceptions (even with available memory) resulting in an unavailable service/process.

Since objects that are greater than 85K in size end up on the large object heap, and you've got a 10:1 size explosion with a DOM reader, you can see it doesn't take much before your xml documents are being allocated from the large object heap.

XmlDocument is very easy to use. Its only real drawback is that it loads the whole xml document into memory to process. Its seductively simple to use.

XmlReader is a stream based reader so will keep your process memory utilization generally flatter but is more difficult to use.

XPathDocument tends to be a faster, read-only version of XmlDocument, but still suffers from memory 'bloat'.