It seems April is the month of custom implementations of the XmlReader. The first entry was Daniel Cazzulino's XPathNavigatorReader. As Daniel writes

There are many reasons why developers don't use the XPathDocument and XPathNavigator APIs and resort to XmlDocument instead... XPathNavigator is a far superior way of accessing and querying data because it offers built-in support for XPath querying independently of the store, which automatically gain the feature and more importantly, because it abstracts the underlying store

There are some problems with the XPathNavigator as implemented in the .NET Framework in v1.0 and v1.1. For the most part the APIs in the .NET Framework that work with XML mostly operate on instances of XmlReader or to a lesser extent XmlNode not  XPathNavigator. Also there are some basic features one would expect from an XML API such as the ability to get the XML as a string which don't exist on the class. Daniel solves a number of these problems by implementing the XPathNavigatorReader which is a subclass of XmlTextReader implemented of over an XPathNavigator. This way you can pass an XPathNavigator to APIs expecting an XmlReader or XmlTextReader and get some user friendly functions like ReadInnerXml().

The second custom XmlReader I've seen this month is Oleg Tkachenko's XIncludingReader which is featured as part of his article on the MSDN entitled Combining XML documents with XInclude which provides a brief overview of XInclude and shows how to use the XIncludingReader which implements the XInclude 1.0 Last Call Working Draft from November 10, 2003. From the article

The key class within XInclude.NET is the XIncludingReader, found in the GotDotNet.XInclude namespace. The primary design goal was to build pluggable, streaming pipeline for XML processing. To meet that goal, XIncludingReader is implemented as an XmlReader, which can be wrapped around another XmlReader. This architecture allows easy plugging of XInclude processing layer into a variety of applications without any major modifications.

XML Inclusion process is orthogonal to XML parsing, validation, or transformation. That effectively means it's up to you when to allow XML Inclusion happen: after parsing, but before validation; or after validation, but before transformation, or even after transformation

The design of the XIncludingReader highlights the composability that was our goal when we originally shipped the XmlReader. One can layer readers on top of each other augmenting their capabilities as needed. We will definitely be emphasizing this more in Whidbey.

The third custom reader I've seen this month is the XPathReader. Nothing has been published about this class so far but I'm in the process of putting together an article about it which should show up on the MSDN XML Developer Center at the end of this week or early next week. The whet your appetite imagine an XmlReader that allows you to read XML in a forward-only, streaming manner but allows you to match XPath expressions based on the subset put forward by Arpan Desai in his paper Introduction to Sequential XPath. The following is a sample of how the XPathReader can be used

XPathCollection xc = new XPathCollection();
int query1 = xc.Add('//book/title');

XmlTextReader reader = new XmlTextReader("books.xml");
XPathReader xpathReader = new XPathReader(reader, xc);

while (xpathReader.ReadUntilMatch()){

   Console.WriteLine("Title={0}", xpathReader.ReadString());
}

I should be done with getting the article reviewed and the like in the next few days. April's definitely been the month of the XmlReader.