mono/web/xml-classes

* XML Classes

** Abstract

  XML library is used by several areas of Mono such as ADO.NET and XML 
  Digital Signature (xmldsig). Here I write about System.Xml.dll and
  related tools. This page won't include any classes which are in other
  assemblies such as XmlDataDocument.

  Note that current corlib has its own XML parser class (Mono.Xml.MiniParser).

  Basically System.XML.dll feature is almost finished, so I write this
  document mainly for bugs and improvement hints.


** System.Xml namespace


*** Document Object Model (Core)

  DOM feature has already implemented. There is still missing feature.

	<ul>
		* ID constraint support is problematic because W3C DOM does not 
		  specify handling of ID attributes into non-adapted element. 
		  (MS.NET also looks incomplete in this area).
	</ul>

*** Xml Writer

  Here XmlWriter almost equals to XmlTextWriter. If you want to see
  another implementation, check XmlNodeWriter.cs and DTMXPathDocumentWriter.cs
  in System.XML sources.

  XmlTextWriter is completed, though it looks a bit slower than MS.NET (I 
  tried 1.1).

*** XmlResolver

  Currently XmlTextReader uses specified XmlResolver. If nothing was supplied,
  then it uses XmlUrlResolver. XmlResolver is used to parse external DTD,
  importing XSL stylesheets and schemas etc.

  However, XmlUrlResolver is still buggy (mainly because System.Uri is also
  incomplete yet) and this results in several loading error.

  XmlSecureResolver, which is introduced in MS .NET Framework 1.1 is basically
  implemented, but it requires CAS (code access security) feature. We need to
  fixup this class after ongoing CAS effort works.

  You might also be interested in an improved <a href="http://codeblogs.ximian.com/blogs/benm/archives/000039.html">XmlCachingResolver</a> by Ben Maurer.

*** XmlNameTable

  NameTable itself is implemented. However, it should be actually used in
  several classes. Currently it makes sense if compared names are both in
  the table, they should be simply compared using ReferenceEquals(). We
  have done where it seems possible e.g. in XmlNamespaceManager (in .NET 
  1.2 methods; if the build is not NET_1_2, it will be used internally).

  NameTable also needs performance improvement.

*** Xml Stream Reader

  When we are using ASCII document, we don't care which encoding we are using.
  However, XmlTextReader must be aware of the specified encoding in XML
  declaration. So we have internal XmlStreamReader class (and currently
  XmlInputStream class. This may disappear since XmlStreamReader is enough to
  handle this problem).

  However, there seems some problems in these classes on reading network
  stream (especially on Linux). This should be fixed soon, if we found the
  actual reason.

*** XML Reader

  XmlTextReader, XmlNodeReader and XmlValidatingReader are almost finished.

	<ul>
		* All OASIS conformance test passes as Microsoft does. Some 
		  W3C tests fail, but it looks better.
		* Entity expansion and its well-formedness check is incomplete.
		  It incorrectly allows divided content models. It incorrectly
		  treats its Base URI, so some dtd fails.
		* I won't add any XDR support on XmlValidatingReader. (I haven't
		  ever seen XDR used other than Microsoft's BizTalk Server 2000,
		  and Now they have 2002 with XML Schema support)
	</ul>

  XmlTextReader and XmlValidatingReader should be faster than now. Currently
  XmlTextReader looks nearly twice as slow as MS.NET, and XmlValidatingReader
  (which uses this slow XmlTextReader) looks nearly three times slower. (Note
  that XmlValidatingReader won't be slow as itself. It uses schema validating
  reader and dtd validating reader.)


**** Some Advantages

  The design of Mono's XmlValidatingReader is radically different from
  that of Microsoft's implementation. Under MS.NET, DTD content validation
  engine is in fact simple replacement of XML Schema validation engine.
  Mono's DTD validation is designed fully separate and does validation
  as normal XML parser does. For example, Mono allows non-deterministic DTD.

  Another advantage of this XmlValidatingReader is support for *any* XmlReader.
  Microsoft supports only XmlTextReader.

  I added extra support interface named "IHasXmlParserContext", which is
  considered in XmlValidatingReader.ResolveEntity(). Microsoft failed to 
  design XmlReader to support pluggable use of XmlReader (i.e. wrapping use
  of other XmlReader) since XmlParserContext is required to support both
  entity resolution and namespace manager. (In .NET 1.2, Microsoft also 
  supported similar to IHasXmlParserContext, named IXmlNamespaceResolver,
  but it still does not provide any DTD information.)

  We also have RELAX NG validating reader. See mcs/class/Commons.Xml.Relaxng.


** System.Xml.Schema

*** Summary

  Basically it is completed. We can compile complex and simple types, refer to
  external schemas, extend or restrict other types, or use substitution groups.
  You can test how current schema validation engine is (in)complete by using
  standalone test module 
  (see mcs/class/System.XML/Test/System.Xml.Schema/standalone_tests).
  At least in my box, msxsdtest fails only 30 cases with bugfixed catalog.

*** Schema Object Model

  Completed, except for some things to be fixed:

	<ul>
		* Complete facet support. Currently some of them is missing.
		  Recently David Sheldon is doing several fixes on them.
		* ContentTypeParticle for pointless xs:choice is incomplete
		  (It is because fixing this arose another bugs in 
		  compilation. Interestingly, MS.NET also fails around here,
		  so it might be nature of ContentTypeParticle design)
		* Some derivation by restriction (DBR) handling is incorrect.
	</ul>

*** Validating Reader

  XML Schema validation feature is (currently) implemented on
  Mono.Xml.Schema.XsdValidatingReader, which is internally used in
  XmlValidatingReader.

  Basically this is implemented and actually its feature is almost complete,
  but I have only did validation feature testing. So we have to write more 
  tests on properties, methods, and events (validation errors).


** System.Xml.Serialization

  Lluis rules ;-)

  Well, in fact XmlSerializer is almost finished and is on bugfix phase.

  However, we appliciate more tests. Please try 
  
  	<ul>
		* System.Web.Services to invoke SOAP services.
  		* xsd.exe and wsdl.exe to create classes.
	</ul>

  And if any problems were found, please file it to bugzilla.

  Lluis also built interesting standalone test system placed under
  mcs/class/System.Web.Services/Test/standalone.

  You might also interested in genxs, which enables you to create custom
  XML serializer. This is not included in Microsoft.NET. 
  See <a 
  href="http://primates.ximian.com/~lluis/blog/archives/000120.html">here</a>
  and mcs/tools/genxs for the details.


** System.Xml.XPath and System.Xml.Xsl

  There are two implementations for XSLT. One (and historical) implementation
  is based on libxslt (aka Unmanaged XSLT). Now we uses fully implemented
  managed XSLT. To use Unmanaged XSLT, set MONO_UNMANAGED_XSLT environment
  value (any value is acceptable).

  As for Managed XSLT, we support msxsl:script.

  It would be nice if we can support <a href="http://www.exslt.org/">EXSLT</a>.
  <a href="http://msdn.microsoft.com/WebServices/default.aspx?pull=/library/en-us/dnexxml/html/xml05192003.asp">Microsoft has already done it</a>, but it
  is not good code since it depends on internal concrete derivatives of
  XPathNodeIterator classes.

  In general, .NET's "extension objects" (including msxsl:script) is not
  usable to return node-sets, so if we support EXSLT, it has to be done
  internally inside our System.XML.dll. Volunteers are welcome.

  Our managed XSLT implementation is still inefficient. For some kind of
  transformation, XslTransform.Load() and .Transform() looks slower than MS.


** System.Xml and ADO.NET v2.0

  Microsoft introduced the first beta version of .NET Framework 1.2 runtime
  and sdk (and Visual Studio Whidbey). They are now available on MSDN
  _subscriber_ download (i.e. it is not publicly downloadable yet). It
  contains several new classes.

  There are two assemblies related to System.Xml v2.0; System.Xml.dll and
  System.Data.SqlXml.dll (here I treat sqlxml.dll as part of System.Xml v2.0,
  but note that it is also one of the ADO.NET 2.0 feature). There are several 
  namespaces such as MS.Internal.Xml and System.Xml. Note that .NET Framework
  is pre-release version and MS.Internal.Xml namespace apparently shows that
  it is not in stable status as yet.

  System.Xml 2.0 contains several features such as:

  <ul>
		* XPathNavigator2 and XPathDocument2
		* XML Query
		* XmlAdapter
		* XSLT IL generator (similar to Apache XSLTC) - it is 
		  internal use
  </ul>

  Tim Coleman started ADO.NET 2.0 related works. Currently I have no plan to
  implement System.Xml v2.0 classes and won't touch with them immediately, 
  but will start in some months. If any of you wants to try this frontier,
  we welcome your effort.

*** XPathNavigator2

  System.Xml v2.0 implementation will be started from XPathDocument2 and
  XPathNavigator2 implementations. First, its document structure and basic
  navigation feature will be implemented. And next, XPath2 engine should
  be implemented (XPathNavigator2 looks very different from XPathNavigator).

  It is once described as to contain schema validation feature, but MS
  guys said that they have removed that feature (It is just a beta version,
  so anything might happen).

*** XML Query

  XML Query is a new face XML data manipulation language (well, at least new
  to .NET world). It is similar to SQL, but intended to manipulate and to
  support XML. It is similar to XSLT, but extended to support new features
  such as XML Schema based datatypes.

  XML Query implementation can be found mainly in System.Xml.Query and 
  MS.Internal.Xml.Query namespaces. Note that they are in 
  System.Data.SqlXml.dll.

  MSDN documentation says that there are two kind of API for XML Query: High
  Level API and Low Level API. At the time of this beta version, the Low Level
  API is described not released yet (though it may be MS.Internal.Xml.*
  classes). However, to implement the High Level API, the Low Level API will 
  be used. They looks to have interesting class structures in MS.Internal.Xml 
  related stuff, so it would be nice (and I will) start to learn about them.

  They looks to have IL generator classes, but it would be difficult to
  start from them.

*** System.Data.Mapping

  System.Data.Mapping and System.Data.Mapping.RelationalSchema are the
  namespaces for mapping support between database and xml. This is at
  stubbing phase (incomplete as yet).

*** XmlAdapter

  XmlAdapter is used to support XML based query and update using 
  XPathDocument2 and XPathNavigator2. This class is designed to synthesize
  ADO.NET and System.Xml. It connects to databases, and querys data in XML
  shape into XPathDocument2, using Mapping schema above. This must be
  done after several classes such as XPathDocument2 and MappingSchema.


** Miscellaneous Class Libraries

*** RELAX NG

  I implemented an experimental RelaxngValidatingReader. It is still not
  complete, for example some simplification stuff (see RELAX NG spec 
  chapter 4; especially 4.17-19) and some constraints (especially 7.3).

  It now supports custom datatype handling. Right now, you can use XML
  schema datatypes ( http://www.w3.org/2001/XMLSchema-datatypes ) as well
  as RELAX NG default datatypes (as used in relaxng.rng).

  I am planning improvements (giving more kind error messages, supporting
  compact syntax and even object mapping), but it is still my wishlist.


** Tools

*** xsd.exe

  See <a href="ado-net.html">ADO.NET page</a>.

  Microsoft has another inference class from XmlReader to XmlSchemaCollection
  (Microsoft.XsdInference). It may be useful, but it won't be so easy.


** Miscellaneous

*** Mutual assembly dependency

  Sometimes I hear complain about System.dll and System.Xml.dll mutual
  dependency: System.dll references to System.Xml.dll (e.g. 
  System.Configuration.ConfigXmlDocument extended from XmlDocument), while 
  System.Xml.dll vice versa (e.g. XmlUrlResolver.ResolveUri takes System.Uri).
  Since they are in public method signatures, so at least we cannot get rid
  of these mutual references.

  Nowadays System.Xml.dll is built using incomplete System.dll (lacking
  System.Xml dependent classes such as ConfigXmlDocument). Full System.dll
  is built after System.Xml.dll is done.

  Also note that you still need System.dll to run mcs.