mono/web/xml-classes

* XML Classes Status and Tasks

** 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.

** Status

*** System.Xml namespace


**** Document Object Model (Core)

  DOM implementation has finished and our DOM implementation scores better
  than MS.NET as to the NIST DOM test results (it is ported by Mainsoft
  hackers and in our unit tests).

**** 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

  XmlUrlResolver is implemented.

  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. 
  If even one time download is not acceptable, you can use <a href="http://primates.ximian.com/~atsushi/XmlStoredResolver.cs">this one</a>.

  [2.0] XmlDataSourceResolver is not implemented as yet.

**** XmlNameTable

  NameTable is implemented, but also needs performance improvement.
  It affects on the whole XML processing performance so much.
  Optimization hackings are welcome. There is also a <a 
  href="http://bugzilla.ximian.com/show_bug.cgi?id=59537">bugzilla entry</a> 
  for this matter.

**** 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 parse 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). If anyone
		  contributes an implementation, it would be still nice.
	</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 wouldn't be so 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 (this bug is fixed in .NET 2.0 beta,
  taking shape of XmlReader.Create()).

  <del>I added extra support interface named "IHasXmlParserContext", which is
  considered in XmlValidatingReader.ResolveEntity(). </del><ins>This is now
  made as internal interface.</ins> Microsoft failed to design XmlReader
  so that XmlReader cannot be subtree-pluggable (i.e. wrapping use of other
  XmlReader) since XmlParserContext shoud be supplied for DTD information
  support (e.g. entity references cannot be expanded) and namespace manager.
  (In .NET 2.0, Microsoft also supported similar to IHasXmlParserContext,
  named IXmlNamespaceResolver, but it still does not provide DTD information.)

  We also have RELAX NG validating reader (described later).


*** System.Xml.Schema

**** Summary

  Basically it is completed. You can test how current schema validation engine
  is complete (incomplete) 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 -
  this score is better than that of Microsoft implementation. But instead,
  we need performance boost. There should be many points to improve
  schema compilation and validation.

**** 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
		  (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

  Basically this is implemented and actually its feature is 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 manpages for details. Code files are in mcs/tools/genxs.

  Lluis also created "sgen", that based on XmlSerializer.GenerateSerializer().
  Code files are in mcs/tools/sgen.

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

  There are two XSLT implementations. One and historical implementation is
  based on libxslt (aka Unmanaged XSLT). Now we uses fully implemented and
  managed XSLT by default. 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 tried to do some of them</a>, 
  but it is not successful because of System.Xml.Xsl design problem:

	<ul>
		* In general, .NET's "extension objects" (including 
		  msxsl:script) is not useful to return node-sets (MS XSLT
		  implementation rejects just overriden XPathNodeIterator, 
		  but accepts only their hidden classes. And are the same 
		  in Mono though classes are different)

		* In .NET's extension object design, extension function name 
		  is a valid method name that cannot contain some characters
		  such as '-'. That is, implementing EXSLT in C# is impossible.
	</ul>

  So if we support EXSLT, it has to be done inside our System.XML.dll.
  Microsoft developers are also aware of this problem and some of them wish
  to have EXSLT support in WinFX (not whidbey). If anyone is interested
  in it, it would be nice.

  Our managed XSLT implementation is slower than MS XSLT for some kind of
  stylesheets, and faster for some.


*** 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).
  See mcs/class/Commons.Xml.Relaxng/README for details.

  Currently we have

	<ul>
		* Custom datatype support. 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).

		* RELAX NG Compact Syntax support, though not yet stable.
		  See Commons.Xml.Relaxng.Rnc.RncParser class.
</ul>


** System.Xml v2.0

  Microsoft released the first public beta version of .NET Framework 2.0,
  available from <a href="http://www.microsoft.com/downloads/details.aspx?familyid=916EC067-8BDC-4737-9430-6CEC9667655C&displaylang=en">MSDN</a>.
  It contains several new classes.

  There are two assemblies related to System.Xml v2.0; System.Xml.dll and
  System.Data.SqlXml.dll. Most of the important part are in System.Xml.dll
  (you will find that the core part of XQuery is in System.Xml.dll; For
  example, see some classes in MS.Internal.Xml namespace), so I'll still 
  spot only on System.Xml.dll.

  Note that .NET Framework is pre-release version, so they are subject
  to change. Actually many of the pre-released classes vanished.

  System.Xml 2.0 contains several features such as:

	<ul>
		* new XPathNavigator and XPathDocument
		* Factory method based and strongly-typed XmlReader
		* XML Schema design changes
		* XSD Inference
		* XQuery implementation
		* Well-documented and improved XmlSerializer.
		* XSLT IL generator (similar to Apache XSLTC) - it is 
		  internal use
	</ul>

*** System.Xml 2.0

**** XmlReader/XmlWrier Factory methods

  In .NET 2.0, XmlTextReader, XmlNodeReader, XmlValidatingReader are
  obsolete and XmlReader.Create() is recommended (there is however no
  alternative way to create XmlNodeReader). Similarly, there are
  XmlWriter.Create() overloads.

  Currently, Microsoft's XmlWriter.Create() is unreliable and maybe there
  will be changes. So basically XmlWriter.Create() is supposed to be done
  after the next beta version of .NET 2.0.

  Some of XmlReader.Create() overloads are implemented, with limited
  XmlReaderSettings support.


**** Typed XmlReader/XmlWriter

  In .NET 2.0, XmlReader is supposed to support strongly-typed data reading.
  They are based on W3C "XML Schema Datatypes" Recommendation and "XQuery 1.0
  and XPath 2.0 Data Model" Working Draft.

  Some of XmlReader.ReadValueAsXxx() and XmlWriter.WriteValue() overloads are
  implemented, though incompletely. They are based on internal XQueryConvert.


**** Sub-tree handling in XmlReader/XmlWriter/XPathNavigator

  Currently XmlReader.ReadSubtree(), XmlWriter.WriteSubtree() and
  XPathNavigator.ReadSubtree() are implemented, though not so stable.
  They are based on Mono.Xml.SubtreeXmlReader and
  Mono.Xml.XPath.XPathNavigatorReader classes.


*** System.Xml.Schema 2.0

  Since .NET 1.x is not so compliant with W3C XML Schema specification,
  Microsoft had to redesign System.Xml.Schema classes. We also have to
  change many things.

  1) It does not expose XmlSchemaDatatype (except for obsolete members).
  Primitive types are represented as XmlSchemaSimpleType instances (thus
  there are ElementSchemaType, AttributeSchemaType, BaseXmlSchemaType that
  replace some existing properties).

  2) "XQuery 1.0 and XPath 2.0 Data Model" datatypes (such as
  xdt:dayTimeDuration) are newly supported. They are partially implemented
  yet.

  3) schema structures are now bound in parent-child relationship. It is
  not yet implemented. With related to it, there seems bunch of schema
  compilation bugfixes.

  4) XmlSchemaCollection is not used anymore to represent effective set of
  schemas. Instead, new XmlSchemaSet class is used. It should affect on
  schema compilation design. In fact, I've implemented XmlSchemaCollection
  as more conformant to W3C specification, but there are still many changes
  required.


**** XSD Inference

  In .NET 2.0, there is an XML Schema inference implementation. Now that
  XmlSchemaSet is basically implemented, it can be separately done by anyone.
  Volunteer efforts are welcome here.


*** System.Xml.XPath 2.0

**** Editable XPathDocument

  in .NET 2.0 XPathDocument is supposed to be editable. Currently we provide
  fast document table model based implementation (DTMXPathNavigator), but
  by that design change, we (and they) cannot provide fast read only
  XPathNavigator from XPathDocument anymore.

  Currently, new XPathDocument implementation is provided. The actual
  implementation is Mono.Xml.XPath.XPathDocument2, that is simple dom-like
  tree model. XPathDocument2 implements the same interfaces as XPathDocument
  does. And XPathDocument delegates most of the methods to that class (for
  example, XPathDocument.CreateEditor() calls XPathDocument2.CreateEditor()).

  Currently Mono.Xml.XPath.XPathDocument2 is unstable (it does not pass
  the standalone XSLT tests as well as existing DTMXPathDocument does). So
  it did not replace existing XPathDocument implementation, but you can use
  new implementation by explicitly setting environment value
  USE_XPATH_DOCUMENT_2 = yes. Currently it supports (well, is supposed 
  to support) basic editor feature such as AppendChild(). Other members
  are untested (such as RejectChanges()).

**** extra stuff - XPathEditableDocument

  Currently we provide another IXPathEditable; XPathEditableDocument. That is
  based on the idea that handles XmlDocument as editor target. It is
  implemented as Mono.Xml.XPath.XPathEditableDocument. We might provide this
  class as extra set (might be different mono-specific XML assembly).


**** System.Xml.XQuery and System.Xml.Xsl 2.0

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

  XQuery implementation can be found mainly in System.Xml.Query and 
  MS.Internal.Xml.Query namespaces. The implementation is mostly
  in System.Xml.dll. System.Data.SqlXml.dll just invokes the actual XQuery
  processor which resides in System.Xml.dll using reflection.

  XQuery implementation will be done in these steps:

	<ul>
		* XQuery syntax parser that parses xquery string to AST 
		  (abstract syntax tree). -> done.

		* XQuery AST compiler into runnable query command
		  (static context)

		* XQuery (dynamic context) runtime = XQuery expression evaluator
		  + sequence iterator.

		* Applied expression classes for XQuery/XPath 2.0 functions and
		  operators.

		* XQuery data model and (mainly) conversion support.
	</ul>


*** Relax NG and DSDL in Mono 1.2

  Currently we support only RELAX NG as one part of ISO DSDL effort. There
  is existing Schematron implementation (NMatrix Project: <a 
  href="http://sourceforge.net/projects/dotnetopensrc/">
  http://sourceforge.net/projects/dotnetopensrc/</a>). With a few changes,
  it can be used with mono.
  
  We also don't have multi-language based validation, namely Namespace-based
  Validation Dispatch Language (NVDL). To support unwrapping, one special
  XmlReader implementation is required (other schema validation support will
  be done by ReadSubtree()). Note that we had seen RELAX Namespace, Modular
  Namespace (MNS) and Namespace Routing Language (NRL) - that is,
  standardization effort is still ongoing (though NVDL looks mostly the same
  as NRL).

  In Mono 1.2, there might be improvements on Commons.Xml.Relaxng.

	<ul>
		* Currently RelaxngPattern.Compile() provides cheap compilation
		  error information. At least it can provide error location.
		  Also, the type of error should be kind of 
		  RelaxngGrammarException.

		* Right now there is no ambiguity detection implementation that
		  would be useful for RelaxngPattern based xml serialization (if
		  there is need).

		* Because of lack of ambiguity detection, currently
		  XmlMapping could not be provided. But If anyone is
		  interested in such effort, integration with XmlSerializer
		  would be interesting task.
	</ul>


** 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.

  Note that you still need System.dll to run mcs.


  Atsushi Eno <[email protected]>
  last updated 08/03/2004