* 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 XmlCachingResolver by Ben Maurer.
If even one time download is not acceptable, you can use this one.
[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 bugzilla entry
for this matter.
**** XML Reader
XmlTextReader, XmlNodeReader and XmlValidatingReader are almost finished.
* 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.
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()).
I added extra support interface named "IHasXmlParserContext", which is
considered in XmlValidatingReader.ResolveEntity(). This is now
made as internal interface. 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:
* 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.
**** 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
* System.Web.Services to invoke SOAP services.
* xsd.exe and wsdl.exe to create classes.
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 here
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 EXSLT.
Microsoft has tried to do some of them,
but it is not successful because of System.Xml.Xsl design problem:
* 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.
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
* 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.
** Plans
*** System.Xml v2.0
Microsoft released the first public beta version of .NET Framework 2.0,
available from MSDN.
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; 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 that they are subject
to change.
System.Xml 2.0 contains several features such as:
* 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
**** XmlReader.Create() and ReadValueAsXxx()
In .NET 2.0, XmlTextReader, XmlNodeReader, XmlValidatingReader are
obsolete and XmlReader.Create() is recommended (there is however no
alternative way to create XmlNodeReader).
As for typed XmlReader support, we need something like XQueryConvert that
is like XmlConvert but based on XQuery 1.0 and XPath 2.0 data model.
**** XML Schema design changes
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 some.
**** XSD Inference
In .NET 2.0, there is an XML Schema inference implementation. Once
XmlSchemaSet is implemented, it can be separately done by anyone.
Volunteer efforts are welcome here.
**** New XPathNavigator
System.Xml v2.0 implementation will be started from new XPathNavigator
implementation. First, its document structure and basic navigation feature
will be implemented. And next, XPath2 engine should be implemented
(XPathNavigator2 looks very different from XPathNavigator).
My current plan is to implement editable interface for XmlDocumentNavigator
(XPathNavigator from XmlDocument). Our XPathDocument is index-based
implementation, and thus it cannot be modified to be editable as it is.
There are some trivial tasks such as schema validation (we have
XPathDocumentReader that just wraps XPathNavigator, and our
XmlValidatingReader can accept any XmlReader).
**** XQuery 1.0
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. Note that they are in
System.Data.SqlXml.dll The implementation is mostly
in System.Xml.dll.
XQuery implementation will be done in these steps:
* XQuery syntax parser that parses xquery string to AST (abstract
syntax tree).
* XQuery AST compiler into runnable query command (not done yet)
* 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.
**** 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:
http://sourceforge.net/projects/dotnetopensrc/). 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. 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.
* 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).
** Tools
*** xsd.exe
See ADO.NET page.
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
last updated 07/11/2004