.NET & XML

Other Ways to Constrain XML Structure

Although DTDs also provide a means of constraining XML, and validation using DTDs is supported by .NET, they do not provide as much control over XML content as XML Schema does. For example, a DTD cannot specify the required order of elements or attributes, nor can it enforce the data type of element or attribute content. XML Schema was actually designed to make up for DTD’s lack of functionality, and does so quite well.

A common complaint about XML Schema, however, is that it is actually too complex. RELAX NG was developed concurrently with XML Schema, has been adopted as a standard by the Organization for the Advancement of Structured Information Standards (OASIS), and has been accepted as a draft international standard of the International Organization for Standardization (ISO). RELAX NG actually began its life as two competing validation languages, RELAX and TREX, which were merged in 2001. RELAX NG is as capable of describing XML Structures as XML Schema, and arguably simpler to use. However, .NET does not support validation with RELAX NG.

There is nothing to keep some enterprising developer from building a RELAX NG validator for .NET, of course. Also, James Clark’s Trang processor (http://www.thaiopensource.com/relaxng/trang.html) lets you work in RELAX NG and convert your results to W3C XML Schema.