Should I unit test XML schema?

Question

I am in the very early stages of writing XML schema for my work's enterprise application. The XML to be validated represents an application - similar to Winforms - forms, grids, menus etc but without layout.

The prime purpose of the XSD is not so much to validate the XML but to add design-time discoverability to the XML file, so that one would get IntelliSense for available elements and attributes.

As I am writing up the schema I have found myself doing elements of TDD and validating a document against the schema, changing either elements/attributes in the document or in the schema to make validation fail to make sure I am writing the schema correctly.

This brings me to the question of whether or not I should be unit-testing the schema, just to throw a few permutations of the XML at it and make sure it behaves how it's supposed to.

It would certainly make sense to me as my XSD-fu is abysmal and I want to be even more sure the XSD, which in effect IS a specification by itself, is correct.

Answer 1

Generally I find it very difficult to test XSD schemas:

• For XSD often the modeling of your domain is the key, often I didn't have problems with the XSD constructs itself but I analyzed the domain wrong.
• The generated model isn't based on the XSD itself, but also on the binding configuration (e.g. JAXB for Java). So in the end you are testing too much.
• Such tests depending on lots of things tend to break often, especially when you refactor the XSD.

In the end to improve XSD quality I prefer:

• Have early reviews of XSDs (by colleagues or QA). Having actual people look at them (both XSD and XML instances) discovered flaws which would never been found out by automated testing.
• Do Integration testing on the generated XML instances. These integration tests can be automated.