I used XML Schema in a recent project, and encountered a few things that,

if improved, would make it a lot more powerful.

My application has a complicated configuration file that has to be 
carefully checked by the application before proceeding. I wrote an XML 
Schema for the config file, in order to get XmlValidatingReader to do 
some of the checking so I wouldn't have to code it myself. Using all 
of the capabilities of XML Schema that I could, I was able to reduce 
the amount of code I had to write from 560 lines to 220 lines. But 
with a few more general capabilities in XML Schema, I could have 
eliminated all of my checking code.

Here is what I found missing:

1) I needed a "contiguous" restriction. Yes, I can create sequences, 
and yes, I can create unique and key restrictions, but there was no 
way to say that there must not be gaps. For example, I could have a 
number ranging from 7 to 20, and I could establish a key on that 
number, which made sure that, say, the number 8 wasn't used twice, but 
I couldn't enforce that the numbers had to be sequential (7, 8, 9, 
etc.). It sure would have been useful to be able to say no gaps are 
allowed.

2) I needed a way to say that, if a number was used, then there had to 
be a keyref to that number. Why? Because my program is mapping one set 
of numbers to another (actually, they're columns in tables--I'm 
mapping input columns to output columns, with no gaps, and I need to 
make sure that every output column is mapped to by at least one input 
column). I couldn't have the numbers 1, 2, 3, and 4, and another set 
of numbers 1, 2, 3, with 4 left out. A general notion of ref counts 
would be really useful. It could have min and max ref counts, which 
would allow all kinds of flexible uses. A ref count with a min of 1 
and max of 1 would mean that every key must have exactly one keyref to 
it. A ref count with a min of 1 and no max would mean that every key 
must have at least one keyref to it (which was the semantics I 
needed). A ref count with a min of 0 would mean that a key didn't have 
to be referred to, and so forth.

3) I needed a way to say that the max value for some attribute could 
not exceed the value of some other attribute. Generalized, it would be 
really useful to be able to have basic expressions for comparision 
(equal, not equal, greater than, less than, greater than or equal, 
less than or equal) of arbitrary fields in the schema.

4) It would be really nice to be able to specify error messages for 
error conditions in the XML Schema. I am currently relying on the 
error messages from the XML reader, but they tend to be pretty 
cryptic. Previously I had written my own messages, which were 
application-specific and quite informative. For example:

     theLogger.ConfigFileError("Specified config file contains output 
column heading base name '" + outputHeadingBaseName +
      "' with forbidden characters (only a-z, A-Z, and 0-9 are 
allowed).");

whereas now my application outputs messages like:

The 'output-heading-base-name' attribute has an invalid value 
according to its data type. An error occurred at file:///C:/Documents 
and Settings/<filename goes here>, (21, 48).

I would like to be able to hook my messages into the schema to 
override the default messages.

Received on Tuesday, 26 July 2005 13:47:52 UTC