Linking a XML document to its schema

[dbowen at es.com]

> We are developing an application which will use XML as a syntax for
> communicating data between two parts of that application.
> 
> This XML uses a DTD, both for the (admittedly limited) validation it
> provides, and to supply default values for attributes, so the 
> receiving
> part of the application needs to be able to find the DTD 
> before it can make
> sense of the XML.
> 
...
> What should I do?  If I put:
> 
>    <!DOCTYPE Linestyle SYSTEM "file://C:/MyApp/MyApp.dtd">
> 
> in the file, the XML will be unusable on any machine which 
> happens to have
> installed my application in another directory.

We have run into a similar issue.  In our case, we are creating a new MS
Windows Application (not Web based at all), and using an XML based file
format.  We are using the MSXML parser to load in and validate documents
according to our schema (or DTD).  We wanted to keep our Schemas (or DTDs)
stored in a central location, and have the data files reference them no
matter where our application is installed (or where the OS is installed).

The best answer we've gotten for our case was to create a handler for our
own Asynchronous Pluggable Protocol.  In our case, a reference to the schema
(or DTD) would look like
"our-custom-protocol://Some/Path/in/the/Registry/CLSID".  We then have a
CLSID to a COM object that delivers the schema (or DTD) to the protocol
handler.  Since MSXML uses URL monikers, we can do this.  Its not completely
implemented yet, but that's the plan.

I realize that this is very Microsoft/Windows specific, but we haven't found
a better way yet.  Does anyone else use an XML based file format for a
normal desktop application?  If so, how do you handle centralizing your
schema(s) and DTD(s)?

Thanks,

Daniel Bowen
Software Engineer
Evans & Sutherland
dbowen at es.com

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