when to validate XML while being created programmatically..

This is related to validating a brand new XML instance
WHILE being created programmatically:
 Do you generally perform validation at the very end
of completely creating an XML instance? If Yes, how
would you correct these validation errors
programmatically? It looks like a daunting task.

 OR

Does it even make sense to perform on-the-fly
validation while constructing a new XML instance? If
Yes, how would you tell the system when the instance
is even incrementally valid before turning ON the
validation. I can imagine there must be an API to
enable/disable on-the-fly validation during the
creation of XML instance. I might be wrong as it may
not make sense.

OR

Does the application design needs to ensure that the
generated XML is always valid?

-D


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