Via Intellisense, I've found that the DocumentFormat.OpenXml.Packaging.WordprocessingDocument class exposes public methods ToFlatOpcString()/ToFlatOpcDocument(), which (as the name suggests) converts the OpenXml document to a string/XDocument in Flat OPC format.
The API documents, however, make no reference to these methods - does this mean they are at risk of being removed by Microsoft in a later update? Is it safe to rely on these undocumented public methods?
Answers to your questions:
- Does this mean they are at risk of being removed by Microsoft in a later update?
No api documentation does not imply they are more at risk of being removed by Microsoft than methods that have documentation.
- Is it safe to rely on these undocumented public methods
Absolutely.
More info:
In 2014, Microsoft made the announcement that the OpenXml library is to follow the open source model. You can download all of the code and review these methods via the OpenXml GitHub site. You can even fork the code and work off of your new repository to safeguard against the case where Microsoft did decide to deprecate methods.
A quick search shows the method has documentation in the GitHub repo.