[quote user="Maury"]They are most certainly in the DgnPlatformNET API[/quote]
I'm pleased to read that.
My reason for carping about non-documented functions is simple. For many years, Bentley Systems have had the policy that, unless a function is both published and documented, it's not supported. In this case, it's not enough for the method to exist in Bentley.GeometryNET.SolidPrimitive— it must also be documented.
In other words, Bentley.GeometryNET.SolidPrimitive could disappear in MicroStation CONNECT Update 1, and we would have no redress. That's why it's important, for us third-party developers, to have APIs that are both public and documented.