We are providing the source code for the free version of the Sincpac-C3D which was originally created for learning purposes in 2009. It was updated to use strictly the .NET API referencing Civil 3D 2020 between 2021 and 2024 as time permitted. It is offered, again, as a learning tool and may contain bugs. If any are encountered, they should be relatively simple to correct by the end user. We do not claim that the code is perfect, or even the best way to code as it was developed by amateur programmers, who are also real world users of Civil 3D, while they were learning to code themselves.
Please let us know if you find the code helpful in your quest for leaning to code for Civil 3D.