3d modelling – How do I Import/export .OBJ and .STL files in C#?

I am automating the process of optimizing 3D models for printing purposes. Basically, I generate an Wavefront OBJ file and dump it in some folder. An automated process then has to do some processing, like merging materials, rotating and resizing it to a fixed size. And then it generates multiple STL files as exports, which is based on the separate parts in the original OBJ file. The file would have no materials, as it’s printed with UV resin in generally white or grey colors.
Now, the OBJ file can easily be parsed and processed using regular expressions and some other tricks I know, but it feels like I’m reinventing the wheel. Also, I want to export as STL file, while my processing trick will just generate a different OBJ file. And searching for 3D modelling libraries for C# mostly ends up with gaming libraries, which is NOT what I’m looking for. I don’t need anything to visualize the model, but just to process a model, fix it and save it in a different format.
There doesn’t seem to be any (open source) libraries that will process 3D models without displaying them. Am I right?