Introduction

NetOcc is Open CASCADE Technology 8.0.1's C++ API in C#, generated from OCCT's headers. Names don't change: gp_Pnt is gp_Pnt, in namespace OCC.Core.gp. OCCT's reference manual, its forum posts and C++ samples apply as they are, once you know how signatures map.

OCCT's layers

Each layer builds on the ones below. The guide follows them bottom-up.

flowchart BT
    values["<b>Values</b><br/>gp, math, Bnd"]
    geometry["<b>Geometry</b><br/>Geom, Geom2d, GeomAPI"]
    topology["<b>Topology</b><br/>TopoDS, TopExp, BRep"]
    algorithms["<b>Algorithms</b><br/>BRepPrimAPI, BRepAlgoAPI, BRepFilletAPI,<br/>BRepMesh, ShapeFix, BRepGProp"]
    exchange["<b>Files</b><br/>STEPControl, IGESControl,<br/>StlAPI, RWGltf"]
    documents["<b>Documents</b><br/>TDocStd, TDF (OCAF),<br/>XCAFDoc (XCAF)"]
    views["<b>3D views</b><br/>V3d, AIS, OpenGl"]
    values --> geometry --> topology --> algorithms
    algorithms --> exchange
    algorithms --> documents
    algorithms --> views
    documents --> exchange

A typical program

flowchart LR
    make["Build<br/>primitives, sweeps"] --> combine["Combine<br/>booleans, fillets"]
    read["Read<br/>STEP, IGES, BRep"] --> check
    combine --> check["Check and repair<br/>BRepCheck, ShapeFix"]
    check --> measure["Measure<br/>volume, distances"]
    check --> mesh["Mesh<br/>BRepMesh"]
    check --> write["Write<br/>STEP, IGES, BRep"]
    mesh --> stl["STL, glTF, OBJ"]
    mesh --> view["Show<br/>AIS, V3d"]

Packages by task

Task	Namespaces (OCC.Core.*)	Guide
Points, vectors, transformations	gp, math, Bnd	Geometry
Curves and surfaces	Geom, Geom2d, GC, GeomAPI, GCPnts	Geometry
Shapes and their parts	TopoDS, TopExp, TopTools, BRep, BRepTools	Topology
Building shapes	BRepBuilderAPI, BRepPrimAPI, BRepOffsetAPI	Modeling
Booleans, fillets, offsets	BRepAlgoAPI, BRepFilletAPI, BRepOffsetAPI	Modeling
Properties, checks, repair	BRepGProp, BRepBndLib, BRepExtrema, BRepCheck, ShapeFix, ShapeUpgrade	Analysis and repair
Triangles	BRepMesh, Poly	Meshing
Files	STEPControl, IGESControl, StlAPI, RWStl, RWGltf, RWObj	Files
Assemblies, colors, names	XCAFDoc, XCAFApp, STEPCAFControl	Assemblies
Application data, undo	TDocStd, TDF, TDataStd, TNaming	Documents
3D views	V3d, AIS, OpenGl, Aspect, Graphic3d	3D views

Reading the examples

Every example on this site is a region of a test in netocc-documentation/samples, compiled against the NetOcc package on each change and run, except the on-screen 3D views, which need a window and a GPU. They use C# 12 (collection expressions, using declarations); on .NET Framework set <LangVersion>latest</LangVersion> or spell those out.

Lengths are plain numbers: OCCT has no unit type, and its file readers and writers assume millimetres unless told otherwise.