Understanding the SolidWorks B-Rep Model

What Is B-Rep?

B-Rep (Boundary Representation) is how SolidWorks internally represents solid geometry. Instead of storing the solid as a volume, it stores the boundaries — the faces, edges, and vertices that define the shape's surface.

Understanding B-Rep is the key to advanced API operations: measuring distances, detecting collisions, recognizing features, and manipulating geometry directly.

Topology vs Geometry

This is the single most important distinction in the SolidWorks API, and it's poorly explained everywhere:

• Topology = the structure (what's connected to what)
• Geometry = the math (what shape things are)

Topology Objects (Structure)

Geometry Objects (Math)

The Hierarchy

Body (IBody2)
+-- Face (IFace2)          // GetFaces()
|   +-- Surface (ISurface) // GetSurface()
|   +-- Loop (ILoop2)      // GetLoops()
|       +-- Edge (IEdge)   // GetEdges()
|           +-- Curve (ICurve)   // GetCurve()
|           +-- Vertex (IVertex) // GetStartVertex() / GetEndVertex()
|               +-- Point (IMathPoint) // GetPoint()

Key Relationships

• A face always has one underlying surface. But a surface can be infinite — the face defines the bounded region
• An edge always has one underlying curve. The edge defines the bounded segment of that curve
• A face has one or more loops. The outer loop defines the boundary. Inner loops are holes
• Each edge is shared by exactly two faces (or one face, if it's a free edge on a surface body)

Checking Face Types

To determine what geometric shape a face is:

ISurface surface = face.GetSurface();
if (surface.IsPlane()) { /* flat face */ }
else if (surface.IsCylinder()) { /* cylindrical face — holes, shafts */ }
else if (surface.IsCone()) { /* tapered face */ }
else if (surface.IsSphere()) { /* spherical face */ }
else if (surface.IsTorus()) { /* fillet face */ }
else { /* B-spline or other complex surface */ }

Practical Applications

• Distance measurement: Use ISurface.GetClosestPointOn() or IBody2.GetClosestPoint() to find minimum distances between bodies
• Hole recognition: Find cylindrical faces, check if they're internal (face normal points inward), measure the radius from ISurface.CylinderParams
• Flat face detection: Find all planar faces for machining operation planning
• Edge length calculation: Use ICurve.GetLength() for total edge length (useful for weld length estimation)
• Collision detection: Use IBody2.Operations2 with intersection to check if two bodies overlap

Tips

• Always check for null when navigating B-Rep — edges can have null start/end vertices (closed loops like circles)
• Face normals point outward on solid bodies. Use IFace2.GetNormal() to determine face orientation
• B-Rep traversal is the fastest way to analyze geometry — it doesn't require feature recognition or rebuilds
• When comparing faces or edges across operations, use persistent IDs (IEntity.GetPersistReference3) rather than object references, which may change after rebuilds