/*
Description:
  Gets the indices of the trims and edges used by a brep face.
Parameters:
  brep              - [in] The brep.
  face_index        - [in] Index of the face in brep.m_F[].
  face_trim_indices - [out] Indices into brep.m_T[] of the face's trims.
  face_edge_indices - [out] Indices into brep.m_E[] of the face's edges.
Returns:
  true if successful, false if face_index is out of range.
*/
bool GetFaceTrimsAndEdges(
  const ON_Brep& brep,
  int face_index,
  ON_SimpleArray<int>& face_trim_indices,
  ON_SimpleArray<int>& face_edge_indices
)
{
  face_trim_indices.Empty();
  face_edge_indices.Empty();

  // Get the face at the given index
  const ON_BrepFace* face = brep.Face(face_index);
  if (nullptr == face)
    return false;

  // Walk the loops of the face
  for (int fli = 0; fli < face->LoopCount(); fli++)
  {
    const ON_BrepLoop* loop = face->Loop(fli);
    if (nullptr == loop)
      cusing System.Collections.Generic;
using Rhino.Geometry;

static class BrepFaceTopologySample
{
  /// <summary>
  /// Gets the indices of the trims and edges used by a Brep face.
  /// </summary>
  /// <param name="brep">The Brep.</param>
  /// <param name="faceIndex">Index of the face in brep.Faces.</param>
  /// <param name="faceTrimIndices">Indices into brep.Trims of the face's trims.</param>
  /// <param name="faceEdgeIndices">Indices into brep.Edges of the face's edges.</param>
  /// <returns>true if successful, false if the face index is out of range.</returns>
  public static bool GetFaceTrimsAndEdges(Brep brep, int faceIndex, out List<int> faceTrimIndices, out List<int> faceEdgeIndices)
  {
    faceTrimIndices = new List<int>();
    faceEdgeIndices = new List<int>();

    // Get the BrepFace at the given index
    if (null == brep || faceIndex < 0 || faceIndex >= brep.Faces.Count)
      return false;
    BrepFace face = brep.Faces[faceIndex];

    // Walk the loops of the face
    foreach (BrepLoop loop in face.Loops)
    {
      // Walk the trims of the loop
      foreach (BrepTrim trim in loop.Trims)
      {
        faceTrimIndices.Add(trim.TrimIndex);

        // Get the edge the trim uses.
        // Singular trims have no edge, so trim.Edge is null for them.
        BrepEdge edge = trim.Edge;
        if (null == edge)
          continue;
        faceEdgeIndices.Add(edge.EdgeIndex);
      }
    }

    return true;
  }
}