public int GetPatchStartPointIndex( 
   int patchIndex
)

// standard geometry properties
bool hasZ = multipatch.HasZ;
bool hasM = multipatch.HasM;
bool hasID = multipatch.HasID;
bool isEmpty = multipatch.IsEmpty;
var sr = multipatch.SpatialReference;

// number of patches (parts)
int patchCount = multiPatch.PartCount;
// number of points
int pointCount = multiPatch.PointCount;

// retrieve the points as MapPoints
ReadOnlyPointCollection points = multipatch.Points;
// or as 3D Coordinates
IReadOnlyList<Coordinate3D> coordinates = multipatch.Copy3DCoordinatesToList();


// multipatch materials
bool hasMaterials = multiPatch.HasMaterials;
int materialCount = multiPatch.MaterialCount;


// multipatch textures
bool hasTextures = multiPatch.HasTextures;
int textureVertexCount = multiPatch.TextureVertexCount;

// normals
bool hasNormals = multiPatch.HasNormals;


// properties for an individual patch (if multipatch.PartCount > 0)
int patchPriority = multiPatch.GetPatchPriority(patchIndex);
esriPatchType patchType = multiPatch.GetPatchType(patchIndex);

// patch points
int patchPointCount = multiPatch.GetPatchPointCount(patchIndex);
int pointStartIndex = multiPatch.GetPatchStartPointIndex(patchIndex);
// the patch Points are then the points in multipatch.Points from pointStartIndex to pointStartIndex + patchPointCount 

// if the multipatch has materials 
if (hasMaterials)
{
  // does the patch have a material?  
  //   materialIndex = -1 if the patch does not have a material; 
  //   0 <= materialIndex < materialCount if the patch does have materials
  int materialIndex = multipatch.GetPatchMaterialIndex(patchIndex);


  // properties for an individual material (if multipatch.MaterialCount > 0)
  var color = multipatch.GetMaterialColor(materialIndex);
  var edgeColor = multipatch.GetMaterialEdgeColor(materialIndex);
  var edgeWidth = multipatch.GetMaterialEdgeWidth(materialIndex);
  var shiness = multipatch.GetMaterialShininess(materialIndex);
  var percent = multipatch.GetMaterialTransparencyPercent(materialIndex);
  var cullBackFace = multipatch.IsMaterialCullBackface(materialIndex);

  // texture properties
  bool isTextured = multipatch.IsMaterialTextured(materialIndex);
  if (isTextured)
  {
    int columnCount = multipatch.GetMaterialTextureColumnCount(materialIndex);
    int rowCount = multipatch.GetMaterialTextureRowCount(materialIndex);
    int bpp = multipatch.GetMaterialTextureBytesPerPixel(materialIndex);
    esriTextureCompressionType compressionType = multipatch.GetMaterialTextureCompressionType(materialIndex);
    var texture = multipatch.GetMaterialTexture(materialIndex);
  }
}

// texture coordinates (if multipatch.HasTextures = true)
if (hasTextures)
{
  int numPatchTexturePoints = multiPatch.GetPatchTextureVertexCount(patchIndex);
  var coordinate2D = multiPatch.GetPatchTextureCoordinate(patchIndex, 0);

  ICollection<Coordinate2D> textureCoordinates = new List<Coordinate2D>(numPatchTexturePoints);
  multiPatch.GetPatchTextureCoordinates(patchIndex, ref textureCoordinates);
}


// patch normals (if multipatch.HasNormals = true)
if (hasNormals)
{
  //  number of normal coordinates = multipatch.GetPatchPointCount(patchIndex)
  Coordinate3D patchNormal = multipatch.GetPatchNormal(patchIndex, 0);
  ICollection<Coordinate3D> normalCoordinates = new List<Coordinate3D>(patchPointCount);
  multipatch.GetPatchNormals(patchIndex, ref normalCoordinates);
}