GetPolyLineCentroid

public static Point2d GetPolyLineCentroid(Polyline polyline)
    {
     double area = polyline.Area;
     double cx = 0.0;
     double cy = 0.0;
     double X0 = 0.0;
     double Y0 = 0.0;
     double X1 = 0.0;
     double Y1 = 0.0;

     for (int i = 0; i < polyline.NumberOfVertices - 1; i++)
     {
      X0 = polyline.GetPoint2dAt(i).X;
      Y0 = polyline.GetPoint2dAt(i).Y;
      X1 = polyline.GetPoint2dAt(i + 1).X;
      Y1 = polyline.GetPoint2dAt(i + 1).Y;

      cx += (X0 + X1) * (X0 * Y1 - X1 * Y0);
      cy += (Y0 + Y1) * (X0 * Y1 - X1 * Y0);
     }

     // last Point
     int Lv = polyline.NumberOfVertices - 1;
     X0 = polyline.GetPoint2dAt(Lv).X;
     Y0 = polyline.GetPoint2dAt(Lv).Y;
     X1 = polyline.GetPoint2dAt(0).X;
     Y1 = polyline.GetPoint2dAt(0).Y;

     cx += (X0 + X1) * (X0 * Y1 - X1 * Y0);
     cy += (Y0 + Y1) * (X0 * Y1 - X1 * Y0);

     cx /= 6 * area;
     cy /= 6 * area;

     return new Point2d(cx, cy);
    }

public static Point2d GetPolyLineCentroid(DBObjectCollection objs)
{
    Solid3d Solid = new Solid3d();
    Solid.Extrude(((Region)Region.CreateFromCurves(objs)[0]), 1, 0);
    Point2d centroid = new Point2d(Solid.MassProperties.Centroid.X, Solid.MassProperties.Centroid.Y);
    Solid.Dispose();
    return centroid;
}