public static Rhino.Commands.Result MoveGripObjects(Rhino.RhinoDoc doc)
    {
        // The following example demonstrates how to move a surface's grip objects.
        // In this sample, all grips will be moved a fixed distance of 0.5 units
        // in the normal direction of the surface at that grip location.

        Rhino.DocObjects.ObjRef objRef;
        Rhino.Commands.Result   rc = Rhino.Input.RhinoGet.GetOneObject("Select surface for control point editing", false, Rhino.DocObjects.ObjectType.Surface, out objRef);
        if (rc != Rhino.Commands.Result.Success)
        {
            return(rc);
        }

        Rhino.DocObjects.RhinoObject obj = objRef.Object();
        if (null == obj)
        {
            return(Rhino.Commands.Result.Failure);
        }

        Rhino.Geometry.Surface srf = objRef.Surface();
        if (null == srf)
        {
            return(Rhino.Commands.Result.Failure);
        }

        // Make sure the object's grips are enabled
        obj.GripsOn = true;
        doc.Views.Redraw();

        Rhino.DocObjects.GripObject[] grips = obj.GetGrips();
        for (int i = 0; i < grips.Length; i++)
        {
            Rhino.DocObjects.GripObject grip = grips[i];

            // Calculate the point on the surface closest to our test point,
            // which is the grip's 3-D location (for this example).
            double u, v;
            if (srf.ClosestPoint(grip.CurrentLocation, out u, out v))
            {
                // Compute the surface normal at a point
                Rhino.Geometry.Vector3d dir = srf.NormalAt(u, v);
                dir.Unitize();

                // Scale by our fixed distance
                dir *= 0.5;

                // Move the grip to a new location
                grip.Move(dir);
            }
        }

        // Altered grip positions on a RhinoObject are used to calculate an updated
        // object that is added to the document.
        doc.Objects.GripUpdate(obj, false);
        doc.Views.Redraw();

        return(Rhino.Commands.Result.Success);
    }