How to convert one list to multidimensional array?

Question

public JsonResult AllListofMainDivisionDesignation() {
List<HrDesignation> designation = new List<HrDesignation>();
     var designationgroup = db.HrDesignationGroup.Where(p=>p.Rank==2).ToList();
     foreach (var des in designationgroup)
    {         var desig = db.HrDesignation.Where(i=> i.DesignationGroupId == des.DesignationGroupId).OrderBy(r => r.Rank).ToList();
 designation.AddRange(desig);}
 HrDesignation[][] arrays = designation.Select(a =>a.ToArray()).ToArray();
 return Json(arrays, JsonRequestBehavior.AllowGet);
 }