public void PopulateDoctorDropdownList()
        {
            int centerId = Convert.ToInt32(Session["centerId"]);

            doctorDropDownList.DataSource     = manager.PopulateDoctorDropDownList(centerId);
            doctorDropDownList.DataTextField  = "DoctorName";
            doctorDropDownList.DataValueField = "Id";
            doctorDropDownList.DataBind();
        }