public bool ThemHoaDonDV(string maKhachHang, string maDV, int soLuongDV, DateTime ngaySuDung)
{
string ngaySuDungFormatted = ngaySuDung.ToString("MM-dd-yyyy");
db.ExecuteQuery <HoaDonDV>("Insert into HoaDonDV values('" + maKhachHang + "','" + maDV + "'," + soLuongDV + ",'" + ngaySuDungFormatted + "')");
db.SubmitChanges();
return(true);
}
