private void cbxLoaiPhong_SelectedIndexChanged(object sender, EventArgs e)
{
string loaiPhong = cbxLoaiPhong.GetItemText(cbxLoaiPhong.SelectedItem);
RoomAccess data = new RoomAccess();
DataTable dt = new DataTable();
dt = data.loadIDCost(loaiPhong);
cbxMaPhong.DisplayMember = "roomno";
cbxMaPhong.DataSource = dt;
cbxtienPhong.DisplayMember = "price";
cbxtienPhong.DataSource = dt;
txtCostPerDay.Text = cbxtienPhong.GetItemText(cbxtienPhong.SelectedItem);
cost = float.Parse(txtCostPerDay.Text);
caculteMoney();
}