private void checkBox1_CheckedChanged(object sender, EventArgs e) { if (checkBox1.Checked) { textBoxDiachi.Visible = false; textBoxKhachhang.Visible = false; label9.Visible = false; comboKhachhang.Visible = true; List <Khachhang> lst = APIs.LstKhachhang(); foreach (Khachhang item in lst) { comboKhachhang.Items.Add(item.Tenkhachhang); } } else { textBoxDiachi.Visible = true; textBoxKhachhang.Visible = true; label9.Visible = true; comboKhachhang.Visible = false; } }