private void button1_Click(object sender, EventArgs e)
 {
     DanhSachKhachHang f = new DanhSachKhachHang();
     f.Show();
 }
 private void DanhSachKhachHangToolStripMenuItem_Click(object sender, EventArgs e)
 {
     DanhSachKhachHang frmDanhSạhKhachHang = new DanhSachKhachHang();
     frmDanhSạhKhachHang.Show();
 }