private void kháchHàngToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (kh == null)
     {
         kh = new Khachhang_View();
         kh.StartPosition = FormStartPosition.CenterScreen;
         kh.WindowState   = FormWindowState.Maximized;
         kh.MdiParent     = this;
         kh.Show();
     }
     else
     {
         kh.Activate();
     }
 }
        private void bbtKhachhang_ItemClick(object sender, ItemClickEventArgs e)
        {
            Form frm = this.Kiemtratontai(typeof(Khachhang_View));

            if (frm != null)
            {
                frm.Activate();
            }
            else
            {
                Khachhang_View kh = new Khachhang_View();
                kh.MdiParent = this;
                //dmk.StartPosition = FormStartPosition.CenterParent;
                kh.WindowState = FormWindowState.Maximized;
                kh.Show();
            }
        }