private void btnAbout_Click(object sender, EventArgs e)
 {
     DialogResult Logout = new DialogResult();
     string strLogout = "Bạn muốn đăng xuất?";
     Logout = MessageBox.Show(strLogout, "Quản Lý Khách Sạn", MessageBoxButtons.YesNo, MessageBoxIcon.Warning);
     if (Logout == DialogResult.Yes)
     {
         frmDangNhap DN = new frmDangNhap();
         DN.Show();
         this.Hide();
         this.Dispose();
     }
 }
 private void dangnhapToolStripMenuItem_Click(object sender, EventArgs e)
 {
     frmDangNhap child = new frmDangNhap();
     child.MdiParent = this;
     child.Show();
 }