private void btnLogin_Click(object sender, EventArgs e)
 {
     if (TaiKhoanData.CheckLogin(txtUsername.Text.Trim().ToString(), txtPassword.Text))
     {
         lblInvalid.Visible = false;
         this.Visible       = false;
         frmMaintainSpeciality form = new frmMaintainSpeciality();
         form.LoginedUser = txtUsername.Text.Trim().ToString();
         form.Visible     = true;
     }
     else
     {
         lblInvalid.Visible = true;
     }
 }