private void btnDangNhat_Click(object sender, EventArgs e)
 {
     SqlConnection con = new SqlConnection(Connect.GetConnect());
     con.Open();
     SqlDataAdapter da = new SqlDataAdapter();
     DataTable dt = new DataTable();
     SqlCommand cmd = new SqlCommand();
     cmd.Connection = con;
     cmd.CommandType = CommandType.Text;
     cmd.CommandText=   @"select *from TaiKhoan
                     where ID =@ID AND PASS=@PASS";
     cmd.Parameters.Add(new SqlParameter("@ID",SqlDbType.VarChar,10)).Value = txtUser.Text;
     cmd.Parameters.Add(new SqlParameter("@PASS", SqlDbType.VarChar, 50)).Value = txtPass.Text;
     da.SelectCommand = cmd;
     da.Fill(dt);
     if (dt.Rows.Count > 0)
     {
         id = txtUser.Text;
         if (txtPass.Text == "admin")
         {
             frmMenu fm = new frmMenu();
             fm.Show();
             this.Hide();
         }
         else
         {
             frmSach_DG fs = new frmSach_DG();
             fs.Show();
             this.Hide();
         }
     }
     else
         if (MessageBox.Show("Đăng nhập thất bại, bạn có muốn đăng nhập lại không?", "Thông báo", MessageBoxButtons.YesNo) == DialogResult.Yes)
         {
             txtUser.Focus();
             txtUser.Clear();
             txtPass.Clear();
         }
         else
         {
             Close();
             System.Windows.Forms.Application.Exit();
         }
     con.Close();
 }
 private void btnThoat_Click(object sender, EventArgs e)
 {
     frmMenu ft = new frmMenu();
     ft.Show();
     this.Hide();
 }
 private void btnThoat_Click(object sender, EventArgs e)
 {
     frmMenu fm = new frmMenu();
     fm.Show();
     this.Close();
 }
 private void buttonX1_Click(object sender, EventArgs e)
 {
     frmMenu nb = new frmMenu();
     nb.Show();
     this.Hide();
 }