教教我做登录验证吧

这是我登录部分：

public static SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=qq520;Integrated Security=True");
    private void button1_Click(object sender, EventArgs e)
    {

    if (cboId.Text.Trim() == "<请输入帐号>")
    {
    cboId.Text = "";
    }
    string id = cboId.Text.Trim();
    string pwd = txtPwd.Text.Trim();
    if (id == "" || pwd == "")
    {
    MessageBox.Show("请输入QQ号及密码！", "登陆提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
    return;
    }
    string sql = string.Format("select loginPwd from users where id={0}", id);
    try
    {
    object obj = DBHelper.GetFristCells(sql);
    if (obj == null)
    {
    MessageBox.Show("该QQ号不存在,如果你还没有注册，请点申请号码！");
    return;
    }
    if (obj.ToString().Equals(pwd))
    {
    UserHelp.loginId = Convert.ToInt32(id);
    this.Hide();
    LoginForm2 lf2 = new LoginForm2();
    lf2.Show();
    }
    else
    {
    LoginForm1 lg1 = new LoginForm1();
    lg1.Show();
    }
    if (toolStripStatusLabel1.Image != imageList1.Images[1])
    {
    string sql1 = string.Format("update users set userstate='在线' where id={0}", cboId.Text.Trim());
    int a = DBHelper.ExecuteUpdate(sql1);
    }
    else
    {
    string sql2 = string.Format("update users set userState='隐身' where id={0}", UserHelp.loginId);
    int a = DBHelper.ExecuteUpdate(sql2);
    }

    }
    catch (Exception)
    {

    throw;
    }
    }