登录窗口一些小技巧应用，防御性编程以及加密

实例：

private void LoginBtn_Click(object sender, EventArgs e)
        {
            //判断用户名和密码输入情况
            if (UserName.Text.Length <= 0)
            {
                MessageBox.Show("用户名为空，请输入用户名！");
                return;
            }
            if (txtPassword.Text.Length <= 0)
            {
                MessageBox.Show("密码不能为空！");
                return;
            }

            try
            {
                //string conn = dbconfig.readxml();方法一
                string conn = configxml.xmlconn.readxml();
                //使用xml读取配置进行数据库连接替代20151023--(使用app.config默认配置程序连接数据库配置)

                //string conn = ConfigurationManager.ConnectionStrings["connstr"].ConnectionString;
                //使用db.config就是上面这个方法来写，使用ConfigurationManager但要先引用System.configuration;方法二
                Helpers.SqlHelper db = new Helpers.SqlHelper(conn);
                DataTable dt = db.ExecuteDataTable("select Id,username,password,ErrorTimes,sign_time from T_users where username = @userName", new SqlParameter("@username", UserName.Text));

                //防御性编程，判断过滤条件
                if (dt.Rows.Count <= 0)
                {
                    MessageBox.Show("输入的用户名不存在或密码错误，请重新输入！");
                    return;
                }

                if (dt.Rows.Count > 1)
                {
                    MessageBox.Show("用户名重复！");
                    return;
                }
                //读取行记录
                DataRow row = dt.Rows[0];
                //显性转换类型
                string dbpassword = (string)row["password"];
                long Id = (long)row["Id"];
                int ErrorTimes = (int)row["ErrorTimes"];
                DateTime dbtime = (DateTime)row["sign_time"];

                //用Timespan函数计算间隔时间差
                DateTime TxtTime = DateTime.Parse(DateTime.Now.ToString());
                TimeSpan timespan = TxtTime - dbtime;

                if (ErrorTimes >= 5)
                {
                    if (timespan.TotalMinutes > 5) //如果超过5分钟即解除锁定
                    {
                        db.ExecuteNonQuery("update T_users set ErrorTimes = 0 where Id = @Id", new SqlParameter("@Id", Id));
                    }
                    else
                    {
                        MessageBox.Show("因密码输错次数过多，该账号已经被禁止登录，请与管理员进行联系或稍候再试!");
                    }
                }
                else
                {
                    if (dbpassword != Encrypt.GetMD5_32(txtPassword.Text))
                    {
                        {
                            db.ExecuteNonQuery("update T_users set ErrorTimes = ErrorTimes+1,sign_time = '" + DateTime.Now + "' where Id = @Id", new SqlParameter("@ID", Id));
                            MessageBox.Show("密码错误，请重新输入!");
                            txtPassword.Clear();//输入错误自动清空输入框
                            return;
                        }
                    }
                    else
                    {
                        db.ExecuteNonQuery("update T_users set ErrorTimes = 0 where Id = @Id", new SqlParameter("@Id", Id));
                        GlobalInfo.LoginUser = UserName.Text.ToString();
                        MISMain d = new MISMain();
                        d.Show();
                        this.Hide();
                    }
                }
            }

            catch
            {
                ServerConfig sc = new ServerConfig();
                sc.Show();
            }
        }