protected void btnAdd_Click(object sender, EventArgs e)
    {
        // retrieve user info
        UserEntity user = new UserEntity(txtUserName.Text);

        using (DataAccessAdapter adapter = new DataAccessAdapter())
        {
            adapter.FetchEntity(user);
        }

        // user exist
        if (!user.IsNew)
        {
            // check for valid password
            if (user.Password == txtPassword.Text)
            {
                SessionHelper.AddUserObject(user);
                Response.Redirect("Default.aspx");
            }

            // wrong password!
            else
            {
                this.lblLoginValidate.Visible = true;
            }
        }

        // user doesn't exist!
        else
        {
            lblLoginValidate.Visible = true;
            txtUserName.Text         = string.Empty;
            txtPassword.Text         = string.Empty;
        }
    }