只有在true时才从UpdatePanel完全回发

Question

我正在尝试创建一个登录系统，如果用户键入默认密码，它将提示更新密码。

控件都在 UpdatePanel 中，我可以通过在onLoad上使用 UpdatePanel 来强制完全回发控件：

ScriptManager.GetCurrent(Me).RegisterPostBackControl(btnClose)

但是，我有另一个按钮是 btnLogin ，这将检查键入的密码是否为默认密码。如果不是，那么我将做一个完整的回发。以下是我的设计师代码：

<div id="aboutModal" class="modal fade" tabindex="-1" role="dialog" aria-hidden="true" data-backdrop="static" data-keyboard="false">
    <div class="modal-dialog">
    <div class="modal-content">
        <div class="modal-body">
                            <h3>Sign in with your username and password</h3>
                            <br/>
                            <p class="text-justify">
                                <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
                                    <ContentTemplate>
                                    <asp:TextBox ID="txtUsername" runat="server" class="form-control" placeholder="Username"/> 
                                     <br />         
                                    <asp:TextBox ID="txtPassword" runat="server" class="form-control" placeholder="Password"  textmode="Password" />
                                    <br />
                                    <asp:TextBox ID="txtNewPassword" runat="server" class="form-control" placeholder="Update Password"  textmode="Password" Visible="false"/>
                                        <br />
                                        <asp:Button ID="btnLogin" runat="server" class="btn btn-primary" Text="Sign in" onclick="btnLogin_Click" /> 
                                        &nbsp;&nbsp;&nbsp;
                                        <asp:Button ID="btnClose" runat="server" Text="Close" class="btn btn-primary"  Visible ="true" OnClick="btnClose_Click"/>
                                    </ContentTemplate>
                                </asp:UpdatePanel>

                            </p>     
            <br/>

        </div>
    </div>
    </div>
</div>

这是我背后的代码：

 If dbPwd = "pwd123" Then

                    txtNewPassword.Visible = True
                    txtUsername.Text = username
                    txtPassword.Text = pwd


                Else

                    Dim signIn As Control = FindControl("signIn")
                    Dim logOut As Control = FindControl("logOut")

                    signIn.Visible = False
                    logOut.Visible = True

                    Session("loginDetails") = dbUsername & "," & dbGroup & "," & dbModify & "," & dbRead & "," & dbWrite
                    'MsgBox(Session("loginDetails"))
                    txtPassword.Text = ""
                    txtUsername.Text = ""
                    dbUsername = ""
                    dbPwd = ""

                    btnLogin.Visible = True

                End If

问题是如何有条件地将其注册为完全回发的控件？