问无法使用vb.net访问asp.net中选项卡容器中动态创建的控件

EN

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" CombineScripts="true">
</asp:ToolkitScriptManager>
<div>
   <asp:UpdatePanel ID="UpdTabContainer" ChildrenAsTriggers="false" UpdateMode="Conditional" runat="server">
    <ContentTemplate>
   <asp:TabContainer ID="TabContainer1" runat="server" AutoPostBack="true">
        <asp:TabPanel ID="TabUserList" runat="server" HeaderText="UserList">
            <ContentTemplate>
                <asp:UpdatePanel ID="UpdUserList" runat="server" UpdateMode="Conditional">
                    <ContentTemplate>
                        <asp:ListBox ID="ListBox1" SelectionMode="Single" AutoPostBack="true" OnSelectedIndexChanged="UserChanged" runat="server">
                            <asp:ListItem Text="User 1" Value="1"></asp:ListItem>
                            <asp:ListItem Text="User 2" Value="2"></asp:ListItem>
                            <asp:ListItem Text="User 3" Value="3"></asp:ListItem>
                        </asp:ListBox>
                    </ContentTemplate>
                    <Triggers>
                        <asp:AsyncPostBackTrigger ControlID="TabContainer1" EventName="ActiveTabChanged" />
                    </Triggers>
                </asp:UpdatePanel>
            </ContentTemplate>
        </asp:TabPanel>
    </asp:TabContainer>
    </ContentTemplate> 
    </asp:UpdatePanel>
</div>

Public Class TabContainerSample
    Inherits System.Web.UI.Page

    Property CreatedTabIDs As List(Of Int32)
        Get
            If Session("CreatedTabIDs") Is Nothing Then
                Session("CreatedTabIDs") = New List(Of Int32)
            End If
            Return DirectCast(Session("CreatedTabIDs"), List(Of Int32))
        End Get
        Set(value As List(Of Int32))
            Session("CreatedTabIDs") = value
        End Set
    End Property

    Private Sub TabContainerSample_Init(sender As Object, e As System.EventArgs) Handles Me.Init
        For Each userID In CreatedTabIDs
            AddTab(userID)
        Next
    End Sub

    Private Sub AddTab(tabID As Int32)
        Dim ucLogin = DirectCast(Page.LoadControl("LoginControl.ascx"), LoginControl)
        ucLogin.UserID = tabID
        AddHandler ucLogin.LoggedIn, AddressOf userLoggedIn
        Dim newTabPanel = New AjaxControlToolkit.TabPanel
        newTabPanel.ID = String.Format("Tab{0}", tabID)
        newTabPanel.HeaderText = String.Format("TabPanel {0}", tabID)
        newTabPanel.Controls.Add(ucLogin)
        TabContainer1.Tabs.Add(newTabPanel)
    End Sub

    Protected Sub UserChanged(sender As Object, e As EventArgs)
        Dim tabID = Int32.Parse(DirectCast(sender, ListBox).SelectedValue)
        If Not CreatedTabIDs.Contains(tabID) Then
            AddTab(tabID)
            CreatedTabIDs.Add(tabID)
            Me.UpdTabContainer.Update()
        End If
    End Sub

    Private Sub userLoggedIn(login As LoginControl)
        ' do something '
    End Sub

End Class

<%@ Control Language="vb" AutoEventWireup="false" CodeBehind="LoginControl.ascx.vb" Inherits="WebApplication1.LoginControl" %>
<asp:UpdatePanel ID="UpdLoginControl" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
    User-ID: <asp:Label ID="LblUserID" runat="server" Text="0"></asp:Label>
    <br /><asp:TextBox ID="TxtUserName" runat="server"></asp:TextBox>
    <br /><asp:TextBox ID="TxtPassord" runat="server"></asp:TextBox>
    <br /><asp:Button ID="BtnLogin" runat="server" Text="Login" />
    </ContentTemplate>
</asp:UpdatePanel>

Public Class LoginControl
    Inherits System.Web.UI.UserControl

    Public Event LoggedIn(sender As LoginControl)

    Public Property UserID As Int32
        Get
            Return Int32.Parse(Me.LblUserID.Text)
        End Get
        Set(value As Int32)
            Me.LblUserID.Text = value.ToString
        End Set
    End Property

    Private Sub BtnLogin_Click(sender As Object, e As System.EventArgs) Handles BtnLogin.Click
        Dim userName = TxtUserName.Text
        Dim password = TxtPassord.Text
        ' validate UserName and Password, if valid you can raise your custom LoggedIn-Event'
        Dim loggedIn As Boolean = True
        If loggedIn Then
            RaiseEvent LoggedIn(Me)
        End If
        Me.UpdLoginControl.Update()
    End Sub
End Class