c# - SelectedIndex 始终为 0 启用回发，检查！回发似乎不起作用

Question

我正在与 DropDownList's 斗争SelectedIndex。它似乎总是以0. 我正在根据 linq 查询的结果填写下拉列表。从下拉列表中选择一个选项并点击返回Button1, Button1 whichSelectedIndex is always后0。我尝试if (!Page.IsPostback)在页面加载区域中添加，这将更SelectedIndex改为-1.. 下面的相关代码

<script runat="server">
    int dropDownCounter = 0;
    public string levelselected = null;

    void Level_Index_Changed(Object sender, EventArgs e) 
    {
        roledropdown.Controls.Clear();
        CheckBox chkList1;
        chkList1 = new CheckBox();
        chkList1.Text = (LevelOptions.SelectedIndex.ToString());
        chkList1.CssClass = "roleCheckBox";
        chkList1.ID = "xxxx";
        roledropdown.Controls.Add(chkList1);
        roledropdown.Controls.Add(new LiteralControl("<br>"));
        UpdatePanel.Update();
    }


    protected void Page_Load(object sender, EventArgs e)
    {
        AsyncScriptManager.RegisterAsyncPostBackControl(LevelOptions);
        AsyncScriptManager.RegisterAsyncPostBackControl(Button1);

        var levels = new List <SelectListItem>();

        foreach(var x in userRoleQuery)
        {
        //Add the levels to a dropdown menue
        ListItem gg = new ListItem();
        gg.Text = "Hello";
        gg.Value = (dropDownCounter.ToString());
        if (dropDownCounter == 0) 
        {
            gg.Selected = true;
            levelselected = (x.LevelId.ToString());
        }
        LevelOptions.Items.Add(gg);
    }
    dropDownCounter++;
}
</script>

<asp:DropDownList ID="LevelOptions" runat="server" AutoPostBack="True" OnSelectedIndexChanged="Level_Index_Changed">
</asp:DropDownList>
<asp:UpdatePanel ID="UpdatePanel" runat="server" UpdateMode="Conditional">
  <ContentTemplate>
    <asp:Panel ID="roledropdown" runat="server">
    </asp:Panel>
  </ContentTemplate>
  <Triggers>
    <asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click" />
  </Triggers>
</asp:UpdatePanel>
<asp:Button ID="Button1" runat="server" Text="Update Both Panels" OnClick="Level_Index_Changed" />

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