如何在更新面板中获取javascript中的下拉列表的值

我只在ddlCountry的事件上绑定ddlstate selectedindechanged。 ddlState javascript 中的下拉值检查，如果它是"-1"，然后生成警报。当我在javascript中检查值时，它向我显示一个空白值"。当我使用 Postbacktrigger 表示 ddlState 时，我可以用 javascript 获取值，但使用 async 的页面平滑度比 Postbacktrigger 更好。这就是我使用异步触发器的原因。我的主要问题是ddlState当我使用异步触发器时，它的值没有进入 javascript，而我可以使用回发触发器获取它。

JavaScript 验证：

function validateForm()
{
    var ddlCountry = document.getElementById('<%=ddlCountry.ClientID%>');
    var ddlState = document.getElementById('<%=ddlState.ClientID%>');
    if (ddlCountry .value == "-1")
    {
        alert("Country  should not be blank.");
        ddlCountry .focus();
        return false;
    }
    if (ddlState .value == "-1")
    {
        alert("State should not be blank.");
        ddlState .focus();
        return false;
    }
    return true;
}

ASPX 代码：

<asp:DropDownList ID="ddlAcqModalityList" runat="server" CssClass="csstextbox" Width="207px" AutoPostBack="true" OnSelectedIndexChanged="ddlAcqModalityList_SelectedIndexChanged">
</asp:DropDownList>
<asp:UpdatePanel ID="updatePanelState" runat="server">
    <ContentTemplate>
        <asp:DropDownList ID="ddlState " runat="server" CssClass="csstextbox" Width="177px">
        </asp:DropDownList>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlCountry" EventName="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>
<asp:Button ID="btnSave" runat="server" Width="80px" OnClientClick="return validateForm();" Text="Save" CssClass="cssbutton" OnClick="btnSave_Click" />

代码隐藏：

protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostBack)
    {
        BindCountry()
        {
            strSQL = @"SELECT Country_ID,Country_Desc
                       FROM Country_Master";
            DataTable dataTableState = null;
            dataTableState = objSqlDbComm.ExecuteDatasetQuery(strSQL).Tables[0];
            var dictioneryCountry = new Dictionary<int, string>();
            foreach(DataRow dr in dataTableStudy.Rows)
            {
                dictioneryCountry .Add(Convert.ToInt32(dr["Country_ID"]), dr["Country_Desc"].ToString());
            }
            ddlCountry.DataTextField = "Value";
            ddlCountry.DataValueField = "Key";
            ddlCountry.DataSource = dictioneryCountry;
            ddlCountry.DataBind();
            ddlCountry.Items.Insert(0, new ListItem("[Select]", "-1"));
            ddlCountry.Items[0].Selected = true;
        }
    }
}
protected void ddlCountry_SelectedIndexChanged(object sender, EventArgs e)
{
    int countryID = Convert.ToInt32(ddlCountry.SelectedItem.Value);
    ifcountryID == -1)
    {
        return;
    }
    strSQL = @"SELECT State_ID,State_Desc
               FROM State_Master
               WHERE countryID = '" + countryID + @"';
    DataTable dataTableState = null;
    dataTableState = objSqlDbComm.ExecuteDatasetQuery(strSQL).Tables[0];
    var dictioneryState = new Dictionary<int, string>();
    foreach(DataRow dr in dataTableStudy.Rows)
    {
        dictioneryState .Add(Convert.ToInt32(dr["State_ID"]), dr["State_Desc"].ToString());
    }
    ddlState.DataTextField = "Value";
    ddlState.DataValueField = "Key";
    ddlState.DataSource = dictioneryState;
    ddlState.DataBind();
    ddlState.Items.Insert(0, new ListItem("[Select]", "-1"));
    ddlState.Items[0].Selected = true;
}