Dropdownlist(ddlState) 值丢失时，在更新面板中的 AsyncPostBackTrigger 之后在

我有ddlCountry下拉列表和ddlState下拉列表。 ddlState 位于更新面板中。在选择国家时，我使用 ddlCountry 的选定索引更改事件填充了其州。我在绑定后将起始值"-1"添加到两个下拉列表中，如果下拉列表内容是 [选择] 值"-1"，我会在 Javascript 中检查，然后发出警报并返回 false。

但是验证适用于 ddlCountry，对于 ddlState，在 ajax 更新面板触发和验证失败后，其值变为"空白"。

我应该如何解决这些问题？我不知道为什么在AsyncPostBackTrigger之后ddlState值丢失。

  //javascript validation
        function validateForm()
                {
                    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 code
<asp:DropDownList ID="ddlCountry " runat="server" CssClass="csstextbox" Width="207px"
                                AutoPostBack="true"  OnSelectedIndexChanged="ddlCountry _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>
            //Code Behind
    protected void Page_Load(object sender, EventArgs e)
        {
               if(!IsPostBack)
            {
                    BindCountry();
                    }
            }
            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;          
                }

  <asp:DropDownList ID="ddlState " runat="server" CssClass="csstextbox" Width="177px" EnableViewState=true>                                                     </asp:DropDownList>