asp.net 后台cs代码怎么操做前台用ajax实现下拉框进行搜索

html页面的两个下拉类别框是联动的，第一个下拉框是后台绑定的，第二个下拉框是用ajax来无刷新动态绑定的，在作查询页面时，选择第一个下拉框以后，第二个下拉框也发生变化，点击搜索以后，第二个下拉类别框没有数据了，

<asp:DropDownList ID="dropTownship" runat="server"></asp:DropDownList>

<asp:DropDownList ID="dropVillage" runat="server" Width="230px">
                    <asp:ListItem Value="">---请选择---</asp:ListItem>
</asp:DropDownList>

查询代码以下

protected void btnSearch_Click(object sender, EventArgs e)
    {
        string strdropTownship = dropTownship.SelectedValue.Trim();
        string strName = txtName.Text.Trim();
        string strdropVillage = dropVillage.SelectedValue.Trim();
        string strWhere = "township=" + Common.UrlEncode(strdropTownship) +
            "&name=" + Common.UrlEncode(strName) +
            "&village=" + Common.UrlEncode(strdropVillage) ;
        Response.Redirect("VillageInforEnrollList.aspx?" + strWhere);
    }

上面的代码中dropVillage.SelectedValue.Trim();读取到永远为空，这是为何呢，由于它的数据不是后台绑定，而是前台经过ajax来实现的，这时要经过Request.Form["dropVillage"].ToString();方式来获取dropVillage下拉列表框的值

Response.Redirect("VillageInforEnrollList.aspx?" + strWhere);这句会从新加载VillageInforEnrollList，执行PageLoad事件

这时参数里含township和village这个两个参数而后经过这两个参数来后台绑定dropVillage，这样查询时dropVillage能够保留原先搜索的值而不是置空。代码以下

if (Request.QueryString["township"] != null && Request.QueryString["township"].ToString().Length > 0)
                {
                    DataTable dtVillage = new SysAreaMg().GetList(" flag='5' and PCode='" + Common.GetEncodeQueryString("township") + "' ").Tables[0];
                    Common.DataBind("AreaName", "AreaCode", dtVillage, dropVillage);//绑定dropVillage
                    if (Request.QueryString["village"] != null && Request.QueryString["village"].ToString().Length > 0)
                    {

                       //绑定dropVillage下拉框默认值
                        dropVillage.Items.FindByValue(Common.GetEncodeQueryString("village")).Selected = true;
                    }
                    else
                    {
                   
                    }
                }

这里来经过先后台来肯定dropVillage下拉框的值