局部刷新&二级联动下拉菜单

前台：

 <td align="left">
       <asp:UpdatePanel ID="UpdatePanel4" runat="server">
        <ContentTemplate>
               <asp:DropDownList ID="ddlFanwei" runat="server" OnSelectedIndexChanged="ddlFanwei_SelectedIndexChanged"
                        AutoPostBack="true">
               </asp:DropDownList>
                         至
               <asp:DropDownList ID="ddlLevel" runat="server">
               </asp:DropDownList>
        </ContentTemplate>
      </asp:UpdatePanel>
  </td>

后台：

 protected void ddlLevel_Bind()
    {
        string SqlString = "select * from Event_level ";
        DataTable dt = DataAccess.SelectBySql("SOC2.ConnectionString.sql", SqlString);
        DataRow dr = dt.NewRow();
        ddlLevel.DataSource = dt;
        ddlLevel.DataValueField = "code";
        ddlLevel.DataTextField = "Level";
        ddlFanwei.DataSource = dt;
        ddlFanwei.DataValueField = "code";
        ddlFanwei.DataTextField = "Level";
        ddlFanwei.DataBind();
        ddlLevel.DataBind();
    }

    protected void ddlFanwei_SelectedIndexChanged(object sender, EventArgs e)
    {
        string sql = "select * from Event_level where code > =  '" + ddlFanwei.SelectedValue +"'";
        DataTable dt2 = DataAccess.SelectBySql("SOC2.ConnectionString.sql",sql);
        DataRow dr2 = dt2.NewRow();
        ddlLevel.DataSource = dt2;
        ddlLevel.DataValueField = "code";
        ddlLevel.DataTextField = "Level";
        ddlLevel.DataBind();
    }