DropDownList 选择新闻类别，点listView下一页时怎么又类别错乱了呢

 <table id="t1" runat="server"  border="0" style="text-align:center;width:98%;" cellpadding="4" cellspacing="1" >
      <tr><td colspan="2">
         <asp:ListView ID="LvNews" runat="server" DataKeyNames="Id" >
                <ItemTemplate>
                     <tr >
                    
                    <td align="center">
                        <%#Eval("NewsType") %>
                    </td>
                    <td align="center">
                        <font color="">
                            <%#Eval("NewsTitle") %></font>
                    </td>
                    <td align="center">
                        <%#Eval("NewsDatetime")%>
                    </td>
                   
                    <td align="center">
                        <a href="NewsEdit.aspx?id=<%#Eval("Id") %>&cat=<%#Eval("NewsCat") %>" title="点击修改该新闻">修改</a>
                    </td>
                    <td>
                        <asp:CheckBox ID="checkgoods" runat="server" Text='<%#Eval("Id") %>' /> 
                    </td>
                </tr>
                </ItemTemplate>
              
                <EmptyDataTemplate>
                    <table id="Table1" runat="server" style="">
                        <tr>
                            <td>
                               没有数据 </td>
                        </tr>
                    </table>
                </EmptyDataTemplate>
              
                <LayoutTemplate>
                    <table id="Table2" runat="server">
                        <tr id="Tr1" runat="server">
                            <td id="Td1" runat="server">
                                <table ID="itemPlaceholderContainer" runat="server" border="0" >
                                     <tr class="thead">
                                    <td  style="width:100px;" >
                                        所属栏目
                                    </td>
                                    <td  style="width:230px;">
                                        标题
                                    </td>
                                    <td style="width:110px;" >
                                        添加日期
                                    </td>
                                    
                                    <td  style="width:80px;" >
                                        修改
                                    </td>
                                    <td style="width:80px">
                                        删除
                                    </td>
                                </tr>
                                <tr ID="itemPlaceholder" runat="server">
                                    </tr>
                                </table>
                            </td>
                        </tr>
                        
                    </table>
                </LayoutTemplate>
               
            </asp:ListView>
           </td> 
         </tr>
         <tr >
            <td align="center">
                <asp:DataPager ID="DataPager1" runat="server" PagedControlID="LvNews" 
                    QueryStringField="pagenum"  
                    PageSize="6" onprerender="DataPager1_PreRender" >
                    <Fields>
                        <asp:NextPreviousPagerField FirstPageText="首页" ShowFirstPageButton="True" 
                            ShowNextPageButton="False" />
                        <asp:NumericPagerField />
                        <asp:NextPreviousPagerField LastPageText="末页" ShowLastPageButton="True" 
                            ShowPreviousPageButton="False" />
                        <asp:TemplatePagerField>
                            <PagerTemplate>
                                <input type="text" id="goPagenum" style="width:30px;height:18px;"/><input type="button" id="goBtn" value="跳转" />
                            </PagerTemplate>
                        </asp:TemplatePagerField>
                    </Fields>
                </asp:DataPager>
                </td>
             <td>
                 <asp:CheckBox ID="CheckBox1" runat="server" />
                 <asp:Button ID="BtnCheck1" runat="server" Text="全选" />
                 <asp:Button ID="BtnDelete1" runat="server" Text="删除"  />
            </td>
        </tr>       
    </table>
后台代码 public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Bind();
            }
        }        protected void Ddlsearch_SelectedIndexChanged(object sender, EventArgs e)
        {
            Bind();
        }
        protected void Bind()
        {
            if (!IsPostBack)
            {
                try
                {
                    string sql = "select * from T_News  order  by NewsDatetime desc";
                    LvNews.DataSource = 韩蒸天下.App_Code.Model.News.GetSet(sql);
                    LvNews.DataBind();
                }
                catch (Exception er)
                {
                    throw new Exception(er.Message);
                }
            }
            else
            {
                if (Ddlsearch.SelectedValue != "所有新闻")
                {
                    string sql = "select * from T_News where NewsType='" + Ddlsearch.SelectedValue + "' order by NewsDatetime desc";
                    LvNews.DataSource = 韩蒸天下.App_Code.Model.News.GetSet(sql);
                    LvNews.DataBind();
                }
                else
                {
                    string sql = "select * from T_News  order  by NewsDatetime desc";
                    LvNews.DataSource = 韩蒸天下.App_Code.Model.News.GetSet(sql);
                    LvNews.DataBind();
                }
            }
        }        protected void DataPager1_PreRender(object sender, EventArgs e)
        {
            LvNews.DataBind();        }