Request.QueryString[""]取不到值

<%@ Page Title="" Language="C#" MasterPageFile="~/common.master" AutoEventWireup="true" CodeFile="List.aspx.cs" Inherits="List" %>

<%@ Register Src="control/NewsCategory.ascx" TagName="NewsCategory" TagPrefix="uc1" %>

/asp:Content

<uc1:NewsCategory ID="NewsCategory2" runat="server" />

<div id="newslist" class="commonfrm">
    <h4>
        <asp:Label ID="lblCaName" runat="server" Text=""></asp:Label>
    </h4>
    <asp:GridView ID="gvNews" runat="server" AutoGenerateColumns="False"
        BorderWidth="0" GridLines="None" >
        <Columns>
            <asp:TemplateField HeaderText="所属类别" HeaderStyle-CssClass="th_category">
                <ItemTemplate>
                    <a class="td_category" href='List.aspx?categoryId=<%#Eval("categoryID") %>'>[<%# Eval("categoryName")%>]</a>
                </ItemTemplate>
                <HeaderStyle CssClass="th_category"></HeaderStyle>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="新闻标题">
                <ItemTemplate>
                    <a href='newscontent.aspx?newsId=<%#Eval("newsID") %>' target="_blank" title='<%# Eval("newsTitle") %>'><%# stringTruncat( Eval("newsTitle").ToString(), 18 , "...") %></a>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="发布时间" HeaderStyle-CssClass="th_time" ItemStyle-CssClass="td_time">
                <ItemTemplate>
                    <asp:Label ID="Label3" runat="server" Text='<%# Bind("newsCreateTime") %>'></asp:Label>
                </ItemTemplate>

                <HeaderStyle CssClass="th_time"></HeaderStyle>

                <ItemStyle CssClass="td_time"></ItemStyle>
            </asp:TemplateField>
        </Columns>

    </asp:GridView>
</div>

/asp:Content

需要获得“所属类别”的categroyId,但没能获得。请高手帮我诊断诊断！！