Repeater中CheckBox多选但是一直为false

 <asp:Button ID="btnDelete" runat="server" Text="删除所有" onclick="btnDelete_Click" /><asp:Repeater ID="repAlbum_List" runat="server">
    <ItemTemplate>
   
    <li>
     <div style="float:left">
     <asp:CheckBox ID="chkchild" name="chkchild" runat="server" Text="" />
   　　<a href="#"><%#Eval("title") %></a><br />　　　
     </div>
 </li>
    </ItemTemplate>
     </asp:Repeater>
   后台：
        protected void btnDelete_Click(object sender, EventArgs e)
        {
            MusicRules mr = new MusicRules();
            for (int i = 0; i < repAlbum_List.Items.Count; i++)
            {
                CheckBox chkchild = (CheckBox)(repAlbum_List.Items[i].FindControl("chkchild"));
                if (chkchild.Checked)
                {                    HiddenField hidid = (HiddenField)repAlbum_List.Items[i].FindControl("hidid");
                    if (mr.DeletePhoto(Convert.ToInt32(hidid.Value)))
                    {
                        int page = Request.QueryString["page"] == null ? 1 : Convert.ToInt32(Request.QueryString["page"]);
                        ScriptManager.RegisterClientScriptBlock(repAlbum_List, GetType(), "xxx", "alert('删除成功！');", true);
                        this.repAlbum_List.DataSource = pr.Search_Photo(this.txtAlbum.Text, page, pagesize, out sumcount);
                        this.repAlbum_List.DataBind();
                    }
                }
               
            }
        }当选中CheckBox时，它还是为Checked=false，求高手，求解释。