Check Box 复选框怎样取到勾选的值
<asp:DataList ID="Show_Admin" Width="600px" align="center" runat="server" > 
  <HeaderTemplate> 
  <table border="0" cellpadding="0" cellspacing="1" bgcolor="#B0C1F2"> 
  <tr> 
  <td width="59" height="25" align="center" bgcolor="#FFFFFF"> <strong>序号 </strong> </td> 
  <td width="147" align="center" bgcolor="#FFFFFF"> <strong>用户名 </strong> </td> 
  <td width="129" align="center" bgcolor="#FFFFFF"> <strong>级别 </strong> </td> 
  <td width="129" align="center" bgcolor="#FFFFFF"> <strong>修改 </strong> </td> 
  <td width="130" align="center" bgcolor="#FFFFFF"> <strong>删除 </strong> </td> 
  </tr> 
  </HeaderTemplate> 
  <ItemTemplate> 
  <tr> 
  <td width="59" height="25" align="center" bgcolor="#FFFFFF"> <%#(AspNetPager1.CurrentPageIndex - 1) * AspNetPager1.PageSize+(Container.ItemIndex + 1)%> </td> 
  <td width="147" align="center" bgcolor="#FFFFFF"> <%#Eval("admin") %> </td> 
  <td width="129" align="center" bgcolor="#FFFFFF"> <%#Eval("isadmin") %> </td> 
  <td width="129" align="center" bgcolor="#FFFFFF"> <a href="Modify_Admin.aspx?id= <%#Eval("id")%>">修改 </a> </td> 
  <td width="130" align="center" bgcolor="#FFFFFF"> <%#Eval("id") %> <input type="checkbox" id="chkItem" value= <%#Eval("id")%>> </td> 
  </tr> 
  </ItemTemplate> 
  <AlternatingItemTemplate> 
  <tr> 
  <td width="59" height="25" align="center" bgcolor="#EFEFEF"> <%#(AspNetPager1.CurrentPageIndex - 1) * AspNetPager1.PageSize+(Container.ItemIndex + 1)%> </td> 
  <td width="147" align="center" bgcolor="#EFEFEF"> <%#Eval("admin") %> </td> 
  <td width="129" align="center" bgcolor="#EFEFEF"> <%#Eval("isadmin") %> </td> 
  <td width="129" align="center" bgcolor="#EFEFEF"> <a href="Modify_Admin.aspx?id= <%#Eval("id")%>">修改 </a> </td> 
  <td width="130" align="center" bgcolor="#EFEFEF" > </td> 
  </tr> 
  </AlternatingItemTemplate> 
  <FooterTemplate> 
  <tr> 
  <td height="25" colspan="5" bgcolor="#FFFFFF" align="center"> <asp:Button ID="Button1" runat="server" Text="删除所选中的" OnClick="Delet_Admin" /> <asp:CheckBox ID="chkHeader"