下面代码中

** <%

if (Session["memberlogin"] == null)

{

%>**

这里报错，提示上下文中不支持代码块，求各位大佬带飞

 <%@ Control Language="C#" AutoEventWireup="true" CodeFile="enter.ascx.cs" Inherits="enter" %>
<link href="images/style.css" rel="Stylesheet" type="text/css" />
<style type="text/css">
    .style1
    {
        width: 221px;
    }
    .top5px
    {}
</style>
<table style="text-align:center; width: 781px; height:200px;" border="0" cellpadding="0" cellspacing="0">
 <tr>
   <td valign="bottom"><h2>注册、登陆和验证模块</h2></td>
 </tr>
</table> 
<table style="text-align:center" border="0" cellpadding="0" cellspacing="0" width="780" class="login bottom6px" runat="server">
  <tr>
# **  <%
#       if (Session["memberlogin"] == null)
#       {    
#   %>**
      <td>
      用户名：<asp:TextBox ID="TextBox_usertext" CssClass="input" runat="server"></asp:TextBox>
      密  码：<asp:TextBox ID="TextBox_userpass" CssClass="input" runat="server" TextMode="Password"></asp:TextBox>
      <asp:ImageButton ID="ImageButton" ImageUrl="~/images/enter.jpg" CssClass="top5px" 
              runat="server" OnClick="ImageButton_Click1" Height="16px" Width="56px" />
      <asp:ImageButton ID="ImageButton2" ImageUrl="~/images/login.jpg" CssClass="top5px" 
              runat="server" OnClick="ImageButton2_Click1" Height="16px" Width="59px" />
      </td>
  <%
      }
      else
      {    
  %>
      <td class="style1">
      欢迎<font color="#FF0000"><%=Session["membername"]%></font>光临
      <a href="" target="_blank">后台管理</a>
      <a href="logout.aspx">注册登录</a>
      </td>
  <%
      }
  %>
  </tr>
</table>

不能在tr标签里插入服务器端代码。这不是asp，你必须在服务器控件里才能用