Global.asax中的Session_End不能用Context

遇到一个问题：Web.Config中设置的Sestion timeout不起作用，设置的timeout时间到期后，Global.asax中的Session_End能够被调用，但在此事件处理函数中，Context总是为null，这样就没有办法利用Context退出登录。
原因如下：

引用

The Session_End event fires when the browser isn't connected to the server -- so there really isn't anyway to clear out the authentication ticket (since that is stored on the client).

What I'd recommend doing instead is changing the timeout duration of the forms-authentication ticket to be 15 minutes.  That way you can avoid having to add a session_end event at all -- and just rely on the browser cookie timing out to force another login.

Q: Do I have a valid HttpContext in Session_End?
A: No, because this event is not associated with any request.

推荐用forms-authentication timeout，在Web.Config中配置如下：

<system.web>
    <authentication mode="Forms">
          <forms timeout="50000000"/>
    </authentication>
</system.web>