有以下两个页面default.aspx和result.aspx，代码如下：
<!-- default.aspx -->
<%@ page language="c#" autoeventwireup="true" masterpagefile="~/default.master"  codefile="default.aspx.cs" inherits="_default" %>
<asp:content id="content1" runat="server" contentplaceholderid="contentplaceholder1">
    &nbsp;<asp:label id="label1" runat="server" text="please input a string here"></asp:label>
    <asp:textbox id="textbox1" runat="server"></asp:textbox>
    <asp:button id="button1" runat="server" text="button" postbackurl="~/result.aspx" /></asp:content>

//default.aspx.cs
using system;
using system.data;
using system.configuration;
using system.web;
using system.web.security;
using system.web.ui;
using system.web.ui.webcontrols;
//using system.web.ui.webcontrols.webparts;
using system.web.ui.htmlcontrols;

public partial class _default : system.web.ui.page
{
    protected void page_load(object sender, eventargs e)
    {
    }
}
 

<!-- result.aspx -->
<%@ page language="c#" masterpagefile="~/default.master" autoeventwireup="true" codefile="result.aspx.cs" inherits="result" title="untitled page" %>
<asp:content id="content1" contentplaceholderid="contentplaceholder1" runat="server">
    <asp:label id="label1" runat="server" text="the string you input in the previous page is"></asp:label>
    <asp:textbox id="textbox1" runat="server"></asp:textbox>
</asp:content>
//result.aspx.cs
using system;
using system.data;
using system.configuration;
using system.collections;
using system.web;
using system.web.security;
using system.web.ui;
using system.web.ui.webcontrols;
using system.web.ui.webcontrols.webparts;
using system.web.ui.htmlcontrols;

public partial class result : system.web.ui.page
{
    protected void page_load(object sender, eventargs e)
    {
        if (previouspage != null)
        {
            textbox tb = (textbox)previouspage.findcontrol("textbox1");
            if (tb != null)
                textbox1.text = tb.text;
        }
    }
}
 

这两个页面都指定了masterpagefile属性。因为该masterpage中的内容无关紧要，就不列出来了。在default.aspx上有两个控件：textbox1用于接受用户的输入，button1用于提交页面，其postbackurl指向result.aspx。在result.aspx.cs的page_load方法中尝试在textbox1中显示用户在前一页面的textbox1中输入的字符串。当执行以下语句时：

textbox tb = (textbox)previouspage.findcontrol("textbox1");

tb的值为null。将以上语句更改为如下代码：

content con = (content)previouspage.findcontrol("content1");
if (con == null)
    return;

textbox tb = (textbox)con.findcontrol("textbox1");

但con的值为null，这样后续的语句也不可能执行了。问题出在哪里呢？

经过一番搜索，在forums.asp.net中找到了答案，以下引用的是bitmask的说法：
...becasue the content controls themselves dissapear after the master page rearranges the page. you can use the contentplaceholders, or the <form> on the masterpage if there are no inamingcontainers between the form and the control you need.

所以以上的代码应该改成：

textbox tb = (textbox)previouspage.master.findcontrol("contentplaceholder1").findcontrol("textbox1");

bitmask还在他的博客上写了一篇文章来阐述findcontrol方法和inamingcontainers接口：
http://www.odetocode.com/articles/116.aspx

http://movingboy.cnblogs.com/archive/2006/07/06/444690.html