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