本文轉自： http://www.sqlsky.com/asp-net/070721/585/

有以下兩個頁面Default.aspx和Result.aspx，代碼如下：
<!--?Default.aspx?-->
<%@?Page?Language="C#"?AutoEventWireup="true"?MasterPageFile="~/Default.master"??CodeFile="Default.aspx.cs"?Inherits="_Default"?%>?
<asp:Content?ID="Content1"?runat="server"?ContentPlaceHolderID="ContentPlaceHolder1">
?????<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;
????????}
????}
}
?

這兩個頁面都指定了MasterPageFile屬性。因為該MasterPage中的內容無關緊要，就不列出來了。在Default.aspx上有兩個控件：TextBox1用于接受用戶的輸入，Button1用于提交頁面，其PostBackUrl指向Result.aspx。在Result.aspx.cs的Page_Load方法中嘗試在TextBox1中顯示用戶在前一頁面的TextBox1中輸入的字符串。當執行以下語句時：

TextBox?tb?=?(TextBox)PreviousPage.FindControl("TextBox1");

tb的值為null。將以上語句更改為如下代碼：

Content?con?=?(Content)PreviousPage.FindControl("Content1");
if?(con?==?null)
????return;

TextBox?tb?=?(TextBox)con.FindControl("TextBox1");

但con的值為null，這樣后續的語句也不可能執行了。問題出在哪里呢？

經過一番搜索，在forums.asp.net中找到了答案，以下引用的是bitmask的說法：
...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.?


所以以上的代碼應該改成：

TextBox?tb?=?(TextBox)PreviousPage.Master.FindControl("ContentPlaceHolder1").FindControl("TextBox1");

bitmask還在他的博客上寫了一篇文章來闡述FindControl方法和INamingContainers接口：
http://www.odetocode.com/Articles/116.aspx

在他的另一篇文章里給出了一張圖片，顯示出了運行時母版頁與內容頁合并后的頁面上各個控件的關系，我冒昧地貼在這里了：

原文： http://odetocode.com/Blogs/scott/archive/2006/01/12/2726.aspx ?

當PreviousPage.FindControl遇到MasterPage，如何查找控件