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"> <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在他的另一篇文章里给出了一张图片,显示出了运行时母版页与内容页合并后的页面上各个控件的关系,我冒昧地贴在这里了:
原文:
http://odetocode.com/Blogs/scott/archive/2006/01/12/2726.aspx