C＃，FindControl已

Question

我很抱歉，但我不明白爲什麼這是行不通的。編譯後，我收到一個「空引用異常」。請幫忙。

public partial class labs_test : System.Web.UI.Page 
{ 
    protected void Button1_Click(object sender, EventArgs e) 
    { 
     if (TextBox1.Text != "") 
     { 
      Label Label1 = (Label)Master.FindControl("Label1"); 
      Label1.Text = "<b>The text you entered was: " + TextBox1.Text + ".</b>"; 
     } 
    } 

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) 
    { 
     Label Label1 = (Label)Master.FindControl("Label1"); 
     Label1.Text = "<b>You chose <u>" + DropDownList1.SelectedValue + "</u> from the dropdown menu.</b>"; 
    } 
} 

和UI：

<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="test.aspx.cs" Inherits="labs_test" Title="Untitled Page" %> 

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server"> 
</asp:Content> 
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server"> 
Type in text and then click button to display text in a Label that is in the MasterPage.<br /> 
This is done using FindControl.<br /> 
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox> 
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Submit" /><br /> 
<br /> 
Choose an item from the below list and it will be displayed in the Label that is 
in the MasterPage.<br /> 
This is done using FindControl.<br /> 
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"> 
<asp:ListItem>Item 1</asp:ListItem> 
<asp:ListItem>Item 2</asp:ListItem> 
<asp:ListItem>Item 3</asp:ListItem> 
</asp:DropDownList> 
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>  
</asp:Content> 

Answer 1

致謝Mr. Atwood himself，這裏是遞歸版本的方法。我還建議在控制測試爲空，我包括如何更改代碼來做到這一點爲好。

protected void Button1_Click(object sender, EventArgs e) 
{ 
    if (TextBox1.Text != "") 
    { 
     Label Label1 = FindControlRecursive(Page, "Label1") as Label; 
     if(Label1 != null) 
      Label1.Text = "<b>The text you entered was: " + TextBox1.Text + ".</b>"; 
    } 
} 

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) 
{ 
    Label Label1 = FindControlRecursive(Page, "Label1") as Label; 
    if (Label1 != null) 
     Label1.Text = "<b>You chose <u>" + DropDownList1.SelectedValue + "</u> from the dropdown menu.</b>"; 
} 

private Control FindControlRecursive(Control root, string id) 
{ 
    if (root.ID == id) return root; 
    foreach (Control c in root.Controls) 
    { 
     Control t = FindControlRecursive(c, id); 
     if (t != null) return t; 
    } 
    return null; 
} 

Answer 2

FindControl只搜索在不久的孩子（技術上下一NamingContainer），而不是整個控件樹。由於Label1不是Master的直接子，Master.FindControl不會找到它。取而代之的是，你要麼需要做FindControl於眼前的父控件，或者做一個遞歸控制搜索：

private Control FindControlRecursive(Control ctrl, string id) 
{ 
    if(ctrl.ID == id) 
    { 
     return ctrl; 
    } 
    foreach (Control child in ctrl.Controls) 
    { 
     Control t = FindControlRecursive(child, id); 
     if (t != null) 
     { 
      return t; 
     } 
    } 
    return null; 
} 

（請注意，這是方便爲extension method）。

Answer 3

當Label1的存在母版頁上：

如何講的內容頁面，您的主頁是

<%@ MasterType VirtualPath="~/MasterPages/PublicUI.Master" %> 

然後製作方法，在主像

public void SetMessage(string message) 
{ 
    Label1.Text = message; 
} 

並在後面的頁面代碼中調用它。

Master.SetMessage("<b>You chose <u>" + DropDownList1.SelectedValue + "</u> from the dropdown menu.</b>"); 

當Label1的內容頁

上存在如果它僅僅是在同一頁上，只需撥打Label1.Text = someString; 或者由於某種原因需要使用FindControl，請將您的Master.FindControl更改爲FindControl