Question

对不起，但我不明白为什么这不起作用。编译后，我收到一个“空引用异常”。请帮忙。

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

由Mr. Atwood himself提供，这是该方法的递归版本。我还建议在控件上测试null，并且我还包括如何更改代码来执行此操作。

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

当母版页上存在Label1时：

如何告诉您的母版页

的内容页面

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

然后在主人中制作一个方法

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

然后在页面代码中调用它。

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

当内容页面上存在Label1时

如果它只是在同一页面上，只需调用Label1.Text = someString; 或者如果由于某种原因需要使用FindControl，请将Master.FindControl更改为FindControl

Answer 3

FindControl仅搜索直接子项（技术上搜索下一个NamingContainer），而不搜索整个控制树。由于Label1不是Master的直接子项，Master.FindControl将找不到它。相反，您需要在直接父控件上执行FindControl，或者执行递归控件搜索：

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;
}

（注意这很方便extension method）。

C＃，FindControl

3 个答案: