C#，FindControl

Question

我很抱歉，但我不明白为什么这个不起作用。编译后，我收到一个“空引用异常”。请帮帮忙。

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

感谢Mr. Atwood himself，下面是该方法的递归版本。我还建议在控件上测试null，我还介绍了如何更改代码来实现这一点。

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Label1存在于母版页上时为：

如何告诉内容页您的母版页在哪里

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

然后在master中创建一个方法，如下所示

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

并在page的代码中调用它。

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

当Label1存在于内容页上时为

如果只是在同一页上，只需调用Label1.Text = someString；或者，如果出于某种原因需要使用FindControl，请将Master.FindControl更改为FindControl

Answer 3

FindControl只搜索直接的子级(从技术上讲，搜索到下一个NamingContainer)，而不是整个控件树。因为Label1不是Master的直接子对象，所以Master.FindControl不会定位它。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;
}

(请注意，作为extension method，这很方便)。