dynamic - asp.net FindControl Recursively

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dynamic - asp.net FindControl Recursively

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

dynamic - asp.net FindControl Recursively

This is a really weird one - I will do my best to explain.

I have a basic master page:

<%@ Master Language="VB" CodeFile="MasterPage.master.vb" Inherits="master_MasterPage" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <asp:ContentPlaceHolder ID="head" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
        </asp:ContentPlaceHolder>
        <asp:PlaceHolder ID="PH1" runat="server" />
        <asp:PlaceHolder ID="PH2" runat="server" />
    </div>
    </form>
</body>
</html>

And a standard child page:

  <%@ Page Title="" Language="VB" MasterPageFile="~/master/MasterPage.master" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="master_Default" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
</asp:Content>

I have the following extension methods for finding controls recursively:

Option Strict On
Option Explicit On

Imports System.Runtime.CompilerServices
Imports System.Web.UI

Public Module ExtensionMethods

    <Extension()> _
    Public Function FindControlRecursively(ByVal parentControl As System.Web.UI.Control, ByVal controlID As String) As System.Web.UI.Control

        If parentControl.ID = controlID Then
            Return parentControl
        End If

        For Each c As System.Web.UI.Control In parentControl.Controls
            Dim child As System.Web.UI.Control = FindControlRecursively(c, controlID)
            If child IsNot Nothing Then
                Return child
            End If
        Next

        Return Nothing

    End Function

    <Extension()> _
    Public Function FindControlIterative(ByVal rootControl As Control, ByVal controlId As String) As Control

        Dim rc As Control = rootControl
        Dim ll As LinkedList(Of Control) = New LinkedList(Of Control)

        Do While (rc IsNot Nothing)
            If rc.ID = controlId Then
                Return rc
            End If
            For Each child As Control In rc.Controls
                If child.ID = controlId Then
                    Return child
                End If
                If child.HasControls() Then
                    ll.AddLast(child)
                End If
            Next
            rc = ll.First.Value
            ll.Remove(rc)
        Loop

        Return Nothing

    End Function

End Module

I have a control with a listview:

<%@ Control Language="VB" AutoEventWireup="false" CodeFile="control-1.ascx.vb" Inherits="controls_control_1" %>
<p>
    Control 1</p>
<asp:ListView ID="lv" runat="server">
    <ItemTemplate>
        <div>
            <asp:Literal ID="Name" runat="server" Text='<%#Eval("Name") %>' />
            <asp:LinkButton ID="TestButton" runat="server">Test</asp:LinkButton>
        </div>
    </ItemTemplate>
</asp:ListView>

That is databound:

Partial Class controls_control_1
    Inherits System.Web.UI.UserControl

    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        If Not Page.IsPostBack Then

            Dim l As New List(Of Person)
            Dim j As New Person
            j.Name = "John"
            l.Add(j)

            lv.DataSource = l
            lv.DataBind()

        End If

    End Sub

End Class

Public Class Person
    Public Property Name As String
End Class

I have a second control that is very basic:

<%@ Control Language="VB" AutoEventWireup="false" CodeFile="control-2.ascx.vb" Inherits="controls_control_2" %>
<p>Control 2</p>

In my child page, I have the following code to load the controls:

Option Strict On
Option Explicit On

Partial Class master_Default
    Inherits System.Web.UI.Page

    Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init

        Dim controlInstance1 As System.Web.UI.Control = LoadControl("~/controls/control-1.ascx")
        controlInstance1.ID = "control_1"

        Dim zone As System.Web.UI.Control = Me.Master.FindControlRecursively("PH1")

        zone.Controls.Add(controlInstance1)

        Dim controlInstance2 As System.Web.UI.Control = LoadControl("~/controls/control-2.ascx")
        controlInstance2.ID = "control_2"

        Dim zone2 As System.Web.UI.Control = Me.Master.FindControlRecursively("PH2")

        zone2.Controls.Add(controlInstance2)

    End Sub

End Class

This loads the controls, but if I click the Test button in the listview, the page loses the data in the listview after postback.

If I change the FindControlRecursively calls to FindControlIterative, when I click the test button, the data in the listview is retained after the postback.

Anybody have any idea what the FindControlRecursively call might be doing to cause the listview to lose it's data? This only happens if control-2 is added to the page - if it is not, and control-1 is loaded using FindControlRecursively, data is retained correctly after postback.

Thanks in advance...this one is driving me nuts, and it took me a while to figure out where exactly it was breaking down.

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1 Answer

深蓝 · Answer 1 · 2021-10-23T21:38:10+0000

Why don't you simply expose properties that return PH1 and PH2, because the master has the reference of them and you don't need to iterate all child controls of master:

Public ReadOnly Property Container1 As PlaceHolder
    Get
        Return Me.PH1
    End Get
End Property

Public ReadOnly Property Container2 As PlaceHolder
    Get
        Return Me.PH2
    End Get
End Property

You can access them:

Dim ph1 As PlaceHolder = DirectCast(Me.Master, myMaster).Container1
Dim ph2 As PlaceHolder = DirectCast(Me.Master, myMaster).Container2

Another problem is this line:

controlInstance1.ID = "control_2"

You are setting only controlInstance1's ID twice, but that doesn't cause your issue.

Your main problem is that you are adding the controls to the placeholders in Page_Init instead of Page_Load. Therefore the UserControls can't load their ViewState and the ListView is empty. Recreate them in Page_Load and it will work.

Edit: But i must admit that i don't know why your iterate extension wins over the recursive. The reference on the placeholders are the same, they shouldn't work both, weird.

summary:

it works with my properties,
putting all in page's load event handler instead init
with your iterate-extension(for whatever reasons)

It also works if you add both UserControls at last, after you've found the placeholders via FindControlRecursively.

zone.Controls.Add(controlInstance1)
zone2.Controls.Add(controlInstance2)

I'm losing motivation on this, but i'm sure you'll find the answer here. Controls.Add loads it's parent's ViewState into all childs and therefore it depends on when you add the controls, also the index of the controls in their parent controls must be the same on postback to reload the ViewState.

Your recursive extension method touches control1's ID after you've added it to PH1(while searching PH2), the iterative extension does not. I assume that this corrupts it's ViewState in Page_Init.

Conclusion Use properties instead

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dynamic - asp.net FindControl Recursively

dynamic - asp.net FindControl Recursively

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