Cannot find control with FindControl [duplicate] - asp.net

Question

This question already has answers here:
find a control in current page
(3 answers)
Closed 7 years ago.
I want to find the control ddlMaster.
Here is the excerpt aspx rah_sync_output.aspx in question:
<%# Page Language="vb" MasterPageFile="~/admin/Admin.master" AutoEventWireup="false" CodeBehind="rah_sync_output.aspx.vb" Inherits="TCDS.Web.admin.RahSyncOutput" EnableEventValidation="false" %>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:Button ID="btnExport" runat="server" Text="Export To Excel" OnClick = "ExportToExcel" /><br />
Locations
<asp:DropDownList ID="ddlMaster" runat="server" OnSelectedIndexChanged = "FilterChanged" AutoPostBack = "true" AppendDataBoundItems = "true">
</asp:DropDownList>
</asp:Content>
Here is the masterfile /admin/Admin.master excerpt:
<%# Master Language="VB" Inherits="XXX.Web.admin.AdminAdmin" Codebehind="Admin.master.vb" MasterPageFile="~/fullPage.master" %>
<asp:Content ID="headerContent" ContentPlaceHolderID="headContent" runat="server">
<asp:contentplaceholder id="head" runat="server">
</asp:contentplaceholder>
</asp:Content>
<asp:Content ID="menuContent" ContentPlaceHolderID="tcdsMenu" runat="server">
<ul class="ulMenu" style="">
<asp:Literal ID="ltrMenu" Text='' runat="server" />
</ul>
</asp:Content>
<asp:Content ID="tcdsContent" ContentPlaceHolderID="tcdsContent" runat="server">
<div id="content">
<asp:contentplaceholder id="ContentPlaceHolder1" runat="server">
</asp:contentplaceholder>
</div>
</asp:Content>
Here is the masterfile's masterfile fullPage.master excerpt:
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<div class="box">
<div id="divHeader" runat="server">
<asp:Literal ID="ltrAgencyLogo" runat="server" />
<asp:Literal ID="ltrAgencyName" runat="server" />
<a id="xxxicon" class="icnMCLLC" href="http://www.hidden.com" target="mcllc" runat="server">
<img src="/tcds/images/xxx.gif" border="0" alt="Click for technical assistance" width="110" height="36" />
</a>
<div class="bannerIcon"><img src="/tcds/images/icnHelp.gif" border="0" alt="TCDS Help" style='width: 24px;height: 24px;' /></div>
<span class="header"><asp:Literal ID="ltrHeader" runat="server" /></span>
<asp:contentplaceholder id="tcdsMenu" runat="server">
</asp:contentplaceholder>
</div>
<div id="content">
<asp:contentplaceholder id="tcdsContent" runat="server">
</asp:contentplaceholder>
</div>
</div>
</form>
My attempt in rah_sync_output.aspx.vb:
Dim ddl = DirectCast(Master.FindControl("tcdsContent").FindControl("ContentPlaceHolder1").FindControl("ddl" + columnName), DropDownList)
where columnName is "Master" at runtime.
Result is null. I either misunderstand that Master refers to the current master or the master's master. All I want to do is reference controls inside ContentPlaceHolderID="ContentPlaceHolder1"

Answer 1

Master.FindControl("tcdsContent").FindControl("ContentPlaceHolder1").FindControl("ddl" + columnName)
That's one way to do it, but you're probably missing a naming container somewhere in the control hierarchy. It would be easier to find the control recursively.
/// <summary>
/// recursively finds a child control of the specified parent.
/// </summary>
/// <param name="control"></param>
/// <param name="id"></param>
/// <returns></returns>
public static Control FindControlRecursive(this Control control, string id)
{
if (control == null) return null;
//try to find the control at the current level
Control ctrl = control.FindControl(id);
if (ctrl == null)
{
//search the children
foreach (Control child in control.Controls)
{
ctrl = FindControlRecursive(child, id);
if (ctrl != null) break;
}
}
return ctrl;
}