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Editorial Team
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Asked: 2026-05-28T02:47:51+00:00

I’m trying to retrieve a server control in JavaScript. For testing purposes I am

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I’m trying to retrieve a server control in JavaScript. For testing purposes I am calling the JavaScript function from the page load event.

protected void Page_Load(object sender, EventArgs e){
    ClientScript.RegisterClientScriptBlock(GetType(), "js", "confirmCallBack();", true);
}

And my JavaScript function is 

function confirmCallBack() {
    var a = document.getElementById('<%= Page.Master.FindControl("PlaceHolderContent").FindControl("Button1").ClientID %>');
    var b = document.getElementById('<%=Button1.ClientID%>');
}

my problem is that both a and b return null. Even when I view the page source the correct ClientID is returned.

I should add that I’m using master page.

Any ideas.

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1 Answer

  1. Editorial Team

    Gotcha!

    You have to use RegisterStartupScript instead of RegisterClientScriptBlock

    Here My Example.

    MasterPage:

    <%@ Master Language="C#" AutoEventWireup="true" CodeBehind="MasterPage.master.cs"
    Inherits="prueba.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>

    <script type="text/javascript">

        function confirmCallBack() {
            var a = document.getElementById('<%= Page.Master.FindControl("ContentPlaceHolder1").FindControl("Button1").ClientID %>');

            alert(a.value);

        }

    </script>

    <asp:ContentPlaceHolder ID="head" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
        </asp:ContentPlaceHolder>
    </div>
    </form>
</body>
</html>

    WebForm1.aspx

    <%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.Master" AutoEventWireup="true"
    CodeBehind="WebForm1.aspx.cs" Inherits="prueba.WebForm1" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">

</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:Button ID="Button1" runat="server" Text="Button" />
</asp:Content>

    WebForm1.aspx.cs

    using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace prueba
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            ClientScript.RegisterStartupScript(this.GetType(), "js", "confirmCallBack();", true);

        }
    }
}
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