Hidden field : Hidden « HTML Controls « ASP.NET Tutorial

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="Default_aspx" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>HiddenField Control</title>
</head>

<script language=javascript>
  function ChangeHiddenValue()
  {
    alert('Entering ChangeHiddenValue');

    var hdnId = '<%=hdnSecretValue.ClientID%>'
    var hdn = document.getElementById(hdnId);

    var txt = document.getElementById('txtSecretValue');

    hdn.value = txt.value;
    alert('Value changed');
  }
</script>

<body>
    <form id="form1" runat="server">
    <div>
    <h1>HiddenField Control</h1>
      <h2>This page was posted at <% =DateTime.Now.ToString() %>.</h2>
    <asp:HiddenField ID="hdnSecretValue" runat="server" OnValueChanged="hdnSecretValue_ValueChanged" />
      Enter secret value:
      <asp:TextBox ID="txtSecretValue" runat="server" />
      <br />
    <br />
    <input type=button value="Change hidden value" onclick="ChangeHiddenValue()" />
    <asp:Button ID="btnPost" runat="server" Text="Post" />
    <br />
    <br />
    <asp:Label ID="lblMessage" runat="server" Text=""/>
    </div>
    </form>
</body>
</html>

File: Default.aspx.cs

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class Default_aspx : System.Web.UI.Page 
{
   protected void hdnSecretValue_ValueChanged(object sender, EventArgs e)
   {
     HiddenField hdn = (HiddenField)sender;
     lblMessage.Text = "The new value is " + hdn.Value + ".";
   }
 }
4.7.Hidden
	4.7.1.	Hidden field