In the previous posts in this series I introduced the client side state management and one of its techniques - the ViewState. Today I'm going to drill down into the hidden fields technique. As mentioned in the previous post, the ViewState stores its state in a hidden field.
What are Hidden Fields?
Hidden fields technique is widely used in ASP.NET programing. Hidden fields are html input control with hidden type that store hidden data in the html. An example for a hidden field can look like this:
<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />
Hidden Fields Values
Hidden fields store only one value in their value property. The value is saved as a string and therefore in order to use it for other types you need to perform casting. Hidden fields' data is submitted to the server only in HTTP post operation. You can see the stored data easily by using the View Source operation of the browser. You can see it by clicking the right mouse button and then choosing View Source from the menu (if the operation is available). Be aware not to use hidden fields to store confidential data! The values has page context and therefore when you leave a page the data stored in the hidden fields is disposed.
Server Control Hidden Fields
There are two types of server control hidden fields - System.Web.UI.WebControls.HiddenField and System.Web.UI.HtmlControls.HtmlInputHidden. Both types has the same primary Value property to hold the value of the hidden field. You should choose between these types whenever you need to use a server side hidden field (A note - The difference between HtmlControls and WebControls isn't in the context of this post). When you don't use server controls you can use the Request.Form NameValueCollection to get the hidden field value by providing the client
id of the hidden field. For example the code bellow will return the string value of the __EVENTTARGET hidden field:
string eventTarget = Request.Form["__EVENTTARGET"];
Hidden Fields Example
field is inserted into a label control. Pay attention that because I use an html hidden field that isn’t a server control after a second post back it’s value will be empty. The only reason that the label will still show a message is because
the label’s default value of the EnableViewState is true. The web page’s code:
<%@ Page="" Language="C#" AutoEventWireup="true" Codebehind="Default.aspx.cs"
Inherits="AJAXEnabledWebApplication1._Default" EnableSessionState="False" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
var hidden = document.getElementById('HTMLHiddenField');
if (hidden != null)
hidden.value = message;
<form id="form1" runat="server">
<asp:Label ID="lblHTMLHiddenField" runat="server"></asp:Label>
<input id="HTMLHiddenField" type="hidden" name="HTMLHiddenField"/>
<asp:Button ID="btnForPostBack" runat="server" Text="Do Postback" />
On the server side:
public partial class _Default : System.Web.UI.Page
private const string SCRIPT_KEY = "HtmlHiddenFieldScript";
#region Page Events
protected void Page_Load(object sender, EventArgs e)
// Insert message to the label of the html control hidden
// field if there is a value in the html hidden field
string message = Request.Form["HTMLHiddenField"];
lblHTMLHiddenField.Text = message;
protected void Page_PreRender(object sender, EventArgs e)
// Register a startup script in order to fill the html
// hidden field with a message value
if (!ClientScript.IsClientScriptBlockRegistered("HtmlHiddenFieldScript") && !IsPostBack)
ClientScript.RegisterStartupScript(typeof(Page), SCRIPT_KEY, "PutHtmlHiddenFieldValue('Html hidden hello world');", true);
To sum up the post, today I drilled down into the hidden fields technique. This technique is very popular and is used widely in ASP.NET. In the next post I'll continue the tour in the client side state management techniques.