6.3.1 Problem You have a page with a state whose value the page needs to remember between postbacks to determine how the page should be displayed. 6.3.2 Solution Use the RegisterHiddenField method of the Page object to create a hidden text field in the rendered page. Nothing special is required in the .aspx file. Instead, in the code-behind class for the page, use the .NET language of your choice to: -
Programmatically insert a hidden text field into the form using the RegisterHiddenField method of the Page object. -
Use this field to store the value of the state you wish to preserve between postbacks . -
Access the hidden text field on subsequent submissions of the page to the server. Figure 6-3 shows the output of a simple form that preserves the page state using a hidden field. Clicking the Prev/ Next buttons decrements/ increments the value in the hidden field by one. Example 6-11 shows the .aspx file that produces the form. Example 6-12 and Example 6-13 show the companion VB and C# code-behind files that demonstrate how to access the application state data. Figure 6-3. Maintaining page state with a hidden field 
6.3.3 Discussion An approach we favor for remembering the current state of a value between postbacks to the server involves programmatically inserting one or more hidden text fields into a form. A similar technique is commonly used by classic ASP developers, who explicitly place a hidden text field in the form and then set its value in the page code. You can use the same technique in ASP.NET pages. However, ASP.NET, unlike ASP, lets you programmatically insert hidden text fields into a form at runtime. This has two significant advantages. First, all code that you write is kept in the code-behind, allowing the .aspx file to contain only the presentation aspects (hidden fields contain no user interface). Second, the hidden fields are completely decoupled from the .aspx file. Because no hidden fields are explicitly placed in the .aspx file, you do not have the maintenance issues associated with placing the server control in the .aspx file and then declaring it in the code-behind. One negative aspect of using hidden fields for storing state data is the data is stored in plain text. If you do not want the data to be readily visible in the rendered HTML, you should store the data in the ViewState instead, as described in Recipe 6.4. As an example of when you might use hidden fields, suppose your application supports complex sorting within a DataGrid , such as a two-way sort that involves both ascending and descending columns and might even be supported by your own sorting expressions. You can use hidden fields to maintain information about how a user has performed a specific sort so that you can preserve the user's sorting order and choice of sorted columns . Our example that illustrates the solution is kept relatively simple to focus on the concept of storing information in hidden fields each time a page is submitted to the server. The example programmatically inserts a single hidden field into a form and then increments or decrements its value based on the button clicked by the user. Because the hidden field is accessed from many points in the code-behind, a constant at the class level, PAGE_STATE , defines the name of the hidden field that is programmatically inserted into the rendered page. This avoids hardcoding the hidden field name throughout the code and the associated maintenance issues. In the Page_Load method, updatePage , which is the method used to update the page based on the page state, is called passing a value of 0 as the initial page state value. In the updatePage method, a label is updated to indicate the current state value. After updating the page to reflect the current page state, the RegisterHiddenField method of the Page object is used to save the page state value in the rendered page. The rendered page contains the following hidden form variable that can be retrieved when the page is submitted to the server. The name is set to the value of the PAGE_STATE constant, and the value is set to the current page state: <input type="hidden" name="PageState" value="0" />
 | Hidden fields always store the value as a string. This requires any data saved using the technique to be converted to a string when the RegisterHiddenField method is called. | |
In this example, two buttons are provided to increment and decrement the page state, respectively. An event handler is added in the code-behind for each of the button click events. The event handler for the increment button is called btnNextState_Click ; the event handler for the decrement button is btnPrevState_Click . In the event handlers, the current page state is retrieved from the hidden field, adjusted as required, and then the updatePage method is again called to update the page and save the new page state value in the form when the page is rendered. Even though the example presented here is simplistic, it nonetheless demonstrates how easy ASP.NET has made it to persist information that is needed for each round trip between the server and the client. For an example of persisting more complex data between page submittals, refer to Recipe 6.4. 6.3.4 See Also Recipe 6.4 Example 6-11. Maintaining page state with hidden values (.aspx) <%@ Page Language="vb" AutoEventWireup="false" Codebehind="CH06HiddenValuesVB.aspx.vb" Inherits="ASPNetCookbook.VBExamples.CH06HiddenValuesVB" %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <html> <head> <title>Hidden Values</title> <link rel="stylesheet" href="css/ASPNetCookbook.css"> </head> <body leftmargin="0" marginheight="0" marginwidth="0" topmargin="0"> <form id="frmStateHiddenValues" method="post" runat="server"> <table width="100%" cellpadding="0" cellspacing="0" border="0"> <tr> <td align="center"> <img src="images/ASPNETCookbookHeading_blue.gif"> </td> </tr> <tr> <td class="dividerLine"> <img src="images/spacer.gif" height="6" border="0"></td> </tr> </table> <table width="90%" align="center" border="0"> <tr> <td><img src="images/spacer.gif" height="10" border="0"></td> </tr> <tr> <td align="center" class="PageHeading"> Maintaining Page State With Hidden Fields (VB) </td> </tr> <tr> <td><img src="images/spacer.gif" height="10" border="0"></td> </tr> <tr> <td align="center"> <table width="30%" align="center" id="tabState" runat="server"> <tr class="MenuItem"> <td>Current Page State: </td> <td><asp:Label ID="labPageState" Runat="server" /></td> </tr> <tr> <td colspan="2" align="center"> <br /> <asp:ImageButton ID="btnPrevState" Runat="server" ImageUrl="images/buttons/button_prev.gif" /> <asp:ImageButton ID="btnNextState" Runat="server" ImageUrl="images/buttons/button_next.gif" /> </td> </tr> </table> </td> </tr> </table> </form> </body> </html>
Example 6-12. Maintaining page state with hidden values code-behind (.vb) Option Explicit On Option Strict On '----------------------------------------------------------------------------- ' ' Module Name: CH06HiddenValuesVB.aspx.vb ' ' Description: This module provides the code behind for ' CH06HiddenValuesVB.aspx ' '***************************************************************************** Namespace ASPNetCookbook.VBExamples Public Class CH06HiddenValuesVB Inherits System.Web.UI.Page 'controls on the form Protected tabState As System.Web.UI.HtmlControls.HtmlTable Protected labPageState As System.Web.UI.WebControls.Label Protected WithEvents btnPrevState As System.Web.UI.WebControls.ImageButton Protected WithEvents btnNextState As System.Web.UI.WebControls.ImageButton 'The following variable defines the name of the hidden field in the 'form used to track the page state Private PAGE_STATE As String = "PageState" '************************************************************************* ' ' ROUTINE: Page_Load ' ' DESCRIPTION: This routine provides the event handler for the page load ' event. It is responsible for initializing the controls ' on the page. '------------------------------------------------------------------------- Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load If (Not Page.IsPostBack( )) Then 'not a postback so initialize the page state to 0 updatePage(0) End If End Sub 'Page_Load '************************************************************************* ' ' ROUTINE: btnPrevState_Click ' ' DESCRIPTION: This routine provides the event handler for the previous ' state button click event. It is responsible for setting ' the page state back one state. '------------------------------------------------------------------------- Private Sub btnPrevState_Click(ByVal sender As Object, _ ByVal e As System.Web.UI.ImageClickEventArgs) _ Handles btnPrevState.Click Dim pageState As Integer pageState = CInt(Request.Form(PAGE_STATE)) pageState -= 1 updatePage(pageState) End Sub 'btnPrevState_Click '************************************************************************* ' ' ROUTINE: btnNextState_Click ' ' DESCRIPTION: This routine provides the event handler for the next ' state button click event. It is responsible for setting ' the page state ahead one state. '------------------------------------------------------------------------- Private Sub btnNextState_Click(ByVal sender As Object, _ ByVal e As System.Web.UI.ImageClickEventArgs) _ Handles btnNextState.Click Dim pageState As Integer pageState = CInt(Request.Form(PAGE_STATE)) pageState += 1 updatePage(pageState) End Sub 'btnNextState_Click '************************************************************************* ' ' ROUTINE: updatePage ' ' DESCRIPTION: This routine updates the page for the passed page state. '------------------------------------------------------------------------- Private Sub updatePage(ByVal pageState As Integer) 'update the current page state display labPageState.Text = pageState.ToString( ) 'register the hidden field used to persist the current page state RegisterHiddenField(PAGE_STATE, _ pageState.ToString( )) End Sub 'updatePage End Class 'CH06HiddenValuesVB End Namespace
Example 6-13. Maintaining page state with hidden values code-behind (.cs) //---------------------------------------------------------------------------- // // Module Name: CH06HiddenValuesCS.ascx.cs // // Description: This module provides the code behind for // CH06HiddenValuesCS.ascx // //**************************************************************************** using System; using System.Web.UI; using System.Web.UI.WebControls; namespace ASPNetCookbook.CSExamples { public class CH06HiddenValuesCS : System.Web.UI.Page { // controls on the form protected System.Web.UI.HtmlControls.HtmlTable tabState; protected System.Web.UI.WebControls.Label labPageState; protected System.Web.UI.WebControls.ImageButton btnPrevState; protected System.Web.UI.WebControls.ImageButton btnNextState; // The following variable defines the name of the hidden field in the // form used to track the page state private String PAGE_STATE = "PageState"; //************************************************************************ // // ROUTINE: Page_Load // // DESCRIPTION: This routine provides the event handler for the page // load event. It is responsible for initializing the // controls on page. //------------------------------------------------------------------------ private void Page_Load(object sender, System.EventArgs e) { // wire the next and prev button click event handlers this.btnPrevState.Click += new ImageClickEventHandler(this.btnPrevState_Click); this.btnNextState.Click += new ImageClickEventHandler(this.btnNextState_Click); if (!Page.IsPostBack) { // not a postback so initialize the page state to 0 updatePage(0); } } // Page_Load //************************************************************************ // // ROUTINE: btnPrevState_Click // // DESCRIPTION: This routine provides the event handler for the // previous state button click event. It is responsible // for setting the page state back one state. //------------------------------------------------------------------------ private void btnPrevState_Click(Object sender, System.Web.UI.ImageClickEventArgs e) { int pageState; pageState = Convert.ToInt32(Request.Form[PAGE_STATE]); pageState -= 1; updatePage(pageState); } // btnPrevState_Click //************************************************************************ // // ROUTINE: btnNextState_Click // // DESCRIPTION: This routine provides the event handler for the // next state button click event. It is responsible // for setting the page state back one state. //------------------------------------------------------------------------ private void btnNextState_Click(Object sender, System.Web.UI.ImageClickEventArgs e) { int pageState; pageState = Convert.ToInt32(Request.Form[PAGE_STATE]); pageState += 1; updatePage(pageState); } // btnNextState_Click //************************************************************************ // // ROUTINE: updatePage // // DESCRIPTION: This routine updates the page for the passed page state. // //------------------------------------------------------------------------ private void updatePage(int pageState) { // update the current page state display labPageState.Text = pageState.ToString( ); // register the hidden field used to persist the current page state RegisterHiddenField(PAGE_STATE, pageState.ToString( )); } // updatePage } // CH06HiddenValuesCS }
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