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Another Simple Wait Page

, 11 Oct 2006 CPOL
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An article to demonstrate to use asynchronous delegate call and client script callback to build long wait page

Sample Image - screenshot.jpg


Sometime you need to process a lengthy job in your web form. It is a better idea to build a wait page instead of letting your user just stare at the screen doing nothing.


There are a lot of solutions over Internet to accomplish this purpose. My solution isn't new, the initial idea is from Brian Dunnington's article Building a Better Wait Page in code project. I have been using this approach for long time in ASP.NET 1.1. Since the release of ASP.NET 2.0, Microsoft makes a lot easier to implement client side callback, I decided to expand this solution to include AJAX function, so that we don't need to refresh the page from time to time.

Using the code

There are 3 main areas you need to consider in this solution

  • Asynchronous delegate call
  • Hook up with client callback
  • Client script

Asynchronous delegate call

    /// <span class="code-SummaryComment"><summary>

Hook up with client callback

In order to hook with client callback, you have to implement ICallbackEventHandler interface

public partial class Process : System.Web.UI.Page, ICallbackEventHandler {
 protected string CallBackEventReference;

Then we need to prepare script to reference client function

string ScriptRef = this.ClientScript.GetCallbackEventReference(

CallBackEventReference = ScriptRef;

This will create a client function like WebForm_DoCallback('__Page','CheckStatus',ClientCallBack,this,ClientCallBack,true);

From this point, you need to implement 2 functions of this interface

    /// <span class="code-SummaryComment"><summary>

Client script

On the client side, we need to prepare some javascript functions with talk with server.

function ClientCallBack(Result, Context) {
    if (Result != "") {
        window.location.href = Result;

In order to make broswer to check server process status, we also need a timer to make client script to run at some certain interval.

<body onload="startClock();">
// check server every 5 seconds, adjust this value to your own preference
var interval = 5
var x = interval
function startClock(){
    x = x - 1;
    setTimeout("startClock()", 1000);
    if(x == 0){
       <%= this.CallBackEventReference %>

So what is happnening now? The client script will be triggered every 5 seconds to talk with server. And ICallbackEventHandler.GetCallbackResult() method on server side will be called every 5 seconds. In this method it checks Session object value, if asynchronous call finishes the process, it will call the MyCallback to set Session object to a not null value returned from process result to make ICallbackEventHandler.GetCallbackResult() be able to capture the result.

Points of Interest

My first explore of client callback, I hope to add more features later.


  • 2006-10-11 Initial version.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Hardy Wang
Canada Canada
No Biography provided

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Comments and Discussions

GeneralIs it possible to use this in a wizard Pin
D.Sridhar5-May-08 8:26
memberD.Sridhar5-May-08 8:26 
GeneralRe: Is it possible to use this in a wizard Pin
Hardy Wang5-May-08 8:37
memberHardy Wang5-May-08 8:37 
Generalthe job is load a new page Pin
jrmora7-Mar-08 5:02
memberjrmora7-Mar-08 5:02 
GeneralRe: the job is load a new page Pin
Hardy7-Mar-08 6:07
memberHardy7-Mar-08 6:07 
QuestionSQL Server Session State Pin
Simon Byrne16-May-07 4:37
memberSimon Byrne16-May-07 4:37 
AnswerRe: SQL Server Session State Pin
Hardy4-Jun-07 3:38
memberHardy4-Jun-07 3:38 
Generalbrowser back button Pin
Joaquin Luna9-Dec-06 10:36
memberJoaquin Luna9-Dec-06 10:36 Pin
Papichulo.NET18-Oct-06 11:04
memberPapichulo.NET18-Oct-06 11:04 
GeneralRe: Pin
Christian Graus18-Oct-06 11:39
staffChristian Graus18-Oct-06 11:39 
GeneralRe: Pin
Hardy19-Oct-06 5:15
memberHardy19-Oct-06 5:15 
AnswerRe: Pin
Joaquin Luna8-Dec-06 11:33
memberJoaquin Luna8-Dec-06 11:33 
Well I've never used C# and I only started using Visual Basic last week, but I think I've got the wait page working under VB.   Sorry if there are any bugs I didn't catch, I'm new to the whole 'making websites' business.   Good Luck!

Here is the code for Default.aspx.vb

Imports System
Imports System.Data
Imports System.Configuration
Imports System.Web
Imports System.Web.Security
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Web.UI.WebControls.WebParts
Imports System.Web.UI.HtmlControls

Partial Class defaultclass
      Inherits System.Web.UI.Page

      Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

      End Sub

      Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
            If Not Me.IsValid Then
            End If
      End Sub

      Private Min As Integer

      Public ReadOnly Property Minute() As Integer
                  Min = 0
                  Integer.TryParse(txtMinute.Text, Min)
                  Return Min
            End Get
      End Property

End Class

And here is the code for Process.aspx.vb

Imports System
Imports System.Data
Imports System.Configuration
Imports System.Collections
Imports System.Web
Imports System.Web.Security
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Web.UI.WebControls.WebParts
Imports System.Web.UI.HtmlControls
Imports System.Runtime.Remoting.Messaging

Partial Class Process
      Inherits System.Web.UI.Page
      Implements ICallbackEventHandler

      Protected CallBackEventReference As String
      Private Delegate Function DoJobDelegate(ByVal minute As Integer) As Boolean
      Dim eventArgument As String = ""

      Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
            If Not IsPostBack And Not IsCallback Then
                  Session("NewOrderResult") = Nothing
                  Dim prevPage As defaultclass = PreviousPage
                  If prevPage IsNot Nothing Then
                        lMinute.Text = "Your request will finish in " & prevPage.Minute & " minute(s)."
                  End If
            End If
      End Sub

      Private Function DoJobAsync(ByVal minute As Integer) As IAsyncResult
            Dim doDelegate As New DoJobDelegate(AddressOf dolongjob)
            Dim ar As IAsyncResult = doDelegate.BeginInvoke(minute, New AsyncCallback(AddressOf mycallback), Nothing)
            Return ar
      End Function

      Private Sub mycallback(ByVal ar As IAsyncResult)
            Dim aResult As AsyncResult = ar
            Dim doDelegate As DoJobDelegate = aResult.AsyncDelegate
            Session("NewOrderResult") = doDelegate.EndInvoke(ar)
      End Sub

      Private Function dolongjob(ByVal seconds As Integer) As Boolean
            System.Threading.Thread.Sleep(seconds * 1000)
            If (seconds Mod 2 = 0) Then
                  Return True
                  Return False
            End If

      End Function

      Protected Sub setcallbackstring()
            Dim ScriptRef As String = ClientScript.GetCallbackEventReference( _
                        Me, "'CheckStatus'", "ClientCallBack", "this", "ClientCallBack", True)

            CallBackEventReference = ScriptRef
      End Sub

      Function GetCallbackResult() As String Implements ICallbackEventHandler.GetCallbackResult
            If Session("NewOrderResult") IsNot Nothing Then
                  If Convert.ToBoolean(Session("NewOrderResult")) Then
                        Return "even.htm"
                        Return "odd.htm"
                  End If
                  Return ""
            End If

      End Function

      Sub RaiseCallbackEvent(ByVal eventArgument As String) Implements ICallbackEventHandler.RaiseCallbackEvent
            Me.eventArgument = eventArgument
      End Sub

End Class

Joaquin Luna
Roanoke VA
Questionempty source files? Pin
alex_-_12-Oct-06 4:42
memberalex_-_12-Oct-06 4:42 
AnswerRe: empty source files? Pin
Hardy12-Oct-06 4:46
memberHardy12-Oct-06 4:46 
GeneralThanks ! Pin
dapoussin12-Oct-06 2:19
memberdapoussin12-Oct-06 2:19 

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