Thomas’ Developer Blog

May 11, 2008

Asp.Net AJAX updatepanel postback trick with reCaptcha!

Filed under: AJAX, Asp.Net Framework 2.0, javascript, — Tags: , , , , , , — sanzon @ 7:54 am

You would have no idea how stressful this was trying to figure out!  Finally after a lot of hardwork I was able to figure out a semi hack around the problem.

Here’s the issue.  Everytime you run a partial postback through an updatepanel you run through a script only for that panel, but what if you want to have a clause in that code where you can do a full postpost so that you can maintain your viewstate and keep form data.

In this case we have a recaptcha device and the following .aspx source code:

  <div onkeypress="return no_enter(event,this,'none');" style="width:550px;">
    theme="clean" />
   <asp:UpdatePanel ID="upnlTAB5" UpdateMode="Conditional" runat="server">
     <asp:Label ID="lblResult" runat="server" />
     <br /><br />
     I confirm that all information provided is correct under my best knowledge and that by 
     intentionally providing inaccurate information
     will result in the termination and potential denial of service from this site.<br /><br />
     <div style="text-align:center;">
      <asp:button text="Confirm and Complete Registration" skinID="button" id="btnSubmit" 
               onclick="TAB5_Submit" runat="server" /><br /><br />
      <input type="button" id="btnConfirmation_Disagree" OnClick="javascript:NewUser_Disagree();" 
              class="buttoninput" style="height:20px;" runat="server"
              value="Disagree & Cancel My Registration" /><br /><br />
     <span class="infospan">Please only submit this form once!  This process may take 1 to 2 
          minutes to complete depending on site traffic. 
          If this page fails you may attempt to login and choose to resend your verification email.</span>

Now here is the issue.  You can’t place the recaptcha into the update panel or else it will not load correctly..  Thus if a user attempts do a partial postback on the updatepanel you will loose the recaptcha control.  So what you do is remove it from the update panel.  Seems easy right?  Not always the case.  What if it is incorrect?  Then you still have the exact same recaptcha.  What you will need to do is referesh the page!  Seems easy hey?

Remember they just filled out this long form!  They will have to retype everything else well.  I wish .Net had an easy solution, but sadly there isn’t.  To solve this what you have to do is create a hidden .Net button within a div with the style display none.

Here is the source for the button:

  <div style="display:none;"><asp:button id="btnRefresh" onclick="refresh" text="test" runat="server" /></div>

Now all we have to do is simply go ahead and use some java to trick the button into being submited.  So on our server side code we have:

 Sub TAB5_Submit(ByVal sender As System.Object, ByVal e As System.EventArgs)
  If Page.IsValid Then
   ScriptManager.RegisterStartupScript(Page, Page.GetType(), "Tab5InvalidFields", "alert('PASS');", true)
   ScriptManager.RegisterStartupScript(Page, Page.GetType(), "Tab5InvalidFields", _
         "alert('Confirmation: Invalid CAPTCHA, please try again.');document.getElementById('" _
         & btnRefresh.clientID.tostring & "').click();", true)
  End If
 End Sub

We use the scriptmanager to register a startup script to call a small javascript code.  In this case we alert the user that the information was incorrect for the CAPTCHA.  Now this is the trick!

We use javascript to click that hidden button!  That button will reload on the serverside and the viewstate will handle the rest!  So once the button is clicked we run the code:

 Sub refresh(ByVal sender As System.Object, ByVal e As System.EventArgs)
  tbxNewPassword.Attributes.Add("Value", tbxNewPassword.text)
  tbxConfirmNewPassword.Attributes.Add("Value", tbxConfirmNewPassword.text)
 End Sub

This sub reloads the page.  it can be blank if you wish, but you should take the time to refill the password boxes manually.  You simply create an attribute to reset the password values!  It works really well.

Now you are have to square one and they just have to reenter the captcha code!

Hope this helps, happy coding.



  1. Thank you for your help.

    Comment by Jillian — August 18, 2008 @ 6:42 pm

  2. I don’t understand what are you saying, please explain your trick

    Comment by Narinder — September 11, 2009 @ 8:19 am

  3. I will thankful you.

    Comment by Narinder — September 11, 2009 @ 8:20 am

  4. An Easy Way Out:

    Put recaptcha outside the tag.

    Add following to your source:

    function getCapcha(){
    = document.getElementsByName(“recaptcha_response_field”)[1].value;

    A textbox in the end, outside the Update Panel

    and you are done.

    Comment by Shuhab — February 11, 2010 @ 7:03 pm

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