RadCaptcha control does not support client-side validation. A post back
to the server is always needed in order for the Captcha to perform
validation with the help of the Telerik RadControls DLL deployed in the
server. The CaptchaValidate server side event can be used to cancel the
default validation mechanism
of the captcha control and to specify your own logic for validating the
RadCaptcha. RadCaptcha neither makes any external calls nor requires any
web service calls to perform the validation mechanism.
Please note if you dynamically create the Captcha control, you must add it to the
page on every postback, and before the ViewState is loaded. That is if
you add the control in the Page.Load, it is too late, because the
ViewState has been loaded, and the Captcha will always invalidate the
code and I suggest you use the Page.Init method instead.
Suppose you are using a RadAjaxPanel or an Updatepanel in your
application, RadCaptcha does not properly validate the code entered if
your code is not properly written and maintained like an ajax request
executed before the Captcha control is validated. In such scenarios
RadCaptcha should be placed inside the Updatepanel (Reason : The
RadCaptcha always returns false, because after clicking the "Post
Back" control such as a button, a new Captcha code is generated, but not
shown on the
image. RadCaptcha is designed to refresh its image on every postback
request, no matter if the ajax request affects the image itself.To
resolve the issue, please set UpdateMode="Always" to the UpdatePanel
containing the RadCaptcha control).
You can also have a look at this
documentation on Using RadCaptcha in a WebFarm/WebGarden Environment.
Hope this helps,