Hello Fred,

Note that RegisterStartupScript() will put the <script> block just before the closing </form> tag and the code in the function will be executed in window.onload - i.e. when the page is loaded. All ASP.NET AJAX controls however (including ours), are registered on the page on a later stage - in Sys.Application.Load - you can easily verify that by putting ASP.NET AJAX controls on a page and examining the HTML dump once the page is loaded in the browser. Basically, what happens is that your code is outputted and executed on the page, before the Ajax controls are rendered, which causes the error.

To summarize, for your code to work, you need to execute it on a later stage. Please check this blog post for more information on the subject and several suggested approaches. If you still experience problems after that, please provide a sample project where I can examine your exact setup. I am asking this because as you will see in the blog, the approaches differ, depending on the setup and logic.



Best wishes,
Georgi Tunev
the Telerik team