Can we use "async void" event handlers in web user control? I tried and failed with "An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked <%@ Page Async="true" %>.". And I'm pretty sure that I have enabled async for the container page.